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Despite lower‐than‐average total precipitation in the western states of the U.S., the 2021 "precipitation roller coaster" defined as large precipitation swings has pointed to a strong hydroclimatic intensity (HYINT). Here we examine the 2021 HYINT using an index—a product of the average precipitation intensity (INT) and dry spell length (DSL). HYINT exhibited an extremely high value in the western U.S. in 2021. INT and DSL contribute differently to the 2021 HYINT, with large spatial variability. Overall, the 2021 extreme HYINT in central California and Utah is tied more to large INT, than to DSL. Meanwhile, the historical trends in INT and DSL may have contributed to the extreme 2021 HYINT event. The fraction of attributable risk framework reveals that the 2021 extreme HYINT is more likely to occur with anthropogenic forcing (e.g., 7.3 times more likely for HYINT exceeding 1.3) than natural forcing alone. Plain Language Summary: The western U.S. is a hotspot for studying climate change impacts on the hydrological cycle. Despite lower‐than‐average total precipitation in 2021, the contrasting dryness and wetness in the western U.S. has been widely reported as a "precipitation roller coaster." In this paper we quantified the "precipitation roller coaster" using an index (hydroclimatic intensity [HYINT])—a product of average precipitation intensity during wet days and dry spell length (DSL). The study found that the 2021 extreme HYINT event was largely attributable to the combined impacts of precipitation intensity and DSL in California and Utah, with precipitation intensity playing a more important role. In contrast, the 2021 precipitation event in other western states exhibited divergent contributions from precipitation intensity and DSL. The southwestern U.S. has been identified as a hotspot for increasing HYINT, which is tied more to the increasing DSL than the precipitation intensity. The trends in DSL and precipitation intensity may have played a key role in driving the 2021 extreme HYINT event. Using climate model experiments with and without anthropogenic forcing, an extreme HYINT event in the western U.S. is more likely to occur with anthropogenic forcing. Key Points: Hydroclimatic intensity (HYINT) exhibited extremely high values in parts of the western U.S. in 2021, mainly caused by average precipitation intensityHYINT shows a significant rising trend in most of the southwestern U.S. mainly tied to a rising dry spell length trendThe extreme HYINT event is more likely to occur under anthropogenic forcing than natural forcing alone [ABSTRACT FROM AUTHOR]

Californian hydroclimate is strongly seasonal and prone to severe water shortages. Recent changes in climate trends have induced shifts in seasonality, thus exacerbating droughts, wildfires, and adverse water shortage effects on the environment and economy. Previous studies have examined the timing of the seasonal cycle shifts mainly as temperature driven earlier onset of the spring season. In this paper, we address quantitative changes in the onset, amounts, and termination of the precipitation season over the past 6 decades, as well as the large‐scale atmospheric circulation underpinning the seasonal cycle changes. We discover that the onset of the rainy season has been progressively delayed since the 1960s, and as a result the precipitation season has become shorter and sharper in California. The progressively later onset of the rainy season is shown to be related to the summer circulation pattern extending into autumn across the North Pacific, in particular, a delay in the strengthening of the Aleutian Low and later southward displacement of the North Pacific westerlies. Plain Language Summary: The rainy season over California is projected to show a distinct sharpening of the mean seasonal cycle, with winter precipitation increasing, and both autumn and spring precipitation decreasing. Our analysis of the past 6 decades of data for California suggests autumn decrease is already underway. A delayed start of the rainy season of 27 days since 1960s can exacerbate seasonal droughts and prolong the wildfire season. This delay occurs due to a number of conditions that controls precipitation: the summer circulation pattern has been extending throughout November across the North Pacific, and the wintertime strengthening of the Aleutian Low is delayed. Accordingly, the southward migration of the North Pacific jet stream as well as extratropical storm tracks are delayed, which marks the start of the California rainy season, are delayed. More work, using climate models, will be needed to provide a better understanding of atmospheric conditions across the North America and the North Pacific. However, our findings provide observational evidence for the projected rainfall change over California and inform ongoing discussion about the drying/wetting tendencies of the rainy season in California. Key Points: Rainfall data over the last 6 decades suggests a progressively delayed onset to the rainy season over CaliforniaA corresponding delay occurs in the transition from summer to wintertime circulation over the North PacificObserved autumn trends appear consistent with projected future shortening of the California rainy season [ABSTRACT FROM AUTHOR]

The variability of the summer North Pacific Subtropical High (NPSH) has substantial socioeconomic impacts. However, state‐of‐the‐art climate models significantly disagree on the response of the NPSH to anthropogenic warming. Inter‐model spread in NPSH projections originates from models' inconsistency in simulating tropical precipitation changes. This inconsistency in precipitation changes is partly due to inter‐model spread in tropical sea surface temperature (SST) changes, but it can also occur independently of uncertainty in SST changes. Here, we show that both types of precipitation uncertainty influence the NPSH via the Matsuno‐Gill wave response, but their relative impact varies by region. Through the modulation of low cloud fraction, inter‐model spread of the NPSH can have a further impact on extra‐tropical land surface temperature. The teleconnection between tropical precipitation and the NPSH is examined through a series of numerical experiments. Plain Language Summary: The North Pacific Subtropical High (NPSH) is a semi‐permanent high‐pressure system located in the subtropical North Pacific. The variability in the summer NPSH has a significant impact on the monsoon and typhoons over East Asia and the hydroclimate of California. However, future projections of the NPSH using state‐of‐the‐art climate models remain highly uncertain. By evaluating how much individual models deviate from the multi‐model mean at different locations, we find four hot spots of high uncertainty in NPSH projections. Our analysis further reveals that the primary source of model variance in changes in the NPSH is tropical precipitation, which can be attributed to both inter‐model SST‐driven and non‐inter‐model SST‐driven factors. Through numerical experiments, we demonstrate that the teleconnection between tropical precipitation and the NPSH is achieved through wave propagation. Key Points: Model spread in the response of the summer North Pacific Subtropical High (NPSH) to CO2 stems from model spread in simulating tropical processesModel spread in tropical sea surface temperature (SST) changes modulates the NPSH by influencing tropical precipitationModel spread in tropical precipitation changes independent of model spread in SST changes also adds to the uncertainty of the NPSH response [ABSTRACT FROM AUTHOR]

The connection between topography and erosion rate is central to understanding landscape evolution and sediment hazards. However, investigation of this relationship in steep landscapes has been limited due to expectations of: (a) decoupling between erosion rate and "threshold" hillslope morphology; and (b) bias in detrital cosmogenic nuclide erosion rates due to deep‐seated landslides. Here we compile 120 new and published 10Be erosion rates from catchments in the San Gabriel Mountains, California, and show that hillslope morphology and erosion rate are coupled for slopes approaching 50° due to progressive exposure of bare bedrock with increasing erosion rate. We find no evidence for drainage area dependence in 10Be erosion rates in catchments as small as 0.09 km2, and we show that landslide deposits influence erosion rate estimates mainly by adding scatter. Our results highlight the potential and importance of sampling small catchments to better understand steep hillslope processes. Plain Language Summary: In general, erosion rates increase as landscapes steepen. But where landslides are common, this relationship is thought to break down as hillslopes approach their angle of repose. The main tracer for measuring erosion rates, 10Be in sediment, can also be affected by landslides, and models predict it is unreliable for small watersheds in steep landscapes. Here, we compile an extensive data set of 10Be erosion rates from the San Gabriel Mountains of California. We show that slope and erosion rate are coupled well above the soil angle of repose due to systematic exposure of bedrock cliffs, supporting a new conceptual model for steep landscapes. The presence of landslides adds scatter but does not bias 10Be erosion rates, which yield robust results even in small, steep watersheds that have previously been avoided. Key Points: Progressive exposure of bare rock on steeper slopes leads to correlation of 10Be erosion rate and mean hillslope angle up to 47°Deep seated landslide deposits add scatter, but do not systematically bias 10Be erosion rate estimates in the San Gabriel MountainsNo evidence for drainage area dependence of 10Be erosion rates in upland catchments [ABSTRACT FROM AUTHOR]

Accurate observation of hillslope groundwater storage and instantaneous recharge remains difficult due to limited monitoring and the complexity of mountainous landscapes. We introduce a novel storage‐discharge method to estimate hillslope recharge and the recharge ratio—the fraction of precipitation that recharges groundwater. The method, which relies on streamflow data, is corroborated by independent measurements of water storage dynamics inside the Rivendell experimental hillslope at the Eel River Critical Zone Observatory, California, USA. We find that along‐hillslope patterns in bedrock weathering and plant‐driven storage dynamics govern the seasonal evolution of recharge ratios. Thinner weathering profiles and smaller root‐zone storage deficits near‐channel are replenished before larger ridge‐top deficits. Consequently, precipitation progressively activates groundwater from channel to divide, with an attendant increase in recharge ratios throughout the wet season. Our novel approach and process observations offer valuable insights into controls on groundwater recharge, enhancing our understanding of a critical flux in the hydrologic cycle. Plain Language Summary: Groundwater in hilly areas is an important source of water. The amount of rainfall that replenishes groundwater storage is known as groundwater recharge. Because groundwater recharge is challenging to measure directly, we applied a technique that makes it possible to use a more readily observable variable—streamflow, or the water flow in rivers and streams—to calculate how much water is stored in the hillslope as groundwater. This made it possible to use streamflow to estimate how much rainfall becomes groundwater recharge. By understanding the structure of the ground and how moisture is distributed, we were able to determine how the amount of recharge changes over the wet season. Our work improves understanding of how rainfall and plant water use affect groundwater recharge, which is important for managing water resources in mountain landscapes. Key Points: Increases in hillslope groundwater storage can be quantified from storage‐discharge relationsField measurements of groundwater and vadose zone (VZ) storage corroborate seasonality in recharge ratios (recharge per precipitation input)Recharge ratio increases with decreasing plant‐driven VZ (soil and rock) storage deficits, reflecting spatial variations in storage [ABSTRACT FROM AUTHOR]

The sensitivity of atmospheric river (AR)‐induced precipitation to climate change is primarily driven by increases in atmospheric water vapor with warming. However, the rate at which AR‐based precipitation intensifies with warming and whether this rate differs from non‐AR events remains uncertain. This work uses multiple statistical models to estimate regional, extreme precipitation‐temperature scaling rates in California for AR and non‐AR events. Scaling rates are determined using cold‐season daily and hourly precipitation, along with multiple temperature variables to assess robustness of the results. We find that regional scaling rates for ARs are consistently larger than non‐ARs, especially for hourly event maxima (posterior median scale rates of 5.7% and 2.4% per °C for ARs and non‐ARs, respectively). ARs remain near saturated (i.e., high relative humidity) and exhibit more lift and a stronger increase in specific humidity aloft with warming as compared to non‐ARs, helping to explain the difference in precipitation‐temperature scaling rates. Plain Language Summary: Atmospheric rivers are long and narrow conveyor belts of water vapor in the sky that transport moisture in the lower atmosphere. They often cause extreme precipitation that can cause flooding along the west coast of the United States. The rate of intensification of atmospheric river‐based precipitation with warming is a fundamental scientific question with important implications for adapting civil infrastructure to climate change. This study aims to quantify the relationship between atmospheric river‐based extreme precipitation and temperature, and to determine how it differs from other precipitation events. To do so, we utilize multiple statistical methods to quantify cold‐season precipitation‐temperature scaling rates at weather stations across California. In our analysis, we consider different temperature variables, timescales of data, and ways of accumulating precipitation during events to determine the robustness of our results. Overall, we find that extreme precipitation increases faster with warming during atmospheric rivers compared to other types of events. This difference is linked to the fact that atmospheric rivers exhibit a more direct increase in moisture content and lift with warming. These results suggest that extreme precipitation during atmospheric rivers will increase faster than other events as temperatures rise in the future, which could accelerate damage linked to future flooding events. Key Points: Precipitation‐temperature (P‐T) scaling rates during atmospheric rivers (ARs) are larger than other precipitation events in CaliforniaP‐T scaling rates for AR‐based events approach the Clausius‐Clapeyron rate (7% per °C) when using dew point temperatureThe differences in P‐T scaling rates for ARs versus non‐ARs are linked to the sensitivity of moisture content and lifting mechanism to warming [ABSTRACT FROM AUTHOR]

Increased wildfire activity in the western United States has exposed regional gaps in our understanding of postfire debris‐flow generation. To address this problem, we characterized flows in an unstudied area to test the rainfall intensity‐duration control of the hazard. Our rainfall measurements and field observations from the northern Sierra Nevada (California, USA) show that debris flows resulted from a short burst rainfall during a low‐accumulation storm. In contrast, a much higher accumulation storm (∼10 times more rainfall) with lower short‐duration rainfall rates only produced low‐hazard flooding. We conclude that total storm rainfall is not an ideal metric for identifying the rainfall conditions that initiate runoff‐generated debris flows in the first year after wildfire. Rather, a focus on short‐duration (<1 hr), high‐intensity rainfall that can occur during localized thunderstorms, or bands of intense rainfall during prolonged rainstorms, is more beneficial for the purposes of hazard assessment and warning. Plain Language Summary: The rainfall conditions that trigger fast‐moving slurries of water and debris (or "debris flows") after wildfire have been studied extensively in southern California. However, the transferability of these concepts to areas further north along the western margin of the United States in regions that are being affected by increased fire activity is unclear. We present rainfall measurements from contrasting storm types that affected an area burned by the 2021 Dixie Fire in northern California and describe the resultant postfire flows, which ranged from nuisance water‐rich flows to large and dangerous debris flows. Short bursts of rainfall, as opposed to prolonged (multiday) periods of rainfall, initiated the most hazardous flows. We conclude that rainfall intensity measured over short durations, not storm rainfall total, is the most useful measure of rainfall for identifying runoff‐induced debris‐flow hazard potential in the first year after fire. Key Points: Postfire debris flows described in region where they were previously undocumentedFlow type and sediment yield controlled by differences in short‐duration rainfall ratesDebris‐flow initiation not sensitive to storm rainfall totals in first year after fire [ABSTRACT FROM AUTHOR]

In August of 2020, an earthquake sequence initiated within the Maacama fault zone in northern California, raising questions about its relationship with the larger‐scale fault. To investigate the faulting geometry and its implications for physical processes driving seismicity, we applied an integrated, multi‐faceted seismic analysis including waveform‐correlation‐based event detection, relative relocation, and high‐precision focal mechanisms. To determine mechanisms for a large population of very small earthquakes (predominantly M < 1), we combined correlation‐derived polarity analysis with a recently developed technique to determine S/P amplitude ratios from single seismic components. Finally, we applied an iterative stress inversion to distinguish between the likely fault and auxiliary planes. This analysis reveals that although the sequence initiated on a right‐lateral fault, plausibly a strand of the Maacama Fault, it also activated numerous en echelon left‐lateral conjugate faults. Together, these interlocking faults form a fracture mesh, consistent with structures associated elsewhere with fluid‐induced faulting and earthquake swarms. Plain Language Summary: In August of 2020, an earthquake sequence initiated within the Maacama fault zone in northern California, raising questions about its relationship with the larger‐scale fault. To investigate the faulting geometry and its implications for physical processes driving seismicity, we applied multiple high‐resolution seismic analyses to determine precise locations and slip orientations. This analysis reveals complex fracture mesh faulting geometry consisting of interlocking faults with two primary orientations, consistent with structures associated elsewhere with fluids and earthquake swarms. Key Points: A swarm‐like earthquake sequence occurred in 2020 near the Maacama Fault in northern California, with magnitudes up to Mw 4.2We determined precise locations for 3,520 events and focal mechanisms for 3,253 events, expanding the routine catalog of 388 earthquakesAn iterative stress inversion distinguishes likely fault planes, revealing en echelon and conjugate faults forming a fracture mesh [ABSTRACT FROM AUTHOR]

Hazard assessments in monogenetic volcanic fields require estimates of the runout of pyroclastic surges that result from phreatomagmatic explosive activity. Previous assessments used runout distances of 1–4 km, with large cases up to 6 km. Surge deposits at Ubehebe Crater (∼2100 y.b.p., Death Valley, California) have been traced ∼9 km from the crater center, and likely originally extended 1–3 km farther. There is no evidence that the Ubehebe Crater activity was unusually energetic; rather, its distal deposits are better preserved than those at most maar volcanoes because of its young age and the arid environment. Numerical simulations illustrate how low temperatures facilitate long runout of phreatomagmatic surges due to reduced expansion of entrained air compared to hot surges, allowing cool surges to retain higher densities than ambient air. We suggest that hazard assessments for volcanic fields with phreatomagmatic, maar‐forming eruptions should consider runout distances in the range of 10–15 km. Plain Language Summary: Assessments of volcanic hazards in areas prone to small‐volume basaltic volcanoes includes consideration of the lateral extent of devastating pyroclastic surges from eruptions that are caused by magma‐water explosions. Previous assessments considered surge distances up to 6 km, but the deposits from Ubehebe Crater (Death Valley, California, USA) have been traced to 9 km and likely originally extended farther. This was not an unusual eruption, but its deposits are exceptionally preserved due to the arid environment. Pyroclastic surges from magma‐water explosions are likely to be cooler than other volcanic flows, and this facilitates long travel distances and hence far‐reaching potential impacts if the area is inhabited. We suggest that future assessments should consider pyroclastic surge distances of 10–15 km from the vent. Key Points: Hazard assessments in monogenetic volcanic fields have considered pyroclastic surge runout up to 6 km based upon preserved deposits of previous eruptionsSurge deposits from Ubehebe Crater extend to 9 km; this may not be unusual and such surges may have long runout facilitated by their low temperaturesWe suggest that future hazard assessments in monogenetic volcanic fields consider pyroclastic surge runout up to 10–15 km [ABSTRACT FROM AUTHOR]

Episodic tremor and slow slip (ETS) downdip of the subduction seismogenic zone are poorly understood slip behaviors of the seismic cycle. Talc, a common metasomatic mineral at the subduction interface, is suggested to host slow slip but this hypothesis has not been tested in the rock record. We investigate actinolite microstructures from talc‐bearing and talc‐free rocks exhumed from the depths of modern ETS (Pimu'nga/Santa Catalina Island, California). Actinolite deformed by dissolution‐reprecipitation creep in the talc‐free rock and dislocation creep ± cataclasis in the talc‐bearing rock. This contrast results from stress amplification in the talc‐bearing rock produced by high strain rates in surrounding weak talc. We hypothesize that higher strain rates in the talc‐bearing sample represent episodic slow slip, while lower strain rates in the talc‐free sample represent intervening aseismic creep. This work highlights the need to consider fluid‐mediated chemical change in studies of subduction zone deformation and seismicity. Plain Language Summary: Episodic tremor and slow slip (ETS) are poorly understood styles of fault slip that occur just below the locked part of the megathrust fault where large subduction zone earthquakes occur. Chemical reactions at the depths of ETS can produce new and particularly weak minerals such as talc that change fault behavior but are rarely considered in studies of ETS. Modeling of rock deformation suggests that talc should host ETS fault slip, but this hypothesis has not been tested in the rock record. We investigated deformation of the mineral actinolite in talc‐free and talc‐bearing rocks exhumed from the depths of ETS in modern subduction zones on Pimu'nga/Santa Catalina Island (California) as a record of fault slip. Contrasting microstructures and deformation mechanisms in talc‐free and talc‐bearing rocks are best explained by differences in the rate of deformation (strain rate) between the two rock types. Lower strain rates in talc‐free rocks likely record background creeping (aseismic) deformation and higher strain rates in talc‐bearing rocks likely represent episodic slow slip events. This work suggests that talc may be a key host for slow slip and that future studies must account for new rock types formed by chemical reactions to accurately represent subduction zone deformation. Key Points: Contrasting actinolite microstructures in talc‐free and talc‐bearing subduction interface metasomatic rocks record stress variationsStress amplification in actinolite from talc‐actinolite schist results from high strain rate deformation of surrounding talcHigh strain rates reflect episodic slow slip localized in talc‐actinolite schist under high pore fluid pressures during metasomatism [ABSTRACT FROM AUTHOR]

The Eastern Transverse Ranges (ETR) province of California contains a system of E‐W‐trending left‐lateral faults accommodating clockwise block rotation between the San Andreas Fault (SAF) system in the Coachella Valley and the Eastern California Shear Zone (ECSZ) in the Mojave Desert. Building upon established geometric relationships, we estimate that this rotation across the ETR may transfer right‐lateral strain from the SAF to the ECSZ at a time‐averaged rate of ∼4.3–7.7 mm/yr. Through geomorphic mapping and the analysis of 38 10Be surface exposure samples, we derive a long‐term slip rate of 1.26 ± 0.50 (2σ) mm/yr over the late Pleistocene, yet analysis of the displacement record indicates a rate decrease ∼71 kya. While a rate change on this fault could have implications for possible plate boundary reorganization in this area, we argue that this slip rate variability more likely reflects routine fluctuation about a steady lifetime slip rate. Plain Language Summary: Neighboring zones of faulting within a tectonic plate boundary can play different roles in the accommodation and transfer of strain over time. Here, we investigate a zone of faulting that, in the past, has experienced clockwise block rotation, which can serve as a mechanism to communicate strain from one zone to another. We use previously published geometric relationships to determine that the Eastern Transverse Ranges (ETR) province may be transferring 4.3–7.7 mm/yr of strain from the San Andreas Fault zone to the Eastern California Shear Zone. In addition, using mapping and surface exposure geochronology, we estimate the slip‐rate of a specific fault within the ETR to have a long‐term rate of 1.26 ± 0.50 (2σ) mm/yr over the late Pleistocene, with slight variation about that long‐term rate, indicating that seismic hazard may vary over time. Key Points: We estimate a time‐averaged rate of strain transfer through block rotation in the Eastern Transverse Ranges to be between ∼4.3 and 7.7 mm/yrNew geologic slip rate points to a best fit variable slip rate, with a long‐term average slip rate that agrees with regional kinematicsThe Blue Cut Fault presents another example of short‐term strain rate variability being preserved in the geologic and geomorphic record [ABSTRACT FROM AUTHOR]

Because downscaling methods can yield substantially different projections of future climate, it is imperative to constrain these projections with information from existing observations, while also recognizing observational uncertainty. California is a natural test case to develop observational constraints on future projections given prior studies that have purportedly found contrasting spatial patterns in late‐20th‐century trends of summertime daily‐maximum temperature: cooling along the coast and warming inland. Revisiting this claim, we find that coastal cooling is largely confined to non‐homogenized temperature records while homogenized observations show either non‐significant cooling or warming trends throughout the state. This finding has implications for weighting historical and future climate simulations downscaled using localized constructed analogs. Failure to consider out‐of‐sample skill results in weighted and unweighted Representative Concentration Pathway 8.5 temperature trend estimates differing by 2 K/century or more in California. However, weighted mean estimates that properly account for trend uncertainty do not differ significantly from the unweighted mean. Plain Language Summary: Downscaling methods and models yield climate projections on local scales relevant to entities such as cities and individual facilities. Weighted averages of future projections are needed so that the huge amount of existing climate model data is curated to a set that is compact enough to be widely‐accessible to end users. Observations are used to judge which models most faithfully represent historical climate. In California, previous work has ostensibly observed coastal cooling and inland warming over the later part of the 20th century. Upon closer inspection, we determine that this California coastal cooling finding is likely an artifact due to historical changes in observing practices at weather stations. We then evaluate and weight downscaled models using temperature records that show California coastal cooling and separately develop weights using temperature records that do not show such cooling. When we carefully consider the information in the observational data, we find that the two approaches to weighting produce about the same results. However, the weights can significantly differ when observational uncertainty is not considered. Together, these results highlight the importance, even over a heavily‐instrumented area like California, of carefully evaluating historical climate model simulations when producing future, localized climate projections for end users. Key Points: Twentieth‐century California coastal cooling estimates are likely an artifact of historical changes in weather stations' observing practicesErrors in measuring summertime extreme temperature trends have limited effect on model weights for localized extreme temperature projectionsFailure to consider out‐of‐sample skill leads to underestimated uncertainty in future trend estimates of extreme temperature in California [ABSTRACT FROM AUTHOR]

Several large strike slip faults in central and northern California accommodate plate motions through aseismic creep. Although there is no consensus regarding the underlying cause of aseismic creep, aqueous fluids and mechanically weak, velocity‐strengthening minerals appear to play a central role. This study integrates field observations and thermodynamic modeling to examine possible relationships between the occurrence of serpentinite, silica‐carbonate rock, and CO2‐rich aqueous fluids in creeping faults of California. Our models predict that carbonation of serpentinite leads to the formation of talc and magnesite, followed by silica‐carbonate rock. While abundant exposures of silica‐carbonate rock indicate complete carbonation, serpentinite‐hosted CO2‐rich spring fluids are strongly supersaturated with talc at elevated temperatures. Hence, carbonation of serpentinite is likely ongoing in parts of the San Andres Fault system and operates in conjunction with other modes of talc formation that may further enhance the potential for aseismic creep, thereby limiting the potential for large earthquakes. Plain Language Summary: Constraining the relationships between the mechanical properties of rocks and lubricating fluids in tectonic fault zones is critical to understanding their seismogenic potential. We examined the distribution of exhumed mantle rocks and CO2‐rich springs in the San Andreas Fault area and find ubiquitous evidence for carbonate alteration in the creeping sections. Although carbonate alteration has gone to completion in exposed silica‐carbonate rocks, thermodynamic calculations suggest that CO2‐rich spring fluids are supersaturated with talc and magnesite at greater depth where temperatures are high. Because wet talc is a mechanically weak mineral, its formation through carbonation promotes tectonic movements without large earthquakes. Key Points: Carbonate‐altered mantle rocks and CO2‐rich springs are ubiquitous in the creeping section of the San Andreas Fault systemCO2‐rich spring fluids are saturated with talc and magnesite at seismogenic depths where mineral carbonation is likely ongoing todayThe formation of talc facilitates aseismic creep in wet faults and promotes tectonic movements without large earthquakes [ABSTRACT FROM AUTHOR]

We perform seismic attenuation tomography of the shallow structure for the San Jacinto Fault in the Ramona Reservation of southern California. The study uses ambient seismic noise recorded by a linear array of 65 3‐C sensors across the fault. We extract amplitude decay information, with uncertainty, from noise interferometry functions. To account for strong heterogeneities in the complex shallow fault zone structure, we apply a frequency‐dependent amplitude correction for focusing/defocusing effects using inverted phase velocity maps obtained by solving the transport equation. We then estimate the attenuation structure based on the linear station‐triplet method for both Love and Rayleigh waves. The attenuation tomography indicates strong attenuation correlated with known San Jacinto Fault surface traces. The Love wave attenuation tomography reveals an asymmetric damage zone that exists primarily on the fault side with faster seismic velocity, consistent with earthquake ruptures on a fault bimaterial interface with preferred propagation direction to the northwest. Plain Language Summary: The San Jacinto Fault Zone (SJFZ) is part of the boundary between the American and Pacific plates in Southern California. Earthquake ruptures produce rock damage that leads to attenuation of seismic waves during propagation. To study the attenuation structure of the fault zone, we extract amplitude information from background seismic noise recorded by a dense array of sensors across the fault. The results indicate an asymmetric attenuation and damage structure, with higher damage on the side where seismic waves travel faster. The rock damage asymmetry may have implications on the seismic shaking hazard from future large earthquakes on the SJFZ. Key Points: We perform ambient noise attenuation tomography based on Rayleigh and Love waves on a linear array across the San Jacinto Fault northwest of Anza, CaliforniaWe apply a frequency‐dependent amplitude correction for elastic focusing/defocusing effects due to strong lateral velocity variationsThe attenuation tomography reveals an asymmetric damage zone concentrated on the stiffer side of fault [ABSTRACT FROM AUTHOR]

Deformation in transform systems is accommodated by discrete fault slip and distributed off‐fault deformation. Here, we consider how a change in slip behavior along a fault can influence the distribution between on‐ and off‐fault deformation. We use a physical experiment to simplify the geometry, material properties, boundary conditions, and slip history along a strike‐slip fault to directly observe patterns of off‐fault deformation. We document deformation of a silicone slab on a simple shear apparatus using particle image velocimetry (2D) and photogrammetry (3D). The experimental results show regions of topographic highs and lows on either side of the slip transition that grow, evolve, and are displaced with progressive strain. The experimental dilatation field shares similarities with strain fields in central California along the San Andreas fault, which suggests that a change in slip behavior may explain some of the real‐world patterns in short‐ and long‐term deformation. Plain Language Summary: Understanding where and why deformation occurs along a fault is important for quantifying future earthquake hazards and interpreting existing geological structures. In nature, creeping sections of strike‐slip faults move slowly and continually, while locked sections are stationary for long periods and then release large amounts of energy rapidly in earthquake events. A portion of a strike‐slip fault's total motion can be accommodated away from the main trace of the fault leading to permanent deformation. Here, we investigate how a change from locked slip to creeping slip affects deformation away from the fault trace by deforming silicone, an analog for the Earth's crust. Physical models are useful because they allow us to simplify the fault system and isolate a single variable affecting deformation. We track deformation in the experiments in two and three‐dimensions. Our results show a pattern of extension and contraction which can be compared to a real‐world change in slip behavior on the San Andreas fault. Key Points: Physical models are useful for understanding where and why off‐fault deformation may occur on strike‐slip faultsA change in slip behavior causes off‐fault deformation patterns to developExperimental dilatation fields share similarities with strain fields in central California along the San Andreas fault [ABSTRACT FROM AUTHOR]

The contribution of ocean fronts to the properties and temporal evolution of Sea Surface Temperature (SST) structure functions have been investigated using a numerical model of the California Current system. First, the intensity of fronts have been quantified by using singularity exponents. Then, leaning on the multifractal theory of turbulence, we show that the departure of the scaling of the structure functions from a straight line, known as anomalous scaling, depends on the intensity of the strongest fronts. These fronts, at their turn, are closely related to the seasonal change of intensity of the coastal upwelling characteristics of this area. Our study points to the need to correctly reproduce the intensity of the strongest fronts and, consequently, properly model processes such as coastal upwelling in order to reproduce SST statistics in ocean models. Plain Language Summary: Forecasting the evolution of the Earth's climate requires to predict the evolution of the statistical characteristics of essential climate variables such as the Sea Surface Temperature. In this study, it has been found that some of such statistical properties depend on the intensity of the strongest fronts in the ocean. This implies that those ocean, or climate, models that fail to correctly predict their intensity will not be able to correctly reproduce the statistical characteristics of key variables such as temperature. The area analyzed in this study is the California Current system, where the strongest fronts are modulated by the seasonal evolution of the upwelling. Therefore, our results imply that such a system has to be correctly modeled, or parametrized, in order to properly reproduce the statistics of ocean temperatures. Key Points: The intensity of Sea Surface Temperature (SST) fronts is quantified using singularity exponents, which measure the continuity of the fieldAnomalous scaling of SST structure functions is correlated to the the intensity of the strongest frontsThe variability of the strongest fronts depends on the seasonal variability of the coastal upwelling in the area of study [ABSTRACT FROM AUTHOR]

Changes in CO2 emissions from volcanoes may evidence volcanic unrest. We use a multiyear time series of CO2 flux collected at the Horseshoe Lake Tree Kill area on Mammoth Mountain, CA, to understand processes that cause variations in flux from this system. Seasonal variations are systematically lowest during the winter months and reach maximum values during the summer season. A persistent ∼20% reduction in CO2 flux occurred during the Spring of 2017, coincident with the emergence of the area from drought and earthquake swarms in Long Valley Caldera. We used continuous GNSS measurements to calculate seasonal strains and stresses across the Mammoth Mountain area, and resolved resultant stresses onto the Mammoth Mountain Fault, which appears to facilitate gas transport to the surface. The normal stress changes are consistent with seasonal and multiyear changes in CO2 flux, suggesting that fault valving by solid earth processes can alter surface gas fluxes. Plain Language Summary: Changes in the rates of volcanic gas emissions may be harbingers of impending eruptions. New methodologies allow us to measure these changes continuously and in near‐real time. With this information, new and unexpected signals have been observed at Mammoth Mountain, California, such as the seasonal ebb and flow of volcanic CO2 emissions. Understanding the processes that produce these seasonal changes is required to distinguish them from the signs of volcanic unrest. This work investigates a number of potential mechanisms for these seasonal changes, and concludes that they can be explained by the seasonal flexing of the solid earth under changing water and ice loads, and subsurface pressurization due to infiltrating water. This flexing of the solid earth can be measured by GNSS, allowing us to filter these effects from the CO2 flux time series, as well as highlight the ways in which volcanic systems may "breathe" over the course of a year with changing hydrologic conditions. Key Points: Time series of volcanic CO2 fluxes from Mammoth Mountain, California, show seasonal variations between 2014 and 2020Local hydrologic and micro‐meteorological variations cannot account for the amplitude and phase of these variationsLong‐wavelength hydrologic processes produce stress changes in a way that can explain the seasonal variations in CO2 flux [ABSTRACT FROM AUTHOR]

Airborne lidar altimetry can measure the sea surface height (SSH) over scales ranging from hundreds of kilometers to a few meters. Here, we analyze the spectrum of SSH observations collected during an airborne lidar campaign conducted off the California coast. We show that the variance in the surface wave band can be over 20 times larger than the variance at submesoscales and that the observed SSH variability is sensitive to the directionality of surface waves. Our results support the hypothesis that there is a spectral gap between meso‐to‐submesoscale motions and small‐scale surface waves and also indicate that aliasing of surface waves into lower wavenumbers may complicate the interpretation of SSH spectra. These results highlight the importance of better understanding the contributions of different physics to the SSH variability and considering the SSH spectrum as a continuum in the context of future satellite altimetry missions. Plain Language Summary: Using unique measurements of the sea surface height (SSH) taken from an instrument onboard an airplane off the California coast, we examine how the variance of the SSH is distributed across different spatial scales, ranging from hundreds of kilometers to a few meters. We show that surface waves can be 20 times more energetic than motions with larger scales (known as submesocales) and that the direction of surface waves impacts the SSH variability depending on different sampling and averaging strategies. These results highlight the importance of better understanding the contributions of different types of motions and physics to the SSH variability. In particular, we advocate measuring scales from meters to hundreds of kilometers as a continuum to better inform future satellite altimetry missions. Key Points: We present unique broadband airborne lidar observations of sea surface height spanning scales from hundreds of kilometers to a few metersThe sea surface height variance due to surface waves can be over an order of magnitude larger than the variance at submesoscalesThe observed sea surface height is sensitive to the direction of waves which may impact the interpretation of remote sensing measurements [ABSTRACT FROM AUTHOR]

The University of Colorado Airborne Solar Occultation Flux (CU AirSOF) instrument conducted the first suborbital carbon monoxide (CO) mass flux measurements on the scale of large wildfires, showing that the destructive fires in northern California in October 2017 emitted 2,040 ± 316 tonnes CO hr−1. Pyrogenic estimates from seven satellite‐based emission inventories bracket the observed flux, but their range spans a factor of 83. The simulated air quality impacts in the form of ozone and fine particulate matter scale primarily with these uncertain emission amounts, and range from insignificant to very severe. This uncertainty in predicting emissions is reduced to a factor of ∼2 by the CU AirSOF flux measurements, with potential for future improvements. The uncertainty is primarily the result of uncertain vegetation types and sources of radiative power measurements, and to a lesser extent uncertain emission factors and fire diurnal cycles. Plain Language Summary: Wildfire smoke is a major source of air pollution that affects public health and natural areas, but the amounts of vegetation that go up in smoke and the emitted amounts of smoke are not well known, due to a lack of direct measurements. The accuracy of models used to predict smoke impacts on public health in affected communities is significantly impacted by their reliance on uncertain emissions estimates. In this study, a new instrument, the University of Colorado Airborne Solar Occultation Flux (CU AirSOF), measured the amount of carbon monoxide (CO) produced by the destructive fires in northern California during October 2017. These are the first airborne emission measurements on the scale of a large wildfire. The measured CO emissions from the fires fall within the large range among satellite‐based emission estimates, reducing the uncertainty in fire emissions. Air quality impacts in the form of ozone (O3) and fine particulate matter (PM2.5) range from insignificant to very severe, in direct relationship to the uncertain satellite‐based emission estimates. Key Points: First suborbital carbon monoxide flux measurements on the scale of large wildfiresCarbon monoxide fire emission inventories span a factor of 83 for a case study day of the northern California fires in October 2017Predicted ozone and fine particulate matter impacts vary from insignificant to very severe depending primarily on uncertain emission amounts [ABSTRACT FROM AUTHOR]

Changes in groundwater storage in California's Central Valley (CV) are considered partly responsible for vertical uplift surrounding the southern CV and for stress changes on nearby faults. Questions remain regarding the distribution of stress on the central San Andreas fault (CSAF) from recent and historical drawdown of the CV aquifer over the past 150 years (1860–2010). We combine groundwater storage change estimates for the 2006–2010 drought with a three‐dimensional finite element model to estimate Coulomb failure stress change (ΔCFS) on the CSAF. We combine a simple parameterization of historical hydraulic head change for 1860–1960 with a CV hydrological model (1961–2003) to estimate visco‐elastic effects on ΔCFS total and 2010 rate. We find that ΔCFS on the CSAF correlates positively with shallow seismicity and low frequency earthquakes for both short‐term and long term stressing. Unloading uplift rates surrounding the southern CV only partially account for the observed vertical GPS rates. Plain Language Summary: We investigate the spatial and temporal effects of groundwater removal in California's Central Valley on ground surface deformation and on stress changes on the nearby central San Andreas fault (CSAF). In particular, we focus on two outstanding questions: (a) What are the spatial distributions of Coulomb failure stress (ΔCFS) on the CSAF? (b) What are the effects of viscoelasticity on uplift and ΔCSF over time? We focus on the well studied 2006–2010 drought, estimating the change in groundwater storage and the resulting uplift and stress change through a numerical model for the Earth. Viscoelastic effects are examined through an estimate of historical groundwater change from 1860‐1961 and a hydrological model from 1961‐2003 added to the 2006–2010 drought estimates. Here we find that lower crust or upper mantle viscosity variations enhance uplift and stress changes and that CSAF stress changes may accumulate with time, especially with droughts and groundwater consumption. Key Points: We estimate Central Valley groundwater unloading viscoelastic effects on present‐day central San Andreas (CSAF) fault stressingCoulomb stress change for the CSAF during the 2006–2010 drought corresponds positively with seismicity and low frequency earthquakesCumulative Coulomb stress change on the CSAF for 1860–2010 is on the order of ∼0.01–0.02 MPa depending on the lower crustal viscosity [ABSTRACT FROM AUTHOR]

Research regarding self‐maintenance of pool‐riffle river morphology has focused on hydraulics within individual pool‐riffle couplets. Here, we make a scientific leap from one site on river to 702 pool‐riffle couplets across northern California to understand the frequency of the foundational velocity‐reversal hypothesis. A geometrical, mass and energy conservation‐based velocity reversal criterion was used to predict the occurrence of a velocity reversal. Only 18% of all couplets met the established criteria indicating a velocity reversal. At locations with riffle‐to‐pool bankfull width ratios greater than 1.2, more than 50% of couplets met velocity‐reversal conditions. Velocity reversal pools were almost always (89%) associated with channel constrictions while riffles were typically (71%) associated with channel expansions, albeit less often. Therefore, across landscapes, it appears channel constrictions propagate velocity reversal maintenance. Phasing between width and depth variability is central to fluvial morphodynamics and ecology and must be considered in sustainable river eco‐engineering. Plain Language Summary: Deep pools and shallow riffles occur in most river systems and are formed through an interaction between the water flowing through a river and the sediment it carries and deposits. Several more technical hypotheses have been developed as to why pools and riffles form where they do. The original hypothesis, called the velocity reversal hypothesis, suggested that while pools commonly have slow moving water, at higher flood event flows, pools are subjected to greater velocities than riffles, which causes the removal of sediment at the stream bed. While this has been shown to only be true in certain circumstances, we sought to quantify how often velocity reversals do occur. Due to the large field data set, this research presented a unique opportunity to understand how frequently a river process, such as a velocity reversal, occurs across a landscape. We found that velocity reversals are relatively uncommon and occur in only 18% of pool‐riffle pairs. Where reversals do occur, the river narrows and then widens to a greater degree than other locations that are closer to the average width. We suggest that future studies need to focus on the prevalence of river processes in rivers for improved river restoration strategies. Key Points: 702 pool‐riffle couplets from a large field data set were analyzed using a mass and energy conservation‐based velocity reversal criterionVelocity reversals are relatively infrequent, but comprise more than 50% of couplets with riffle‐to‐pool top width ratios greater than 1.2Velocity reversal pools were almost always found at width constrictions and riffles at width expansions, though to a lesser extent [ABSTRACT FROM AUTHOR]

A one‐year's worth of near‐global marine shallow single‐layer cloud top radiative cooling (CTRC) is derived from a radiative transfer model with inputs from the satellite cloud retrievals and reanalysis sounding. The mean cloud top radiative flux divergence is 61 Wm−2, decomposed into the longwave and shortwave components of 73 and −11 W m−2, respectively. Equatorward of 30°N/S, the CTRC is largely a reflection of free‐atmospheric specific humidity distribution: a dry atmosphere enhances CTRC by reducing downward thermal radiation. Consequently, the cooling minimizes in the "wet" tropics and maximizes in the "dry" eastern subtropics. Poleward of 30°N/S, the CTRC decreases slightly due to the colder clouds that emit less effectively. The CTRC exhibits distinctive seasonal cycles with stronger cooling in the winter and has amplitudes of order 10–20 Wm−2 in stratocumulus‐rich regions. The datasets were used to train a machine‐learning model that substantially speeds up the retrieval. Plain Language Summary: Marine low‐lying clouds cool by emitting thermal radiation. The cooling is known as cloud top radiative cooling (CTRC). A change in CTRC can influence the properties of marine clouds via many avenues, ranging from altering the vertical motions of the clouds to changing the clouds' ability to reflect sunlight. Despite the importance of CTRC to the climate system, its climatological characteristics, namely how it varies with space and time, remain unknown. This work fills this knowledge gap. We generate the product of the CTRC over the global ocean using a novel satellite methodology developed in our previous work. Analyses of the data show that the spatial and temporal distributions of the CTRC are largely reflections of the atmospheric humidity: the drier the atmosphere, the stronger the cooling. As a result, the CTRC is weakest in the wet tropics and strongest in the dry eastern subtropical oceans, such as the west of California coast. We also use the CTRC data to train a machine‐learning algorithm that can substantially speed up the calculation of CTRC. Key Points: A one‐year's worth of global marine shallow single‐layer cloud top radiative cooling (CTRC) is derived from satellite and reanalysis dataSpatial and seasonal variations of CTRC are largely reflections of changes in free‐tropospheric humidity and cloud top temperatureA neural network model for the CTRC was trained, which substantially speeds up the retrieval while maintaining good accuracy [ABSTRACT FROM AUTHOR]

Low Velocity Zones (LVZs) with anomalously high Vp‐Vs ratios occur along the downdip extents of subduction megathrusts in most modern subduction zones and are collocated with complex seismic and transient deformation patterns. LVZs are attributed to high pore fluid pressures, but the spatial correlation between the LVZ and the subduction interface, as well as the rock types that define them, remain unclear. We characterize the seismic signature of a fossil subduction interface shear zone in northern California that is sourced from the same depth range as modern LVZs. Deformation was distributed across 3 km of dominantly metasedimentary rocks, with periodic strain localization to km‐scale ultramafic lenses. We estimate seismic velocities accounting for mineral and fracture anisotropy, constrained by microstructural observations and field measurements, resulting in a Vp/Vs of 2.0. Comparable thicknesses and velocities suggest that LVZs represent, at least in part, the subduction interface shear zone. Plain Language Summary: Many subduction zones ‐ places where one tectonic plate goes under another ‐ have areas where seismic waves travel up to three times slower than normal and where the ratio of speeds of two different types of seismic waves is anomalously high. Some researchers have concluded that these Low Velocity Zones (LVZs) at 25–50 km below the surface of the Earth are the undeformed top of a downgoing tectonic plate whereas others suggest that LVZs are zones of intense deformation that allow two tectonic plates to slide past each other. To help resolve this uncertainty, we investigated rocks in a fossil subduction zone that record a history of being subducted and then returned to the surface. We identified the thickness of a zone of deformation and estimated how fast seismic waves would have passed through this zone based on the rock types, how the minerals are oriented, and the presence of fractures, all of which affect seismic speeds. The thicknesses and seismic wave speeds are comparable to modern LVZs, suggesting that LVZs mark zones of deformation between tectonic plates. These results can help us better understand how plates move past each other in modern subduction zones. Key Points: Seismic velocities of a 3 km fossil subduction interface shear zone are comparable to Low Velocity Zones (LVZs) in modern subduction zonesAccounting for fracture and mineral anisotropy in a sediment‐dominated interface shear zone results in highly anomalous seismic velocitiesThe LVZ represents the seismic signature of a distributed interface shear zone composed of mixed lithologies [ABSTRACT FROM AUTHOR]

Drought‐induced pumpage has precipitated dramatic groundwater‐level declines in California's Central Valley over the past 30 yr, but the impacts of aquifer overdraft on water quality are poorly understood. This study coupled over 160,000 measurements of nitrate from ∼6,000 public‐supply wells with a 30 yr reconstruction of groundwater levels throughout the Central Valley to evaluate dynamic relations between aquifer exploitation and resource quality. We find that long‐term rates of groundwater‐level decline and water‐quality degradation in critically overdrafted basins accelerate by respective factors of 2–3 and 3–5 during drought, followed by brief reversals during wetter periods. Episodic water‐quality degradation can occur during drought where increased pumpage draws shallow, contaminated groundwater down to depth zones tapped by long‐screened production wells. These data show, for the first time, a direct linkage between climate‐mediated aquifer pumpage and groundwater quality on a regional scale. Plain Language Summary: It has been established that wells are at risk to run dry when groundwater levels decline during drought, but associated impacts to water quality are poorly understood. This study examined 30 yr of data from California's Central Valley to find dramatic groundwater‐level declines during drought were associated with worsening groundwater quality. This likely happens because increased pumping during drought can draw shallow, contaminated groundwater to depths commonly tapped by public drinking‐water wells. This is the first study of its kind to show direct linkages between drought and groundwater quality at such a broad spatial scale. Key Points: Drought‐induced pumpage has precipitated dramatic groundwater‐level declines in California's Central Valley over the past 30 yrLong‐term rates of groundwater‐level decline and water‐quality degradation in overdrafted basins accelerate by factors of 2–5 during droughtIncreased pumpage during drought can draw shallow, contaminated groundwater down to depths tapped by long‐screened production wells [ABSTRACT FROM AUTHOR]

Strong strain and pore pressure changes are observed after three Mw 4.5+ local and one Mw 7.2 regional earthquake during 2010–2017 in borehole strainmeters near Anza, California. The strain change emerges immediately after the earthquakes and lasts 40–100 days with amplitudes up to 10−7, larger than the coseismic strain offsets. The pore pressure exhibits change immediately after the earthquakes at some boreholes and with a delay of 4–10 days at the others. A joint analysis of the observed postseismic strain and pore pressure change suggests that the postseismic strains could be explained by combined effects of poroelastic deformation due to earthquake‐induced pore pressure change and elastic deformation due to an earthquake‐triggered aseismic slip on a local fault. Our study indicates that, in addition to possible aseismic fault slips triggered by an earthquake, pore pressure changes after the earthquake could be even more important in producing postseismic deformation. Plain Language Summary: Understanding the physical mechanisms producing postseismic underground deformation is important for assessing fault slip budget and seismic hazards. In this study, we seek to clarify the possible roles of aseismic fault slip and underground water in producing postseismic deformation, through a joint analysis of underground deformation and pressure change in the pores of underground water reservoirs observed in the boreholes in southern California following four middle/large‐magnitude earthquakes. We find that both the underground deformation and pore pressure in the underground water reservoir exhibit changes lasting 1–3 months after the earthquakes, with changing amplitudes larger than the coseismic changes. These observations are well explained by a mechanism in which the mainshock earthquakes instantly trigger aseismic slips on the local faults and alter the hydrological conditions in the region; the change of hydrological condition results in a postseismic change of pore pressure in the underground water reservoir and produces poroelastic deformation in the region, while the aseismic fault slips produce elastic deformation. This study indicates that, in addition to possible aseismic fault slips triggered by an earthquake, changes of pore pressure after the earthquake in the underground water reservoir could play an even more important role in producing postseismic deformation. Key Points: We observe the strong months‐long change of strain and pore pressure after four Mw 4.5+ earthquakes in borehole strainmeters at Anza, CaliforniaThe postseismic strains last 40–100 days, and exhibit different trends and larger amplitudes up to 10−7 compared to coseismic strainsPostseismic strain = poroelastic strain by earthquake‐induced pore pressure change + elastic strain by an earthquake‐triggered aseismic slip [ABSTRACT FROM AUTHOR]

Significant sediment flux and deposition in a sedimentary system are influenced by climate changes, tectonics, lithology, and the sedimentary system's internal dynamics. Identifying the timing of depositional periods from stratigraphic records is a first step to critically evaluate the controls of sediment flux and deposition. Here, we show that ages of single‐grain K‐feldspar luminescence subpopulations may provide information on the timing of previous major depositional periods. We analyzed 754 K‐feldspar single‐grains from 17 samples from the surface to ∼9 m‐depth in a trench located downstream of the Mission Creek catchment. Single‐grain luminescence subpopulation ages significantly overlap at least eight times since ∼12.0 ka indicating a common depositional history. These depositional periods correspond reasonably well with the Holocene intervals of wetter than average climate conditions based on hydroclimatic proxies from nearby locations. Our findings imply a first‐order climatic control on sediment depositional history in southern California on a millennial timescale. Plain Language Summary: Various environmental factors such as climate, tectonics, rock types, and internal sedimentary processes may influence sediment generation, delivery, and deposition over 1000‐year timescales. To understand what controls sedimentation, we first seek to understand when periods of significant sediment transport and deposition have occurred in the past. Previous studies have shown that luminescence signals from individual sand grains may preserve information on past sunlight exposure (luminescence bleaching) and burial (luminescence regeneration) history. We use this information to identify the timing of significant depositional periods that occurred prior to the current deposit. We collected 17 samples (n = 754 individual analyses) from a trench located on the Banning strand of the San Andreas Fault system in southern California. Using these samples, we identified at least eight Holocene overlapping luminescence ages of single‐grain subpopulations. These subpopulation ages broadly match with the intervals of wetter than the average climate in the last 12,000 years, indicating a first‐order climatic influence on sediment transport and depositional history in southern California. Key Points: Single‐grain luminescence ages reveal at least eight major depositional periods in the lower Mission Creek catchment during ∼12.0–0.6 kaThese depositional periods correspond reasonably well with the wetter intervals in southern California based on paleoclimatic proxiesThe average interval between intermittent depositional periods decreases from the Mid‐Holocene (∼1.6 ka) to the Late Holocene (∼0.7 ka) [ABSTRACT FROM AUTHOR]

In the study of marine large igneous provinces, investigations often focus on the importance of mantle plumes in generating excess magmatism. Few studies, however, have addressed the role of plate boundary processes in promoting widespread and extensive mantle melting. This study investigates how spreading center geometry may have facilitated the emplacement of Shatsky Rise, an oceanic plateau in the western Pacific Ocean. The largest structure within Shatsky Rise, Tamu Massif, was created ∼140–150 Ma at the Pacific‐Izanagi‐Farallon (PIF) ridge‐ridge‐ridge triple junction. Moreover, the PIF triple junction was one of three triple junctions operating in close proximity at the time, as the nascent Pacific Plate (NPP) began to expand in size. This study suggests that the thermal conditions of the NPP, which likely resulted in a large area of thin lithosphere, could have significantly facilitated widespread mantle melting as well as the magmatic expression of any nearby mantle plume. Plain Language Summary: This study focuses on Shatsky Rise, a massive submarine magmatic plateau in the Pacific Ocean that covers an area comparable to California. Magnetics data show that this large igneous province (LIP) began to form ∼140–150 million years ago, near a setting in the plate tectonic system called a triple junction (TJ). TJs occur when three plate boundaries meet at a point; for Shatsky, the TJ consisted of three mid‐ocean ridges. High magmatic fluxes inferred for Shatsky, and particularly for Tamu Massif in the southern portion of the LIP, led previous investigations to suggest that a mantle plume was important in plateau generation. However, although a large number of studies have examined the role of mantle plumes in LIP formation, few investigations address how plate boundary geometry could enhance LIP magmatism. Focusing on the latter, this study finds that the area where Tamu Massif was formed was characterized by thin lithosphere. A broad region of thin lithosphere could have facilitated widespread mantle melting and promoted mantle plume magmatism. Since there are many cases around the globe where LIPs formed near TJs, the relationship between plate boundary geometry and excess magmatic activity should be evaluated for those LIPs also. Key Points: Spreading center geometry and ridge migration of the Pacific‐Izanagi‐Farallon triple junction contributed to Tamu Massif formationA broad area of thin lithosphere at the Pacific‐Izanagi‐Farallon triple junction promoted spatially widespread mantle and plume meltingThis study provides a mechanism for why marine large igneous provinces are often developed near mid‐ocean ridge triple junctions [ABSTRACT FROM AUTHOR]

Cross‐shore transport of larvae, pollutants, and sediment between the surf zone and the inner shelf is important for coastal water quality and ecosystems. Rip currents are known to be a dominant pathway for exchange, but the effects of horizontal temperature and salinity gradients are not well understood. Airborne visible and infrared imaging performed on the California coast shows warm and cool plumes driven by rip currents in the surf zone and extending onto the shelf, with temperature differences of approximately 1°C. The airborne imagery and modeled temperatures and tracers indicate that warm plumes exhibit more lateral spreading and transport material in a buoyant near‐surface layer, whereas cool plumes move offshore in a subsurface layer. The average cross‐shore extent of warm plumes at the surface is approximately one surfzone width larger than for cool plumes. Future work may explore the sensitivity of nearshore plumes to density patterns, wave forcing, and bathymetry. Plain Language Summary: Waves and currents in the coastal ocean move plankton, pollutants, nutrients, and sediment between the beach and deeper waters, impacting ecosystems and water quality. Rip currents, strong narrow seaward currents caused by breaking waves, provide a conduit for larvae to move offshore and also carry pollutants away from the beach. One common type of rip current formed in channels interrupting sandbars can sometimes be observed as a plume of brown sediment‐laden water moving offshore. Previous studies of this rip‐current type have assumed that the movement of material by these currents is not affected by differences in the water density (temperature and salinity) between shallower and deeper water. However, thermal (infrared) images taken from an aircraft on the coast of California show that the wave‐breaking region near the beach is often significantly cooler or warmer than the ocean immediately offshore. Rip currents in these images and in numerical simulations appear either as warm plumes, which carry material further offshore and are concentrated at the water surface, or cool plumes, which move material offshore under the surface. The results show that differences in water temperature from the beach to deeper water impact how rip currents move material in the coastal ocean. Key Points: Airborne infrared imagery on the CA coast shows cool and warm plumes driven by rip currents that connect the surf zone to the inner shelfThe surface cross‐shore extent of warm plumes is about one surfzone width larger than that of cool plumes in observations and simulationsModeled cool nearshore plumes entering a stratified shelf subduct, whereas warm plumes extend offshore in a near‐surface layer [ABSTRACT FROM AUTHOR]

Understanding and predicting precipitation characteristics on seasonal and longer timescales can help California prepare for long‐term droughts and precipitation extremes. We find that interannual variations in total precipitation across California are primarily determined by precipitation frequency. As was shown previously for total precipitation, the precipitation frequency is linked to the North Pacific jet stream location. This indicates that California precipitation frequency is primarily controlled by where the jet guides precipitate weather systems rather than how moist or energetic the systems are. The jet's position, in turn, depends on the states of the El Niño‐Southern Oscillation (ENSO) and of the Pacific Decadal Oscillation (PDO). We use this to construct a regression model that predicts variations in California's annual total precipitation and precipitation frequency. Up to 20% of the wintertime precipitation variance in Southern California is predictable using decorrelated ENSO and PDO indices in the previous summer. Plain Language Summary: This study shows that the number of rainy days, rather than the rainfall intensity, determines whether a given year is wet or dry in California. How often it rains, in turn, is related to a specific pattern of Pacific sea surface temperatures several months before the rainy season, providing modest predictability of year‐to‐year rainfall variations for Southern California. Key Points: Throughout California, the precipitation frequency determines how wet a water year isThe precipitation frequency in parts of California is determined by the East Pacific jet stream locationUp to 20% of California's annual precipitation variations are predictable given summer SST in the Pacific [ABSTRACT FROM AUTHOR]

The ShakeAlert earthquake early warning (EEW) system issues public alerts in California and will soon extend to Oregon and Washington. The Cascadia subduction zone presents significant new challenges and opportunities for EEW. Initial publications suggested that EEW algorithms based on Global Navigation Satellite System (GNSS) data could provide improved warning for intraslab events and dramatically improved warning for offshore megathrust events, both of which contribute significantly to hazard in Cascadia. We find that some expectations in these publications were unrealistic, and we demonstrate that in general geodetic algorithms would not produce timely warnings for intraslab events nor warning times of two minutes or more for severe shaking from megathrust earthquakes. Nonetheless, lessons from recent earthquakes in Japan and California, for which alerts from seismic algorithms suffered from magnitude saturation and high data latencies, demonstrate the urgent need for rigorous testing of geodetic EEW as a potential complement to seismic EEW. Plain Language Summary: ShakeAlert was initially implemented and optimized in California where most earthquakes occur on shallow faults in the Earth's crust. These earthquakes typically start just a few miles underground and can begin directly underneath population centers. Thus, earthquake early warning (EEW) needs to rely on the very first detections of seismic waves to be able to issue warnings before stronger shaking arrives. In a subduction zone, many damaging earthquakes are located offshore on the plate boundary megathrust, and many inland large earthquakes occur in the subducted plate at depths of 30+ miles. Because some seismic EEW systems have underestimated large earthquakes in the past, approaches are being tested that use GNSS data as the primary input for quantifying the magnitude and length of a large, growing rupture. We discuss realistic expectations from this approach and identify key topics for future research. Key Points: Geodetic earthquake early warning (EEW) algorithms require rigorous testingRegularization operators are a key feature to prevent over alertingGeodetic EEW will likely not help with most intraslab earthquakes [ABSTRACT FROM AUTHOR]

We conducted a novel study to capture the on‐going advancement of mineral weathering within a serpentinite formation by using an integrated approach of multi‐scale quantitative rock magnetic analyses and nano‐resolution geochemical imaging analyses. We studied a suite of rock samples from the Coast Range Ophiolite Microbial Observatory (CROMO) in California to conduct rock magnetic analyses enabling us to determine character of Fe‐bearing minerals and to predict locations of reaction boundaries among various stages of weathering. QEMSCAN® and other electron micro‐imagery analyses highlighted microstructural changes in amorphous minerals, and possible changes in porosity and coincides with the iron‐enrichment region. This iron enrichment indicates initiation of iron (‐oxides) nucleation, resulting in extremely fine gain magnetite formation. This is a newly documented mode of magnetite production in serpentinites and enhances the application of magnetite abundance as a proxy for the degree and extent of water‐rock interaction in mantle peridotite and serpentinite. Plain Language Summary: Accounting for the range of weathering processes on Earth enables us to understand the intertwined processes related to climate change, connect regional and global ecosystems, and evaluate the planet's chemical budget. We used quantitative rock magnetic and micro‐scale analyses to capture the advancement of weathering in mineral phases, particularly in clay‐rich formations and soils in core samples extracted from subsurface rock formation of the Coast Range Ophiolite Microbial Observatory where the characteristics of the clay‐rich serpentinite impact microbial populations in deep biosphere. We capture weathering fronts based on information related to the physical condition of weathering phases within the boreholes and micro‐scale imagery of mineral phases. By focusing, on how iron‐rich regions are formed in the different mineral phases and the corresponding changes where water reacts with minerals, we identify where weathering is currently taking place at a micro‐scale, that is, the foremost front of this chemical phenomena and associating changes in physical conditions (porous spaces in the rocks) that could shed a new light on the habitability of deep biosphere. Key Points: A suite of rock magnetic analyses suggests a sequence of in situ weathering phases within serpentine formations in depthsWe identify a layer within CROMO serpentine formations where seasonal hydrogeology advances weathering processesWe identify changes in porosity structure in mineral phases using micro‐scale imagery that suggests where weathering is currently taking place in serpentine [ABSTRACT FROM AUTHOR]

P‐wave azimuthal anisotropic tomography reveals that the July 6, 2019 Mw 7.1 Ridgecrest earthquake occurred in a region with clockwise crustal rotation. The rotation together with the sinistral slip on the Garlock Fault is a response to the northwest‐trending, dextral shear within the Eastern California Shear Zone due to the relative motion between the Pacific and North America Plates. The hypocentral area of the Ridgecrest mainshock is characterized by a sharp lateral velocity contrast which has a reversal in contrast polarity at about 5 km depth. We find high Vp/Vs ratio structures covering the rupture zones of the Mw 6.4 foreshock and the Mw 7.1 mainshock, which may indicate the existence of fluids in the fault zones. We speculate that fluids and crustal rotation may have played important mechanical roles in causing the 2019 Ridgecrest earthquake sequence. Plain Language Summary: New Eikonal equation‐based seismic tomography methods are developed to construct isotropic and azimuthally anisotropic P‐wave velocity models as well as the Vp/Vs ratio for the source area of the 2019 Ridgecrest earthquake sequence, southern California. Based on the tomographic results, we speculate that crustal rotation and fluids may have led to the occurrence of the 2019 Mw 7.1 Ridgecrest earthquake. Key Points: Crustal rotation and fluids may have led to the occurrence of the 2019 Ridgecrest earthquake sequenceP‐wave azimuthal anisotropy provides the evidence of crustal rotation in the source area of the Mw 7.1 2019 Ridgecrest earthquakeHigh Vp/Vs ratio and low‐velocity structure in the hypocentral region suggests the existence of crustal fluids [ABSTRACT FROM AUTHOR]

Quasi‐decadal variation in various Pacific regions has been recognized for decades but its spatial‐temporal characteristics and generation mechanisms remain poorly understood. Here we show that the sea surface temperature (SST) in the equatorial central Pacific (ECP) exhibits a prominent 11‐year periodicity since 1951 but not before. An episodic‐like quasi‐decadal warm event is initiated in a northeast‐southwest tilted belt from the United States' west coast to the ECP. The initial development involves positive feedback between atmospheric heating‐induced Rossby waves and underlying SST in the tropical North Pacific. The amplification of ECP warming is primarily due to equatorward heat advection and deepening thermocline, while the zonal advective feedback mainly controls its decay. The quasi‐decadal oscillation (QDO) tends to follow the 11‐years solar cycle with a 1‐to‐2‐year phase delay, meanwhile, it possibly links to the nonlinear behavior of ENSO. The cause of intensification of the QDO over the past 70 years, however, remains elusive. Plain Language Summary: The sea surface temperature (SST) at the equatorial central Pacific (ECP) exhibits a prominent 11‐year periodicity over the last 70 years. This quasi‐decadal oscillation (QDO) can significantly influence tropical cyclone activity and continental climate anomalies. This work provides a novel description of the QDO phenomenon as a prominent mode of SST variability in the subtropical North Pacific and equatorial central Pacific (ECP). The initial QDO warming develops over a northeast‐southwestward elongated belt extending from California's off‐coast to the ECP by the interaction between atmospheric heating‐induced weakening trade winds and underlying SST anomalies. The amplification of ECP warming is primarily attributed to equatorward heat transport, while its decay is principally caused by the transport of cold water from the eastern Pacific. The QDO is found to follow the 11‐year solar irradiance cycle with a 1‐to‐2‐year phase delay and potentially link to the decadal variability of El Niño‐Southern Oscillation. However, why the QDO has strengthened since the 1950s remains elusive. Key Points: Equatorial central Pacific's Sea surface temperature exhibits a conspicuous quasi‐decadal oscillation (QDO) since 1951 but not beforeThe evolution of QDO warming involves a sequence of air‐sea interaction mechanisms in the subtropical Pacific and the equatorial PacificThe QDO follows the 11‐year solar cycle with a 1‐to‐2‐year phase delay and potentially links to the nonlinear behavior of ENSO [ABSTRACT FROM AUTHOR]

The 15 May 2020 Monte Cristo Range Mw 6.5 earthquake occurred in the northeast of the Mina deflection, which accommodates approximately a quarter of the relatively dextral motion between the Pacific and North American plates. The Monte Cristo Range event provides an opportunity to study the present‐day regional deformation mechanism and active tectonics. In this study, we investigate the source rupture process of the event using joint inversion of interferometric synthetic aperture radar and broadband seismic data. We find that the rupture propagates almost simultaneously on two main segments. The fault motion changes from the predominantly sinistral slip near the epicenter on the eastern segment to the oblique slip on the western segment, with a maximum coseismic slip of 0.8 m. Our results suggest that the accommodation of slip transfer localized in the northeastern Mina deflection tends to transform from the wrench‐ to extension‐dominated transtension. Plain Language Summary: The relative motion between the Pacific and North American plates is largely focused on the dextral San Andreas fault system. A complex array of strike‐slip and normal faults, known as the Walker Lane and the Eastern California shear zone, accommodates about a quarter of this plate motion. The 2020 Monte Cristo Range earthquake struck the Mina deflection, a right stepover between two parallel but noncoplanar faults. The Mina deflection is considered to transfer slip from the southern to central Walker Lane. Because of the obliquity between the plate motion and the fault orientation, the deformation across the Mina deflection is generally interpreted as wrench‐dominated transtension, commonly characterized by strike slip and block rotation. We reconstruct the spatial and temporal process of the source rupture by integrated geodetic and seismological data analysis and find that the accommodation of slip transfer in the northeast of the Mina deflection has a tendency to evolve into the extension‐dominated transtension, thereby harboring normal faulting. We suggest that this behavior is because the extensional pure shear accumulates more rapidly compared with the wrench simple shear. Key Points: Source rupture process of the 2020 Monte Cristo Range earthquake is derived using joint inversion of geodetic and seismic dataTwo fault segments with varied orientations ruptured, producing oblique and sinistral slip on the western and eastern segments, respectivelyAccommodation of slip transfer in the northeastern Mina deflection tends to transform from wrench‐ to extension‐dominated transtension [ABSTRACT FROM AUTHOR]

Fault creep reduces seismic hazard and serves as a window into plate boundary processes; however, creep rates are typically constrained with sparse measurements. We use differential lidar topography (11–13 year time span) to measure a spatially dense surface deformation field along a 150 km section of the Central San Andreas and Calaveras faults. We use an optimized windowed‐iterative‐closest‐point approach to resolve independent creep rates every 400 m at 1–2 km apertures. Rates vary from <10 mm/year along the creeping fault ends to over 30 mm/year along much of the central 100 km of the fault. Creep rates are 3–8 mm/year higher than most rates from alignment arrays and creepmeters, likely due to the larger aperture of the topographic differencing. Creep is often focused along discrete fault traces, but strain is sometimes distributed in areas of complex fault geometry, such as Mustang Ridge. Our observations constrain shallow seismic moment accumulation and the location of the creeping fault trace. Key Points: Aseismic fault creep rates are measured over 11–13 years in central California by differencing repeat airborne lidar topographyThe Central San Andreas fault creeps at rates over 30 mm/year, indicating limited strain accumulation for release in earthquakesWe describe challenges and new insights regarding applying topographic differencing methods along creeping faults Plain Language Summary: Repeated dense measurements of topography reveal how the Earth's surface shifts and deforms at tectonic plate boundaries. Along some active faults, deformation and offset occur continuously even in the absence of large earthquakes; this is referred to as fault creep which generally proceeds at rates of millimeters to tens of millimeters per year. Along sections of the Central San Andreas and southern Calaveras faults in California, we map rates and patterns of fault creep over 150 km by comparing two older airborne lidar elevation data sets from 2005 and 2007, with newer data collected in 2018. This analysis shows that the San Andreas fault creeps at a mean rate of 22 mm/year between Parkfield, California, and San Juan Bautista, California. We use the mean rate and the variation along the length of the fault to better understand the seismic hazard along the fault, depth variations of fault creep, and how the geology surrounding the fault impacts the distribution of creep. [ABSTRACT FROM AUTHOR]

Paleoseismic rupture histories provide spatiotemporal models of earthquake moment release needed to test numerical models and lengthen the instrumental catalog. We develop a model of the fewest and thus largest magnitude earthquakes permitted by paleoseismic data for the last 1,500 years on the southern San Andreas and San Jacinto Faults, California, USA. The largest geometric complexity appears to regulate the system: Only two ruptures break the San Gorgonio Pass region, followed by episodes of ruptures that could bridge the northern San Jacinto Fault and the San Andreas Fault. When tested against independent data on slip per event, the model produces comparable values indicating the end‐member model does not underpredict rupture rates. Rupture of >85% of the fault length in the historic period between 1800 and 1857 and the subsequent quiescence is similar to epochs of activity in the prehistoric model, suggesting that regional clustering of seismicity could be a trait of the system. Plain Language Summary: Large, damaging earthquakes are rare, so geologic data are essential to supplement the record of earthquakes from seismographs. We use the geologic record to determine the magnitude and timing of past earthquakes on the San Andreas and San Jacinto Faults. A pattern emerges in which the biggest bends and splays control the length of most ruptures. Because the model estimates the largest‐possible earthquakes on this system, it is useful as an upper limit on earthquake size used in engineering and earthquake forecasting. Key Points: End‐member model examines the fewest, but longest, ruptures required by ages of ground‐rupturing earthquakes from 31 paleoseismic sitesRegional clustering emerges, with largest moment release ruptures similar to the historic record with ruptures breaking much of the systemRuptures through San Gorgonio Pass are infrequent, and anticorrelated with episodes when fault sections near Cajon Pass are active [ABSTRACT FROM AUTHOR]

In February 2017, a 5‐day sequence of atmospheric river storms in California, USA, resulted in extreme inflows to Lake Oroville, the state's second‐largest reservoir. Damage to the reservoir's spillway infrastructure necessitated evacuation of 188,000 people; subsequent infrastructure repairs cost $1 billion. We assess the atmospheric conditions, snowmelt, and runoff against major historical events. The event generated exceptional runoff volumes (second largest in a 30‐yr record) partially at odds with the event precipitation totals (ninth largest). We explain the discrepancy with observed record melt of deep antecedent snowpack, heavy rainfall extending to unusually high elevations, and high water vapor transport during the atmospheric river storms. An analysis of distributed snow water equivalent indicates that snowmelt increased water available for runoff watershed‐wide by 37% (25–52% at 90% confidence). The results highlight potential threats to public safety and infrastructure associated with a warmer and more variable climate. Plain Language Summary: In February 2017, extreme runoff into California's second‐largest reservoir, Lake Oroville, and cracks in the reservoir's spillways resulted in evacuations of thousands of people and major repair costs. We analyzed to what extent the atmospheric river storms that caused the extreme runoff were unusual in terms of precipitation, snowmelt, temperature, and moisture in the air. We found that the precipitation amounts were less unusual than the runoff amounts, suggesting that other factors were involved. We also found that snowmelt in the Sierra Nevada mountains above the reservoir was the heaviest on record at many locations, driven by unusually warm temperatures and deep preexisting snowpack before the storms began. Thus, the warm temperatures and record melt likely increased the water available for runoff by about a third during the spillways incident. Our findings are consistent with other studies that suggest that unusually warm temperatures during winter atmospheric river storms in the Western United States are associated with flood risk due to substantial rainfall and snowmelt. Other studies show that climate change is expected to increase the type of flood risk in the 2017 incident. Key Points: The atmospheric river event causing the 2017 Oroville Dam spillways incident was more exceptional for runoff than precipitation totalsHigh rain‐snow elevations, deep antecedent snowpack, and unprecedented snowmelt are shown to explain the discrepancyWe highlight the importance of considering snowmelt, rain‐snow elevations, and climate change in assessing current and future flood risk [ABSTRACT FROM AUTHOR]

The steepness of the beach face is a fundamental parameter for coastal morphodynamic research. Despite its importance, it remains extremely difficult to obtain reliable estimates of the beach‐face slope over large spatial scales (thousands of km of coastline). In this letter, a novel approach to estimate this slope from time series of satellite‐derived shoreline positions is presented. This new technique uses a frequency domain analysis to find the optimum slope that minimizes high‐frequency tidal fluctuations relative to lower‐frequency erosion/accretion signals. A detailed assessment of this new approach at eight locations spanning a range of tidal regimes, wave climates, and sediment grain sizes shows strong agreement (R2 = 0.93) with field measurements. The automated technique is then applied across thousands of beaches in eastern Australia and California, USA, revealing similar regional‐scale distributions along these two contrasting coastlines and highlights the potential for new global‐scale insight to beach‐face slope spatial distribution, variability, and trends. Plain Language Summary: How steep a beach is can dictate the way the beach interacts with the incoming ocean waves and therefore is of paramount importance for coastal scientists and engineers, coastal flood modelers, and swim safety officers. However, despite its importance, it is impractical to obtain reliable estimates of the "typical" beach‐face slope along large lengths of sandy coastlines (hundreds to thousands of km) because of the logistics that would be necessary to visit many sites repeatedly to obtain these measurements. This letter describes a new technique to estimate the beach‐face slope in the absence of field observations, relying instead on long‐term publicly available satellite observations and a global tide model. This technique is then applied to thousands of beaches along the coastlines of eastern Australia and California in the United States. Key Points: A novel remote sensing technique to estimate beach‐face slopes from satellite imagery and modeled tides is presentedTime series of shoreline change are transformed into frequency domain to find the slope that minimizes high‐frequency tidal fluctuationsValidation against in situ data shows high accuracy across sites ranging in grain size, tidal range, and wave climate [ABSTRACT FROM AUTHOR]

Climatic variations in the northeastern (NE) Pacific Ocean have been related to the strength of intermediate water‐driven productivity and global ocean circulation. Productivity variations and ocean circulation during the late Quaternary may have caused changes in the intensity of the oxygen minimum zone (OMZ). To understand these changes in productivity and resultant OMZ fluctuations, we analyzed benthic foraminiferal assemblages from the Cascadia margin which reveal major faunal and ecological shifts associated with the millennial‐scale climate changes during ~32–10 ka. The intermediate waters were highly variable during ~20–14 ka with increased productivity and nutrient supply to the subsurface during circa 15.5–14.5 ka. Our results suggest that suboxic environmental conditions occurred during the warm interstadials, and the Bølling‐Ållerød event was favorable for the dysoxic species. Our results further indicate that paleoclimate variations in the northeast Pacific region were mainly driven by the Pacific Intermediate Water Current. Plain Language Summary: Deep‐sea environmental changes in the northeastern (NE) Pacific have been linked to the primary productivity and poorly ventilated intermediate water. This study aims at documenting the strength of the oxygen minimum zone (OMZ) on the Cascadia margin and its impact on marine biotic community as well as doing a comparison with results from the California margin. The Last Glacial Maximum (LGM) ranges from circa 23 to 18 ka in the proxy records of Cascadia margin sediments (Galbraith et al., 2007, https://doi.org/10.1038/nature06227). Our proxy record suggests weakening of the OMZ during the LGM and a strengthening during ~17–12 ka in response to global climate change. The Cascadia margin experienced intermediate to high organic carbon and low‐oxygen environment during the LGM as is noticeable in this study. The Pacific Intermediate Water was highly variable and productive during circa 15.5–14.5 ka. Our results suggest that paleoclimate variations in the NE Pacific Ocean region were mainly driven by the Pacific Intermediate Water Current. Key Points: The Pacific Intermediate Water Circulation was highly variable between ~20 and 14 kaThe OMZ strengthened during ~17–12 ka in the Cascadia marginThe Bølling‐Ållerød interval is marked by high productivity on the slope off Vancouver Islands [ABSTRACT FROM AUTHOR]

We studied the 2019 Mw6.4 and Mw7.1 Ridgecrest, California, earthquake sequence, using Sentinel‐1 and ALOS‐2 coseismic interferograms and subpixel offsets to retrieve the three‐dimensional (3‐D) surface displacements. By inverting the 3‐D displacements, optimal dip angles of the earthquake faults and the slip model were obtained. The interferometric synthetic aperture radar‐based slip model supplemented with the analysis of GPS data shows that the Mw6.4 event ruptured two orthogonal faults and its major geodetic moment was released by sinistral motion on a NE‐SW trending fault that dips 78° NW. Right‐lateral slip on NW‐SE trending subvertical faults was responsible for the Mw7.1 earthquake. Postseismic analysis with U.S. Geological Survey earthquake catalog and GPS time series at site P595 shows that the postseismic surface deformation following the Mw7.1 event has similar temporal patterns with the postseismic moment release, but requires more energy. This implies that the early aftershocks were likely controlled by rapid afterslip following the mainshock. Key Points: The 3‐D surface displacements of the 2019 Ridgecrest earthquakes are derived from Sentinel‐1 and ALOS‐2 InSAROrthogonal fault ruptures of the Mw6.4 event are revealed by using GPS horizontal measurementsThe rapid decay of aftershocks is explained by the afterslip following the mainshocks [ABSTRACT FROM AUTHOR]

Laboratory experiments report that detectable seismic velocity changes should occur in the vicinity of fault zones prior to earthquakes. However, operating permanent active seismic sources to monitor natural faults at seismogenic depth is found to be nearly impossible to achieve. We show that seismic noise generated by vehicle traffic, and especially heavy freight trains, can be turned into a powerful repetitive seismic source to continuously probe the Earth's crust at a few kilometers depth. Results of an exploratory seismic experiment in Southern California demonstrate that correlations of train‐generated seismic signals allow daily reconstruction of direct P body waves probing the San Jacinto Fault down to 4‐km depth. This new approach may facilitate monitoring most of the San Andreas Fault system using the railway and highway network of California. Plain Language Summary: Even though laboratory experiments report that they should be preceded by detectable precursors, earthquakes remain unpredictable. Indeed, contrary to the lab, scanning natural faults at a few kilometers depth where earthquakes initiate requires operating high‐energy seismic sources continuously in time, which is found to be nearly impossible. In this study, we show that large freight trains generate sufficient seismic energy to travel down to a few kilometers depth and be detected at tens of kilometers from railways. We demonstrate that we can turn this apparently random source of seismic signal into an impulsive virtual seismic source to monitor active faults. We finally estimate that this new approach can be used for monitoring most of the San Andreas Fault system using the railway and highway network of California. Key Points: Freight trains in Southern California are locally equivalent to a magnitude 2.2 earthquake every dayWe use train noise to reconstruct repetitive virtual sources of P waves crossing the San Jacinto Fault at 4‐km depthPredictions of train noise across California show potential for passive monitoring of most of the San Andreas Fault system [ABSTRACT FROM AUTHOR]

Foreshocks have been documented as preceding less than half of all mainshock earthquakes. These observations are difficult to reconcile with laboratory earthquake experiments and theoretical models of earthquake nucleation, which both suggest that foreshock activity should be nearly ubiquitous. Here we use a state‐of‐the‐art, high‐resolution earthquake catalog to study foreshock sequences of magnitude M4 and greater mainshocks in southern California from 2008–2017. This highly complete catalog provides a new opportunity to examine smaller magnitude precursory seismicity. Seventy‐two percent of mainshocks within this catalog are preceded by foreshock activity that is significantly elevated compared to the local background seismicity rate. Foreshock sequences vary in duration from several days to weeks, with a median of 16.6 days. The results suggest that foreshock occurrence in nature is more prevalent than previously thought and that our understanding of earthquake nucleation may improve in tandem with advances in our ability to detect small earthquakes. Plain Language Summary: Earthquakes often occur without warning or detectable precursors. Here we use a new, highly complete earthquake catalog to show that most mainshock earthquakes in southern California are preceded by elevated seismicity rates—foreshocks—in the days and weeks leading up to the event. Many of these foreshock earthquakes are small in magnitude and hence were previously undetected by the seismic network. These observations help bridge the gap between observations of real earth fault systems and laboratory earthquake experiments, where foreshock occurrence is commonly observed. Key Points: We analyze foreshock activity in a catalog of more than 1.8 million earthquakes in southern CaliforniaForeshock occurrence significantly exceeds the background seismicity rate for 72% of candidate mainshocksDurations of elevated foreshock activity range from days to weeks for these sequences [ABSTRACT FROM AUTHOR]

Analysis of 984 induced microearthquakes from The Geysers geothermal reservoir in California reveals that the retrieved moment tensors depend on the frequency band of the inverted waveforms. The observed dependence is more significant for the percentages of the double‐couple, compensated linear vector dipole, and isotropic (ISO) components than for the focal mechanisms. The average root‐mean‐square of the moment tensors obtained in different frequency bands is correlated with spectra of ambient noise. The percentages of double‐couple and ISO components tend to decrease and increase with the upper cutoff frequency (fu), respectively. This suggests that shear rupture radiates energy preferentially in a lower frequency band and tensile rupture in a higher frequency band. Events displaying a strong increase of the ISO with fu are confined within the same depth interval as the injection points. This might be related to the strong thermoelastic effects in the vicinity of injection points that promote opening of small cracks adjacent to the main fractures. Plain Language Summary: Moment tensor (MT) describes shear and tensile motions in the earthquake source. The components of MT are usually assumed to be independent of the frequency. However, this assumption may not satisfy the complex rupture process of induced microearthquakes. We use a novel approach to investigate 984 induced microearthquakes from The Geysers geothermal reservoir in California and find that the retrieved MTs depend on the frequency band of input waveforms. The observed dependence is more significant for the components of MT measuring the proportions of seismic energy than for the components determining fault geometry. The component of MT describing shear motion shows a different frequency dependence than that describing tensile motion, suggesting that these two motions occur on the structures with different spatial scales. A subset of seismic events is identified to have a distinct feature of frequency dependence. These events only occur in the layer where the cool water is injected into the hot reservoir and do not migrate downward as the other events. This might be related to the strong thermal effects in the vicinity of injection points that promote the opening of small cracks adjacent to the main fractures. Key Points: Moment tensor components of induced earthquakes from The Geysers geothermal reservoir in California show significant frequency dependenceDC and ISO components tend to decrease and increase with frequency, respectively, implying different properties of shear and tensile rupturesEvents with a significant increase of ISO component with frequency are confined within the same depth interval as the injection points [ABSTRACT FROM AUTHOR]

Existing major reservoirs in California, with average age above 50 years, were built in the previous century with limited data records and flood hazard assessment. Changes in climate and land use are anticipated to alter statistical properties of inflow to these infrastructure systems and potentially increase their hydrological failure probability. Because of large socioeconomic repercussions of infrastructure incidents, revisiting dam failure risks associated with possible shifts in the streamflow regime is fundamental for societal resilience. Here we compute historical and projected flood return periods as a proxy for potential changes in the risk of hydrological failure of dams in a warming climate. Our results show that hydrological failure probability is likely to increase for most dams in California by 2100. Noticeably, the New Don Pedro, Shasta, Lewiston, and Trinity Dams are associated with highest potential changes in flood hazard. Plain Language Summary: Dams are critical manmade infrastructure that provide resilience against extremes (e.g., droughts and floods) and regulate water resources. In 2017, California experienced a series of flooding events, which triggered incidents such as structural failure of the Oroville Dam's spillway. Because of the large social and economic impacts of such incidents and given the major dams in California have an average age of above 50 years, it is important to evaluate the risk of failure of dams over a planning period in the future. In this study, we inspected the possible impacts of climate change on the future flooding hazard for several major dams in California. Here we show that in the warmer future climate, the risk of dam failure most likely increases for most of the major dams in California. The insights gained from this study highlight the important role of adaptation strategies for the operational management of aging dams in a changing climate, together with adequate and timely maintenance. Key Points: Flood hazard is projected to increase at the upstream of major dams in California due to changes in precipitation extremesFailure probability is projected to increase for most of the California dams in a warming climateHistorical 100‐year flood is 5 times more likely for some biggest California dams under RCP8.5 (high greenhouse gas concentration levels) [ABSTRACT FROM AUTHOR]

Persistent motion of slow‐moving landslides has been linked to entrapment of water in slide masses by weak, low‐permeability shear zones at their basal and lateral margins. This so‐called "bathtub effect" should have remotely sensible effects on soil moisture and vegetation health. Here we assess this effect at a seasonally active earthflow in northern California with analysis of soil properties and nine years (2009–2018) of multispectral satellite imagery. The shear zone has low hydraulic conductivity, and the earthflow shows elevated water content relative to its surroundings. Spectral indices suggest that the earthflow is perennially wetter, and its vegetation thrives for longer into the dry season as a result. The magnitude of the bathtub effect is correlated with cumulative multiyear rainfall, and its surface expression disappears during the driest year of the 2012–2015 drought. These findings demonstrate a new method for identifying and monitoring mechanical and hydrological interactions that enable persistent landslide motion. Plain Language Summary: The ability of some landslides to move slowly and persistently for many years without accelerating catastrophically may be linked to entrapment of rain water in slide masses by slow‐draining boundaries at their sides and bottoms. The existence of this so‐called "bathtub effect" can be tested for directly with soil analysis but should also have visible effects on soil moisture and vegetation health that can be measured remotely using satellite imagery. We examined a seasonally active landslide in northern California with slow‐draining boundaries and elevated water content relative to its surroundings. Nine years (2009–2018) of satellite imagery suggest that the earthflow is perennially wetter and its vegetation thrives for longer into the dry season as a result. The magnitude of this effect is correlated with cumulative multiyear rainfall. These findings demonstrate a new method for identifying and monitoring the hydrology and movement of persistent landslides. Key Points: Persistent movement of earthflows is linked to entrapment of water within weak, low‐permeability shear zonesExcess water stored in earthflows has remotely sensible effects on soil moisture and vegetation health that scale with multiyear rainfall [ABSTRACT FROM AUTHOR]

The unexpected dry 2015–2016 El Niño winter and extremely wet 2016–2017 La Niña winter in California challenged current seasonal prediction systems. Using the Met Office GloSea5 forecast ensemble, we study the precipitation and circulation differences between these seasons and identify processes relevant to California precipitation predictions. The ensemble mean accurately predicts the midlatitude atmospheric circulation differences between these years, indicating that these differences were predictable responses to the strong oceanic forcing differences. The substantial California precipitation differences were poorly predicted with large uncertainty. Notable differences in high‐latitude circulation anomalies associated with internal variability distinguish the ensemble members that successfully simulate precipitation from those that do not. Specifically, accurate representation of the Arctic Oscillation phase differences improves the accuracy of simulated precipitation differences but these differences were not well predicted in the ensemble mean for these seasons. Improved representation of high‐latitude processes such as the Arctic Oscillation and polar‐midlatitude teleconnections could therefore improve California seasonal predictions. Plain Language Summary: California recently experienced two unusual winter seasons. Following a failed rainy season despite the strong 2015–2016 El Niño that typically brings heavy rains, California unexpectedly experienced one of its wettest winter seasons on record during the 2016–2017 La Niña. The seasonal forecast systems were unable to predict these unusual winter precipitation patterns. We examine the ability of a state‐of‐the‐art forecast system to capture the large‐scale atmospheric circulation patterns that influence the track of storms, which bring a majority of winter precipitation to California. We show that the seasonal forecast systems can reproduce the large‐scale circulation differences in the North Pacific‐American domain, but random atmospheric variability can still easily prevent the accurate prediction of regional scale precipitation in extratropical regions such as California. Further, we identify the role of a natural pattern of large‐scale variability in the atmosphere that affects weather in the middle and high latitudes, referred to as the Arctic Oscillation, in controlling the accuracy of California precipitation forecasts. Prioritizing improvements in the representation of these patterns, the processes by which they are predicted and their influence over other regions in the forecasts systems can help improve seasonal predictions, which is important for the management of California's water resources and infrastructure. Key Points: Large‐scale circulation differences across the Pacific Basin are predictable, but California precipitation has high internal variabilityHigher skill in predicting upper‐level (200 mb) circulation anomalies relative to the lower‐level (850 mb) anomalies over the Pacific BasinEnsemble subset with correct Arctic Oscillation phase improves accuracy of predicting California precipitation differences between these seasons [ABSTRACT FROM AUTHOR]

The mountains of the Western United States provide a vital natural service through the storage and release of mountain snowpack, lessening impacts of seasonal aridity and satiating summer water demand. However, climate change continues to undermine these important processes. To understand how snowpack may change in the headwaters of California's major reservoirs, the North American Coordinated Regional Climate Downscaling Experiment is analyzed to assess peak water volume, peak timing, accumulation rate, melt rate, and snow season length across both latitudinal and elevational gradients. Under a high‐emissions scenario, end‐of‐century peak snowpack timing occurs 4 weeks earlier and peak water volume is 79.3% lower. The largest reductions are above Shasta, Oroville, and Folsom and between 0‐ and 2,000‐m elevations. Regional climate model and global forcing data set choice is important in determining historical snowpack character, yet by end century all models show a significant and similar decline in mountain snowpack. Plain Language Summary: Mountains are natural water towers that store snowpack in winter and release it as snowmelt during spring to summer. However, climate change has and continues to undermine this natural service. To answer where and when water resource management may be impacted by a future of low‐to‐no snowpack, we can leverage climate models, which are able to project the future conditions of mountain snowpack under various assumptions of global greenhouse gas emissions. In this study, we use five unique climate models under a high‐emissions scenario to evaluate a set of snowpack measures upstream of 10 California reservoirs. These 10 reservoirs represent nearly half of California's surface storage and by end century could face a 79% reduction in peak snowpack water volume. This work provides detailed guidance on the mountain snow conditions policymakers, water managers, and scientists will encounter in addressing adaptive resiliency in the face of climate change. Key Points: The historical role of California's Sierra Nevada mountain snowpack as a steady source of fresh water is fading due to climate changeAcross nine climate simulations historical snow representation varied but by end century agree on the magnitude and direction of declineUnder a high‐emissions scenario, a 79.3% decline in peak snow upstream of 40% of California's reservoir storage is shown by end century [ABSTRACT FROM AUTHOR]