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Supraglacial lakes have been observed to drain within hours of each other, leading to the hypothesis that stress transmission following one drainage may be sufficient to induce hydro‐fracture‐driven drainages of other nearby lakes. However, available observations characterizing drainage‐induced stress perturbations have been insufficient to evaluate this hypothesis. Here, we use ice‐sheet surface‐displacement observations from a dense global positioning system array deployed in the Greenland Ice Sheet ablation zone to investigate elastic stress transmission between three neighboring supraglacial lake basins. We find that drainage of a central lake can place neighboring basins in either tensional or compressional stress relative to their hydro‐fracture scarp orientations, either promoting or inhibiting hydro‐fracture initiation beneath those lakes. For two lakes located within our array that drain close in time, we identify tensional surface stresses caused by ice‐sheet uplift due to basal‐cavity opening as the physical explanation for these lakes' temporally clustered hydro‐fracture‐driven drainages and frequent triggering behavior. However, lake‐drainage‐induced stresses in the up‐flowline direction remain low beyond the margins of the drained lakes. This short stress‐coupling length scale is consistent with idealized lake‐drainage scenarios for a range of lake volumes and ice‐sheet thicknesses. Thus, on elastic timescales, our observations and idealized‐model results support a stress‐transmission hypothesis for inducing hydro‐fracture‐driven drainage of lakes located within the region of basal cavity opening produced by the initial drainage, but refute this hypothesis for distal lakes. Plain Language Summary: Mass loss from the Greenland Ice Sheet is accelerating, partly due to increasing rates of ice flow to the ocean. Ongoing increases in ice‐sheet surface melting play a complex role in this process: meltwater flows down through conduits and fractures to the ice‐sheet bed, lubricates the ice‐bed interface, and modulates ice‐flow speeds on hourly to decadal timescales. Drainage of supraglacial lakes via hydro‐fracture—water‐driven fracture propagation—produces the highest rates of meltwater flux to the ice‐sheet bed. The geographical range of lakes on the ice sheet has expanded in recent decades, but not all supraglacial lakes drain by hydro‐fracture: whether a hydro‐fracture‐driven drainage occurs in any particular lake basin is controlled by the ice‐sheet stress state. Here, we investigate whether one lake drainage can generate stress changes in neighboring lake basins that trigger hydro‐fracture initiation. We find that the drainage of one lake does lead to tensional stresses that promote hydro‐fracture initiation in a neighboring lake basin, but also that this initial lake drainage leads to a more compressive stress state that inhibits hydro‐fracture initiation in a different neighboring lake basin. Key Points: Drainage of one lake can place neighboring basins in tensional or compressional stress, promoting or inhibiting hydro‐fracture initiationTensional surface stresses are predominantly caused by basal cavity opening, with smaller contributions from basal slipThe first‐order control on elastic stress‐coupling length scales is the region of the bed over which basal cavity opening occurs [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]

The largest slip in great megathrust earthquakes often occurs in the 10–30 km depth range, yet seismic imaging of the material properties in this region has proven difficult. We utilize a dense onshore‐offshore passive seismic dataset from the southernmost Cascadia subduction zone where seismicity in the mantle of the subducted Gorda Plate produces S‐to‐P and P‐to‐S conversions generated within a few km of the plate interface. These conversions typically occur in the 10–20 km depth range at either the top or bottom of a ∼5 km thick layer with a high Vp/Vs that we infer to be primarily the subducted crust. We use their arrival times and amplitudes to infer the location of the top and bottom of the subducted crust as well as the velocity contrasts across these discontinuities. Comparing with both the Slab1.0 and the updated Slab2 interface models, the Slab2 model is generally consistent with the converted phases, while the Slab1.0 model is 1–2 km deeper in the 2–20 km depth range and ∼6–8 km too deep in the 10–20 km depth range between 40.25°N and 40.4°N. Comparing the amplitudes of the converted phases to synthetics for simplified velocity structures, the amplitude of the converted phases requires models containing a ∼5 km thick zone with at least a ∼10%–20% reduction in S wave velocity. Thus, the plate boundary is likely contained within or at the top of this low velocity zone, which potentially indicates a significant porosity and fluid content within the seismogenic zone. Key Points: We observe clear Ps and SP converted phases generated from the top and bottom of the oceanic crust of the subducted Gorda PlatePs and SP converted phases are used to constrain the geometry of the interface of the subducted slabAmplitude of the converted phases indicates that the subduction plate interface is a low shear‐wave velocity zone at seismogenic depth [ABSTRACT FROM AUTHOR]

Submarine landslides that occur offshore are common along the U.S. continental margins. These mass wasting events can trigger tsunamis and hence potentially devastate coastal communities and damage offshore infrastructure. However, the initiation and failure processes of submarine landslides are poorly understood. Here, we identify and locate 85 previously unknown submarine landslides in the Gulf of Mexico from 2008 to 2015. Ten of these landslides failed spontaneously while the remaining 75 were dynamically triggered by passing seismic surface waves from distant earthquakes with magnitudes as small as ∼5. Our observations demonstrate ongoing submarine landslide activity in the Gulf of Mexico where dense energy industry infrastructure is present and that the region is prone to secondary seismic hazard despite the low local seismicity rate. Our results should facilitate future investigations to identify unstable offshore slopes, to illuminate dynamic processes of landslides, and perhaps to apply remote detection technology in tsunami warning systems. Plain Language Summary: Landslides under the ocean are termed submarine landslides. Submarine landslides can pose hazards to coastal communities and offshore infrastructure, including triggering tsunamis and damaging oil platforms, pipelines, and submarine cables. These devastations may further cause environmental damages such as oil spills. Identifying these landslides and understanding their failure processes have both societal significance and intellectual merit. Using 8 years of continuous seismic data, we found 85 previously unknown submarine landslides in the Gulf of Mexico from 2008 to 2015. Ten of these landslides occurred without preceding earthquakes while the remaining 75 were triggered by the passing seismic surface waves from distant earthquakes. Our approach suggests that a remote detection technology for offshore landslides could be applied in tsunami warning systems. Key Points: Abundant submarine landslides were observed in the Gulf of MexicoMany of the landslides were dynamically triggered by remote earthquakesThere is no clear magnitude dependence of the triggering earthquakes [ABSTRACT FROM AUTHOR]

We develop a semiautomated method for estimating with second seismic moments the directivity, rupture area, duration, and centroid velocity of earthquakes. The method is applied to 41 southern California earthquakes with magnitude in the range 3.5–5.2 and provides stable results for 28 events. Apparent source time functions (ASTFs) of P and S phases are derived using deconvolution with three stacked empirical Green's functions (seGf). The use of seGf suppresses nongeneric source effects, improves the focal mechanism correspondence to the analyzed earthquakes, and typically allows inclusion of 5 to 15 more ASTFs compared with analysis using a single eGf. Most analyzed earthquakes in the Trifurcation area of the San Jacinto Fault have directivities toward the northwest, while events around Cajon Pass and San Gabriel Mountain tend to propagate toward the southeast. These results are generally consistent with predictions for dynamic rupture on bimaterial interfaces associated with the imaged velocity contrasts in the area. The second moment inversions also provide constraints on the upper and lower bounds of rupture areas in our data set. Stress drops and uncertainties are estimated for elliptical ruptures using the derived characteristic rupture length and width. The semiautomated second moment method with seGfs can be used for routine application to moderate earthquakes in locations with good station coverage. Key Points: We develop a semiautomated method for estimating finite fault parameters of earthquakes with second seismic momentsResolved rupture directivities are generally consistent with expectations for dynamic ruptures on the imaged velocity contrastsStress drops and uncertainties are estimated for elliptical ruptures using the derived characteristic rupture length and width [ABSTRACT FROM AUTHOR]

Seismic signals from ocean‐solid Earth interactions are ubiquitously recorded on our planet. However, these wavefields are typically incoherent in the time domain limiting their utilization for understanding ocean dynamics or solid Earth properties. In contrast, we find that during large storms such as hurricanes and Nor'easters the interaction of long‐period ocean waves with shallow seafloor features located near the edge of continental shelves, known as ocean banks, excites coherent transcontinental Rayleigh wave packets in the 20‐ to 50‐s period band. These "stormquakes" migrate coincident with the storms but are effectively spatiotemporally focused seismic point sources with equivalent earthquake magnitudes that can be greater than 3.5. Stormquakes thus provide new coherent sources to investigate Earth structure in locations that typically lack both seismic instrumentation and earthquakes. Moreover, they provide a new geophysical observable with high spatial and temporal resolution with which to investigate ocean wave dynamics during large storms. Plain Language Summary: Large storms such as hurricanes and Nor'easters generate strong long‐period ocean waves, which can interact with shallow seafloor features located near the edge of continental shelves known as ocean banks. Such interactions produce seismic sources with equivalent earthquake magnitudes that can be greater than 3.5. These seismic sources are termed "stormquakes," and they can excite coherent seismic wave fields that are well recorded across the North American continent. Stormquake is a newly identified geophysical phenomenon, which involves interactions of atmosphere, ocean, and the solid Earth. Therefore, stormquakes can provide useful information to investigate the Earth structure and ocean wave dynamics. Key Points: Energetic storms can generate stormquakes exciting coherent transcontinental surface wavefieldsThese stormquakes are effective point sources with equivalent earthquake magnitudes that can be greater than 3.5Large continental shelves, ocean banks, and strong storms are the three necessary factors for stormquake generation [ABSTRACT FROM AUTHOR]

Deep earthquakes exhibit strong variabilities in their rupture and aftershock characteristics, yet their physical failure mechanisms remain elusive. The 2018 Mw 8.2 and Mw 7.9 Tonga‐Fiji deep earthquakes, the two largest ever recorded in this subduction zone, occurred within days of each other. We investigate these events by performing waveform analysis, teleseismic P wave backprojection, and aftershock relocation. Our results show that the Mw 8.2 earthquake ruptured fast (4.1 km/s) and excited frequency‐dependent seismic radiation, whereas the Mw 7.9 earthquake ruptured slowly (2.5 km/s). Both events lasted ∼35 s. The Mw 8.2 earthquake initiated in the highly seismogenic, cold core of the slab and likely ruptured into the surrounding warmer materials, whereas the Mw 7.9 earthquake likely ruptured through a dissipative process in a previously aseismic region. The contrasts in earthquake kinematics and aftershock productivity argue for a combination of at least two primary mechanisms enabling rupture in the region. Plain Language Summary: Physical mechanisms of deep earthquakes are poorly understood as their ambient environments inhibit brittle slips, which operate shallow earthquake rupture processes. On 19 August 2018, a moment magnitude 8.2 deep earthquake occurred in Tonga, and 18 days later, another moment magnitude 7.9 deep earthquake occurred about 280 km away. These two events are among the largest deep earthquakes that have ever been recorded. We investigate these two events with a variety of seismological techniques and find that these two earthquakes show distinct rupture characteristics and aftershock productivities. The Mw 8.2 earthquake ruptured fast, whereas the Mw 7.9 earthquake ruptured slowly, despite they both lasted ∼35 s. Our observations show that Tonga can host two types of deep earthquakes with diverse and complex source properties, which is rarely observed. More importantly, our observations suggest that multiple physical mechanisms enabled the rupture propagation for the Mw 8.2 earthquake, and the Mw 8.2 and Mw 7.9 earthquake likely ruptured through different physical processes. Key Points: The Mw 8.2 Tonga earthquake ruptured for 37 s at 4.1 km/s and likely propagated from the cold slab core to warmer surrounding regionsThe Mw 8.2 Tonga earthquake excited high‐frequency seismic radiation spatially coinciding with abundant aftershocksThe Mw 7.9 Fiji earthquake was dynamically triggered in a previously aseismic region and ruptured for 35 s at 2.5 km/s likely through a dissipative process [ABSTRACT FROM AUTHOR]

Great subduction zone earthquakes vary considerably in the updip extent of megathrust rupture. It is unclear if this diversity reflects variations in interseismic strain accumulation owing to the limited number of subduction zones with seafloor monitoring. We use a borehole seismic‐geodetic observatory installed at the updip end of the Cascadia fault offshore Vancouver Island to show that the megathrust there does not appear to slip in triggered tremor or slow‐slip events when subjected to moderate dynamic stress transients. Borehole tilt and seismic data from recent teleseismic M7.6–8.1 earthquakes demonstrate a lack of triggered slow slip above the Mw 4.0 level and an absence of triggered tremor despite shear‐stress transients of 1–10 kPa that were sufficient to trigger tremor on the downdip end of the interface. Our observations are most consistent with a model in which the Cascadia fault offshore Vancouver Island is locked all the way to the trench. Plain Language Summary: Subduction zone thrust faults are generally thought to contain three primary regions in terms of earthquake rupture. Both the shallowest region near the trench (depths <~5–10 km) and the deeper onshore region (depths >~35 km) are thought to fail primarily without earthquakes, while seismic slip is contained primarily within the intervening depth range (~5–35 km). In many subduction zones around the world, the deeper region has been observed to emit small amounts of seismic radiation when nonvolcanic tremor events are triggered by passing seismic waves. Similarly, the shallowest portions of a few subduction zones behave the same way. Here we study the Cascadia subduction zone with newly available subseafloor seismic and geodetic data that indicate a clear difference in behavior between the deep and shallow parts of the fault. This likely indicates that strain is accumulated across most of the breadth of the accretionary prism and that the shallow part of the fault may rupture during great earthquakes. Key Points: New borehole seismic and geodetic data from the updip end of the Cascadia subduction zone show no evidence of triggered tremor or slow slip during the passage of teleseismic surface wavesThere is a clear contrast in the behavior of triggered tremor between the updip and downdip ends of the Cascadia subduction zone at Vancouver IslandBorehole tilt can rule out slow slip events at the Mw 4.0 level [ABSTRACT FROM AUTHOR]