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Fault zone structures at many scales largely dictate earthquake ruptures and are controlled by the geologic setting and slip history. Characterizations of these structures at diverse scales inform better understandings of earthquake hazards and earthquake phenomenology. However, characterizing fault zones at sub‐kilometer scales has historically been challenging, and these challenges are exacerbated in urban areas, where locating and characterizing faults is critical for hazard assessment. We present a new procedure for characterizing fault zones at sub‐kilometer scales using distributed acoustic sensing (DAS). This technique involves the backprojection of the DAS‐measured scattered wavefield generated by natural earthquakes. This framework provides a measure of the strength of scattering along a DAS array and thus constrains the positions and properties of local scatterers. The high spatial sampling of DAS arrays makes possible the resolution of these scatterers at the scale of tens of meters over distances of kilometers. We test this methodology using a DAS array in Ridgecrest, CA which recorded much of the 2019 Mw7.1 Ridgecrest earthquake aftershock sequence. We show that peaks in scattering along the DAS array are spatially correlated with mapped faults in the region and that the strength of scattering is frequency‐dependent. We present a model of these scatterers as shallow, low‐velocity zones that is consistent with how we may expect faults to perturb the local velocity structure. We show that the fault zone geometry can be constrained by comparing our observations with synthetic tests. Fault zones are multi‐scale structures that govern where and how earthquakes happen. Characterizing fault zones at all scales is thus important for understanding earthquake ruptures and earthquake‐related hazards. However, finding and describing fault zones at small scales remains a persistent challenge in earthquake science. We propose a framework for the characterization of fault zones using distributed acoustic sensing (DAS), a recently developed technique that converts fiber optic cables into dense networks of ground motion sensors. Earthquake waves are scattered when they encounter fault zones, and this scattering creates signatures in DAS data that we can use to locate these fault zones. Additionally, the behavior of fault zone scattered waves with frequency may illuminate detailed characteristics of the fault zone. We test this framework using a DAS network in Ridgecrest, CA that recorded aftershocks of the 2019 magnitude 7.1 Ridgecrest earthquake. We use these recordings to map fault zone locations near the network. These locations are close to previously mapped faults but are more accurate. By comparing the behavior of observed fault zone scattered waves with frequency with that of simulations, we can constrain shallow fault zone geometry. We develop a framework for systematically locating fault zones at sub‐kilometer scales using the distributed acoustic sensing‐measured earthquake wavefieldWe present a model for these fault zones and use simulations to show that this model reproduces first‐order observations of scatteringBy comparing observations with synthetics, we use this method to constrain local fault zone geometry We develop a framework for systematically locating fault zones at sub‐kilometer scales using the distributed acoustic sensing‐measured earthquake wavefield We present a model for these fault zones and use simulations to show that this model reproduces first‐order observations of scattering By comparing observations with synthetics, we use this method to constrain local fault zone geometry

Zhan et al. (2014a) reported supershear rupture during the Mw6.7 aftershock of the 2013 Mw8.3 Sea of Okhotsk deep earthquake, relying heavily on the regional station PET, which played a critical role in constraining the vertical rupture dimension and rupture speed. Here we include five more regional stations and find that the durations of the source time functions derived from these stations are consistent with Zhan et al.'s supershear rupture model. Furthermore, to reduce the nonuniqueness of deconvolution and combine the bandwidths of different stations, we conduct a joint inversion of the six regional stations for a single broadband moment‐rate function (MRF). The best fitting MRF, which explains all the regional waveforms well, has a smooth shape without any temporal gaps. The Mw6.7 Okhotsk deep earthquake is more likely a continuous supershear rupture than a dynamically triggered doublet. New regional seismic data confirm the supershear rupture during the Mw6.7 Okhotsk deep earthquakeThe moment‐rate function based on a joint inversion has a smooth shape without any temporal gapsThe Mw6.7 Okhotsk deep earthquake is more likely a continuous rupture than a triggered doublet

The main cause of tsunamis is large subduction zone earthquakes with seismic magnitudes Mw> 7, but submarine volcanic processes can also generate tsunamis. At the submarine Sumisu caldera in the Izu–Bonin arc, moderate‐sized earthquakes with Mw< 6 occur almost once a decade and cause meter‐scale tsunamis. The source mechanism of the volcanic earthquakes is poorly understood. Here we use tsunami and seismic data from the recent 2015 event to show that abrupt uplift of the submarine caldera, with a large brittle rupture of the ring fault system due to overpressure in its magma reservoir, caused the earthquake and tsunami. This submarine trapdoor faulting mechanism can efficiently generate tsunamis due to large vertical seafloor displacements, but it inefficiently radiates long‐period seismic waves. Similar seismic radiation patterns and tsunami waveforms due to repeated earthquakes indicate that continuous magma supply into the caldera induces quasi‐regular trapdoor faulting. This mechanism of tsunami generation by submarine trapdoor faulting underscores the need to monitor submarine calderas for robust assessment of tsunami hazards. Tsunamis are mainly caused by large submarine earthquakes, but submarine volcanic processes can also trigger tsunamis. Disproportionately large tsunami waves have been generated every decade by moderate‐sized volcanic earthquakes at a submarine volcano with a caldera structure, called Sumisu caldera, in the Izu–Bonin arc, south of Japan. Despite the moderate earthquake size, the maximum wave heights of the tsunamis were about a meter, and their source mechanism has been controversial. In this study, we used tsunami and seismic data from a recent earthquake to show that the submarine caldera abruptly uplifts due to brittle rupture of its intra‐caldera fault system driven by overpressure of magma accumulating in its underlying magma reservoir and generates large tsunamis almost once a decade. The atypical source mechanism for tsunami generation suggests that it is important to monitor active submarine calderas for assessing tsunami hazards. Large tsunamis are generated by moderate‐sized volcanic earthquakes at a submarine calderaTsunami and seismic data indicate that abrupt uplift of the submarine caldera by trapdoor faulting causes large tsunamisContinuous magma supply into the submarine caldera induces submarine trapdoor faulting on a decadal timescale Large tsunamis are generated by moderate‐sized volcanic earthquakes at a submarine caldera Tsunami and seismic data indicate that abrupt uplift of the submarine caldera by trapdoor faulting causes large tsunamis Continuous magma supply into the submarine caldera induces submarine trapdoor faulting on a decadal timescale

Fault zone complexities contain important information about factors controlling earthquake dynamic rupture. High‐resolution fault zone imaging requires high‐quality data from dense arrays and new seismic imaging techniques that can utilize large portions of recorded waveforms. Recently, the emerging Distributed Acoustic Sensing (DAS) technique has enabled near‐surface imaging by utilizing existing telecommunication infrastructure and anthropogenic noise sources. With dense sensors at several meters' spacing, the unaliased wavefield can provide unprecedented details for fault zones. In this work, we use a DAS array converted from a 10‐km underground fiber‐optic cable across Ridgecrest City, California. We report clear acausal and coda signals in ambient noise cross‐correlations caused by surface‐to‐surface wave scattering. We use these scattering‐related waves to locate and characterize potential faults. The mapped fault locations are generally consistent with those in the United States Geological Survey Quaternary Fault database of the United States but are more accurate than the extrapolated ones. We also use waveform modeling to infer that a 35 m wide, 90 m deep fault with 30% velocity reduction can best fit the observed scattered coda waves for one of the identified fault zones. These findings demonstrate the potential of DAS for passive imaging of fine‐scale faults in an urban environment. Fault zones are complex networks of fractures that can host earthquakes. The fractured rock surrounding the faults in the top hundreds of meters can amplify earthquake shaking intensity. Therefore, locating and characterizing faults is important for evaluating seismic hazards, especially in urban settings. But it is challenging to identify small hidden faults in the absence of surface evidence or cataloged seismicity. High resolution, high frequency seismic experiments may provide a solution. Distributed acoustic sensing (DAS) is an emerging technique that can turn existing fiber‐optic cables into cost‐effective seismic networks with meter‐scale spacing. In this work, we show how we image the fault zones at shallow depth using seismic noise generated by traffic along a DAS cable in Ridgecrest City, CA. The results can detect and distinguish faults at sub‐kilometer scales. We also show we can use DAS data to characterize fault zone properties. These results demonstrate the potential of DAS in fine‐scale fault imaging without needing earthquakes. Ambient noise interferometry with distributed acoustic sensing captures scattered surface waves from fault zonesThe fault locations mapped with scattered surface waves are generally consistent with previous models but with higher resolutionWe constrain the fault zone geometry and velocity reduction using the amplitudes of the scattered surface waves Ambient noise interferometry with distributed acoustic sensing captures scattered surface waves from fault zones The fault locations mapped with scattered surface waves are generally consistent with previous models but with higher resolution We constrain the fault zone geometry and velocity reduction using the amplitudes of the scattered surface waves

The cross‐correlation of a diffuse or random wavefield at two points has been demonstrated to recover an empirical estimate of the Green's function under a wide variety of source conditions. Over the past two decades, the practical development of this principle, termed ambient noise interferometry, has revolutionized the fields of seismology and acoustics. Yet, because of the spatial sparsity of conventional water column and seafloor instrumentation, such array‐based processing approaches have not been widely utilized in oceanography. Ocean‐bottom distributed acoustic sensing (OBDAS) repurposes pre‐existing optical fibers laid in seafloor cables as dense arrays of broadband strain sensors, which observe both seismic waves and ocean waves. The thousands of sensors in an OBDAS array make ambient noise interferometry of ocean waves straightforward for the first time. Here, we demonstrate the application of ambient noise interferometry to surface gravity waves observed on an OBDAS array near the Strait of Gibraltar. We focus particularly on a 3‐km segment of the array on the continental shelf, containing 300 channels at 10‐m spacing. By cross‐correlating the raw strain records, we compute empirical ocean surface gravity wave Green's functions for each pair of stations. We first apply beamforming to measure the time‐averaged dispersion relation along the cable. Then, we exploit the non‐reciprocity of waves propagating in a flow to recover the depth‐averaged current velocity as a function of time using a waveform stretching method. The result is a spatially continuous matrix of current velocity measurements with resolution <100 m and <1 hr. Ocean currents are challenging to measure because they are complex: flow varies across more than six orders of magnitude in space and time. The wavespeed of ocean surface gravity waves propagating in a current encodes information about the velocity of the current, providing an opportunity to measure current velocity from ocean wave records. In particular, waves propagating along the current move faster than waves propagating against the current, which is termed non‐reciprocity. By cross‐correlating ocean wave records at two locations, we can measure the non‐reciprocity and thereby recover an estimate of the average current velocity. In this study, we employ distributed acoustic sensing to measure ocean surface gravity wave propagation along an ocean‐bottom fiber optic cable. As waves pass over the cable, they exert a small force at the seafloor which deforms the cable and stretches the fiber within. By repeatedly probing the fiber with a laser, we can measure these minute deformations at each point along the fiber. We demonstrate this method on a power transmission cable in the Strait of Gibraltar, monitoring the spatio‐temporal evolution of the tidal current over a period of 4.5 days. Seafloor horizontal strain measured with distributed acoustic sensing is sensitive to ocean surface gravity wave (OSGW) pressureAmbient noise interferometry can be used to measure the dispersion relation of OSGW and infer flow velocityWe resolve the spatio‐temporal pattern of the tidal current along a 3‐km submarine cable segment in the Strait of Gibraltar Seafloor horizontal strain measured with distributed acoustic sensing is sensitive to ocean surface gravity wave (OSGW) pressure Ambient noise interferometry can be used to measure the dispersion relation of OSGW and infer flow velocity We resolve the spatio‐temporal pattern of the tidal current along a 3‐km submarine cable segment in the Strait of Gibraltar

We determined the rupture sequence of the 12 August 2021 Mw8.2 South Sandwich Island earthquake which appears to be a complex sequence in both time and space. Notable tsunamis were recorded by tide gauges at global distances. Given the complexity of this event, we conducted a multiple subevent inversion on broadband seismograms, to resolve its complex variations of fault geometry, location, depth, and temporal characteristics. We found that the rupture initiated as a regular deep thrust earthquake; it then ruptured shallower and triggered a silent and dominantly slow subevent extending ∼200 km to the south, and ended with two other regular subevents. The total duration is ∼260 s, unusually long for an Mw8.2 event. Our result is qualitatively consistent with other moment tensor solutions and the deviant mB–Mwand MS–Mwrelations, and provides a more quantitative space‐temporal pattern of this unusual sequence. The 2021 August South Sandwich Island Mw8.2 earthquake was a surprise, because it was initially reported as a magnitude 7.5 event at a deep depth (47 km) but generated a global‐spreading tsunami that would only be expected for a larger and shallower event. By using seismic data with period as long as 500 s, we revealed a hidden Mw8.16 shallow slow event that happened between clusters of regular ruptures in the beginning and end. Although the slow event contributed 70% of the seismic moment, lasted three minutes, and ruptured a 200‐km section of the plate interface, it is essentially invisible at short or intermediate periods, which explains its anomalously low body‐wave and surface‐wave magnitudes. The 2021 South Sandwich Island earthquake represents an extreme example of the broad spectral behaviors of subduction zone earthquakes and calls for attention in the research and warning of similar events. The South Sandwich Island Mw8.2 earthquake appears to have extended deep depth, yet displayed tsunami earthquake‐like featuresInversions including long‐period data revealed a Mw8.16 slow subevent at shallow depth, connecting regular deep ruptures at the endsThe hybrid of deep and shallow ruptures represents an extreme example of the broad spectral behaviors of subduction zone earthquakes The South Sandwich Island Mw8.2 earthquake appears to have extended deep depth, yet displayed tsunami earthquake‐like features Inversions including long‐period data revealed a Mw8.16 slow subevent at shallow depth, connecting regular deep ruptures at the ends The hybrid of deep and shallow ruptures represents an extreme example of the broad spectral behaviors of subduction zone earthquakes

Small‐scale intraslab heterogeneity is well documented seismically in multiple subduction zones, but its nature remains elusive. Previous efforts have been mostly focusing on the scattering strength at intermediate depth (<350 km), without constraining its evolution as a function of depth. Here, we illustrate that the inter‐source interferometry method, which turns deep earthquakes into virtual receivers, can resolve small‐scale intraslab heterogeneity in the mantle transition zone. The interferometric waveform observations in the Japan subduction zone require weak scattering (<1.0%) within the slab below 410 km. Combining with previous studies that suggest high heterogeneity level (∼2.5%) at intermediate depth, we conclude that the small‐scale intraslab heterogeneity weakens as slabs subduct. We suggest that the heterogeneities are caused by intraslab hydrous minerals, and the decrease in their scattering strength with depth reveals processes associated with dehydration of subducting slabs. Deep earthquakes often generate surprisingly lengthy surface shaking, due to wave trapping and guiding along laminar small‐scale scatterers within subducting slabs. The nature of these scatterers is not well understood, although they have been observed in multiple subduction zones above 350 km. Here, by turning some deep earthquakes into virtual sensors closer to our target, we find that the small‐scale scatterers within the slab core fade substantially as slab subducts, from ∼2.5% above 350 km to <1% below 410 km. The fading signal suggests that the scatterers are caused by heterogeneous hydration of the slab in the outer rise that decreases as the slab core dehydrates. We apply inter‐source interferometry to probe the small‐scale intraslab heterogeneity in the mantle transition zone beneath the Japan SeaThe deep intraslab scattering level needs to be weaker than at intermediate depth to fit the high‐frequency interferometric waveformsThe decrease of intraslab scattering with depth reveals the processes associated with slab dehydration We apply inter‐source interferometry to probe the small‐scale intraslab heterogeneity in the mantle transition zone beneath the Japan Sea The deep intraslab scattering level needs to be weaker than at intermediate depth to fit the high‐frequency interferometric waveforms The decrease of intraslab scattering with depth reveals the processes associated with slab dehydration

Characterizing the large M4.7+ seismic events during the 2018 Kīlauea eruption is important to understand the complex subsurface deformation at the Kīlauea summit. The first 12 events (May 17–May 26) are associated with long‐duration seismic signals and the remaining 50 events (May 29–August 2) are accompanied by large‐scale caldera collapses. Resolving the source location and mechanism is challenging because of the shallow source depth, significant nondouble‐couple components, and complex velocity structure. We demonstrate that combining multiple geophysical data from broadband seismometers, accelerometers, and infrasound is essential to resolve different aspects of the seismic source. Seismic moment tensor solutions using near‐field summit stations show the early events are inflationary. Infrasound data and particle motion analysis identify the source of inflation as the Halema'uma'u reservoir. For the later collapse events, two‐independent moment tensor inversions using local and global stations consistently show that asymmetric slips occur on inward‐dipping normal faults along the northwest corner of the caldera. While the source mechanism from May 29 onwards is not fully resolvable seismically using far‐field stations, infrasound records, and simulations suggest there may be inflation during the collapse. The summit events are characterized by both inflation and asymmetric slip, which are consistent with geodetic data. Based on the location of the slip and microseismicity, the caldera may have failed in a “see‐saw” manner: small continuous slips in the form of microseismicity on the southeast corner of the caldera, compensated by large slips on the northwest during the large collapse events. Characterizing the large seismic events that occurred at the Kīlauea summit is important to understand the subsurface deformation process during the 2018 eruption. There are a total of 62 events where the first 12 events are accompanied with long‐duration seismic signals and the later 50 events are associated with large collapses within the caldera. There are several challenges in characterizing these events due to the complex volcanic environment that can be overcome by using multiple geophysical data sets including seismic waves that travel in the Earth and infrasound that travels in the atmosphere to provide a more complete perspective on the seismic source—its location and how it deforms. While the shallow magma reservoir at the summit experiences an overall deflation throughout the eruption, we found that the reservoir inflates temporarily during the earlier seismic events. For the later collapse events, the caldera slipped on only one side instead of a complete subsidence of the entire caldera which is commonly assumed. Our finding of both inflation and one‐sided slip is consistent with other independent studies and suggests this asymmetric slip may be a common feature for basaltic volcanoes like Kīlauea. We characterized the large seismic events at the Kīlauea summit using particle motion, infrasound, and seismic moment tensor inversionNear‐field seismic observation is essential to resolve the isotropic contribution due to inflation of the Halema'uma'u reservoirTwo‐independent moment tensor inversions show that the caldera collapsed asymmetrically along the northwest corner We characterized the large seismic events at the Kīlauea summit using particle motion, infrasound, and seismic moment tensor inversion Near‐field seismic observation is essential to resolve the isotropic contribution due to inflation of the Halema'uma'u reservoir Two‐independent moment tensor inversions show that the caldera collapsed asymmetrically along the northwest corner

Urban basin investigation is crucial for seismic hazard assessment and mitigation. Recent advances in robust nodal‐type sensors facilitate the deployment of large‐N arrays in urban areas for high‐resolution basin imaging. However, arrays typically operate for only one month due to the instruments' battery life, and hence, only record a few teleseismic events. This limits the number of available teleseismic events for traditional receiver function (RF) analysis‐the primary method used in sediment‐basement interface imaging in passive source seismology. Insufficient stacking of RFs from a limited number of earthquakes could, however, introduce significant biases to the results. In this study, we present a novel Bayesian array‐based Coherent Receiver Function (CRF) method that can leverage datasets from short‐term dense arrays to constrain basin geometry. We cast the RF deconvolution as a sparsity‐promoted inverse problem, in which the deconvolution at a single‐station involves the constraints from neighboring stations and multiple events. We solve the inverse problem using a trans‐dimensional Markov chain Monte Carlo Bayesian algorithm to find an ensemble of RF solutions, which provides a quantitative way of deciding which features are well resolved and warrant geological interpretation. An application in the northern Los Angeles basin demonstrates the ability of our method to produce reliable and easy‐to‐interpret RF images. The use of dense seismic networks and the state‐of‐the‐art Bayesian array‐based CRF method can provide a robust approach for subsurface structure imaging. Basin imaging is very important in urban areas and megacities (e.g., Los Angeles, San Francisco, Seattle, Tokyo) that are densely populated and prone to earthquakes, as basins tend to trap seismic energy, and increase the amplitude and duration of strong ground shaking. Recent advances in nodal‐type sensors facilitate the deployment of a larger number of seismometers (dense arrays) in the urban environment areas for rapid basin surveying. In this study, we developed a novel method that can leverage the dense arrays for urban sub‐surface structure imaging. The imaged sub‐surface structures are presented in probabilistic form, allowing objective assessment of feature identification and geological interpretation. We applied our method to two linear nodal arrays deployed in the greater Los Angeles region to demonstrate the ability of our method to produce reliable and easy‐to‐interpret sediment‐basement imaging. Our method can effectively unleash the power of dense array data for subsurface structure imaging. We developed a novel Bayesian array‐based receiver function method that can leverage dense arrays for urban basin imagingProbabilistic representation of the receiver functions helps objective assessment of feature identification and geological interpretationOur method produces reliable and coherent basin images that can improve our understanding of subsurface structures We developed a novel Bayesian array‐based receiver function method that can leverage dense arrays for urban basin imaging Probabilistic representation of the receiver functions helps objective assessment of feature identification and geological interpretation Our method produces reliable and coherent basin images that can improve our understanding of subsurface structures

We investigate the non‐double‐couple components of 224 M≥ 3.0 earthquakes in the 2019 Mw7.1 Ridgecrest sequence, which occurred on a complex fault system in the Eastern California Shear Zone. Full moment tensors are derived using waveform data from near‐fault and regional stations with a generalized cut‐and‐paste inversion and 3‐D velocity and attenuation models. The results show limited Compensated Linear Vector Dipole components, but considerable explosive isotropic components (5%–15% of the total moments) for approximately 50 earthquakes. Most of these events occur between the Mw6.4 foreshock and 1 day after the Mw7.1 mainshock and are mainly distributed around the rupture ends and fault intersections. The percentage of isotropic components is reduced considerably when data recorded by near‐fault stations are not included in the inversions, highlighting the importance of near‐fault data. The results suggest that high‐frequency damage‐related radiation and other local dilatational processes are responsible for the observed isotropic source terms. Earthquakes occur when rocks below the surface break and move rapidly. Deriving earthquake source mechanisms provides information on the involved physical processes. We examine source mechanisms of M≥ 3.0 earthquakes in the 2019 Mw7.1 Ridgecrest sequence, using waveforms from 39 near‐fault and regional stations. Many earthquakes are found to have considerable isotropic radiation that is not expected for pure slip motion along faults. The isotropic radiation reflects motions normal to the faults that may be caused by complex fault geometry, transient fluid pressure effects, and generation of microcracks in the rupture zones. We systematically investigate the possibility of each mechanism by analyzing the spatiotemporal variations of events with considerable isotropic components. The results suggest that rock damage in the rupture zone likely provides a major contribution to the isotropic radiation. Fifty out of 224 M≥ 3.0 earthquakes show considerable isotropic components not resolved without near‐fault dataEvents with large isotropic components occurred early in the sequence near rupture ends and fault intersectionsRock fracturing in earthquake source volumes likely contributes significantly to the isotropic components Fifty out of 224 M≥ 3.0 earthquakes show considerable isotropic components not resolved without near‐fault data Events with large isotropic components occurred early in the sequence near rupture ends and fault intersections Rock fracturing in earthquake source volumes likely contributes significantly to the isotropic components