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Seismic waveform modeling is a powerful tool for determining earth structure models and unraveling earthquake rupture processes, but it is usually computationally expensive. We introduce a scheme to vastly accelerate these calculations with a recently developed machine learning paradigm called the neural operator. Once trained, these models can simulate a full wavefield at negligible cost. We use a U-shaped neural operator to learn a general solution operator to the 2-D elastic wave equation from an ensemble of numerical simulations performed with random velocity models and source locations. We show that full-waveform modeling with neural operators is nearly two orders of magnitude faster than conventional numerical methods, and more importantly, the trained model enables accurate simulation for velocity models, source locations, and mesh discretization distinctly different from the training dataset. The method also enables convenient full-waveform inversion with automatic differentiation.

The depth of the Los Angeles (LA) basin is a critical factor for seismic hazard assessment and active tectonic studies. By analyzing S‐waves generated by earthquakes below the basin that convert to P‐waves at the sediment‐bedrock interface, we estimate the maximum depth of the LA basin to be 9 km. This estimate depends on the velocities within and below the basin, and the depth presented here is based on the latest community velocity model. To map the basin depth, we use two dense arrays: the Community Seismic Network, a dense network of low‐cost accelerometers in schools across LA region, and a basin‐wide node survey conducted in 2022, consisting of about 300 geophones deployed for a month. Utilizing differential travel times between direct S and Sp converted phases of local earthquakes, we produce a detailed depth map of the LA basin. A basin's depth influences how seismic waves resonate and amplify, affecting ground shaking during earthquakes. This information is vital for designing earthquake‐resistant buildings and infrastructure in the densely populated Los Angeles area, which sits atop a sedimentary basin. We collected data from two dense seismic sensor arrays: the Community Seismic Network, which uses low‐cost sensors placed in schools across the LA region, and a survey in 2022 using about 300 geophones deployed across the basin. By examining the travel times of seismic waves that convert at the sediment‐bedrock interface, we provided an independent measure of the basin's depth with dense coverage. Our findings show that the deepest part of the central Los Angeles Basin reaches over 9 km. New dense arrays covering Los Angeles basin enable better characterization of basin structureSp converted phase provides an independent constraint on basin depthThe deepest part of the central Los Angeles basin has an averaged depth of over 9 km New dense arrays covering Los Angeles basin enable better characterization of basin structure Sp converted phase provides an independent constraint on basin depth The deepest part of the central Los Angeles basin has an averaged depth of over 9 km

We study ground-motion response in urban Los Angeles during the two largest events (M7.1 and M6.4) of the 2019 Ridgecrest earthquake sequence using recordings from multiple regional seismic networks as well as a subset of 350 stations from the much denser Community Seismic Network. In the first part of our study, we examine the observed response spectral (pseudo) accelerations for a selection of periods of engineering significance (1, 3, 6, and 8 s). Significant ground-motion amplification is present and reproducible between the two events. For the longer periods, coherent spectral acceleration patterns are visible throughout the Los Angeles Basin, while for the shorter periods, the motions are less spatially coherent. However, coherence is still observable at smaller length scales due to the high spatial density of the measurements. Examining possible correlations of the computed response spectral accelerations with basement depth and Vs30, we find the correlations to be stronger for the longer periods. In the second part of the study, we test the performance of two state-of-the-art methods for estimating ground motions for the largest event of the Ridgecrest earthquake sequence, namely three-dimensional (3D) finite-difference simulations and ground motion prediction equations. For the simulations, we are interested in the performance of the two Southern California Earthquake Center 3D community velocity models (CVM-S and CVM-H). For the ground motion prediction equations, we consider four of the 2014 Next Generation Attenuation-West2 Project equations. For some cases, the methods match the observations reasonably well; however, neither approach is able to reproduce the specific locations of the maximum response spectral accelerations or match the details of the observed amplification patterns.

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Subducting the buoyant crustal material of an aseismic oceanic ridge has been regarded as a dominant contributor to flat slab subduction. However, normal‐dip subduction is also observed in some cases where ridges are subducting. In this study, we compare the subduction of two ridges on the Nazca Plate: Nazca Ridge (flat slab) and Iquique Ridge (normal‐dip slab). Anisotropy determined by shear wave splitting observation suggests that the low‐velocity anomalies found beneath the ridges are mapping anisotropic structure into isotropic velocities. After a tomographic inversion incorporating anisotropy models for both ridges, we find that the low‐velocity anomalies found beneath the Nazca Ridge are not anisotropic and therefore likely represent warm mantle, and those beneath the Iquique Ridge are caused by anisotropy. We conclude that subslab mantle buoyancy has a larger impact on the subduction angle than the crustal material of the ridge. Understanding the subduction process and how the mantle flows is pivotal in understanding the planetary evolution of Earth. Yet, several subduction characteristics remain unsolved, and the angle of plate subduction, which is often categorized into <30° (shallow), ∼30–35° (normal), >35° (steep), is one of those. Subduction of an oceanic ridge on a plate has been proposed as a cause of a shallow subduction angle since the thick crust of the ridge is less dense than the surrounding mantle. However, normal angles have been observed in some cases where oceanic ridges are subducting. In this study, we compare the Nazca Ridge and the Iquique Ridge, on the Nazca Plate subducting beneath South America. The subduction angles of the Nazca and Iquique Ridges are shallow and normal, respectively. When we incorporate directional variations in seismic wave velocities, which are produced by mantle flow, in seismic tomographic imaging, we find that the subducting oceanic ridge may not be a primary factor producing shallow angle subduction. Instead, the warm mantle beneath the Nazca Ridge may provide the buoyancy to support the Nazca Plate. Comparably, since the mantle beneath the Iquique Ridge is not warm, the subduction angle would stay normal. Despite having different subduction angles, subslab low‐velocity anomalies are found beneath both Nazca Ridge and Iquique RidgeWe introduced hypothetical seismic anisotropy in tomographic inversion to explore the origin of the subslab low‐velocity anomaliesThe low‐velocity anomalies beneath the Nazca Ridge and Iquique Ridge may come from buoyant warm mantle and anisotropy, respectively Despite having different subduction angles, subslab low‐velocity anomalies are found beneath both Nazca Ridge and Iquique Ridge We introduced hypothetical seismic anisotropy in tomographic inversion to explore the origin of the subslab low‐velocity anomalies The low‐velocity anomalies beneath the Nazca Ridge and Iquique Ridge may come from buoyant warm mantle and anisotropy, respectively

The eastern sector of the Trans‐Mexican Volcanic Belt (TMVB) is an enigmatic narrow zone that lies just above where the Cocos plate displays a sharp transition in dipping angle in central Mexico. Current plate models indicate that the transition from flat to steeper subduction is continuous through this region, but the abrupt end of the TMVB suggests that the difference in subduction styles is more likely to be accommodated by a slab tear. Based on a high‐resolution shear wave velocity and radial anisotropy model of the region, we argue that a slab tear within South Cocos can explain the abrupt end of the TMVB. We also quantify the azimuthal anisotropy beneath each seismic station and present a well‐defined flow pattern that shows how mantle material is being displaced from beneath the slab to the mantle wedge through the tear in the subducted Cocos plate. We suggest that the toroidal mantle flow formed around the slab edges is responsible for the existence of the volcanic gap in central Mexico. Moreover, we propose that the temperature increase caused by the influx of hot, less dense mantle material flowing through the tear to the Veracruz area may have significant implications for the thermomechanical state of the subducted slab and explain why the intermediate‐depth seismicity ends suddenly at the southern boundary of the Veracruz basin. The composite mantle flow formed by the movement of mantle material through the slab tears in western and southern Mexico may be allowing the Cocos plate to roll back in segments. The Trans‐Mexican Volcanic Belt (TMVB) is a prominent and enigmatic feature of the subduction system in Mexico. Its volcanic style diversity and oblique orientation to the trench are explained by the large along‐strike variations in the subduction parameters of the Rivera and Cocos plates. However, the abrupt termination of the TMVB on its eastern end with the Pico de Orizaba volcano is puzzling as the current slab model suggests that the transition of the Cocos flat‐slab geometry to normal subduction is smooth through this region. There is evidence that suggests that a tear in the slab might be developing, but it is unclear how this feature can support the unusually large topographic gradient that connects the volcanic high peaks with the Veracruz basin just south of the volcanic front. To provide further insight into the transition anatomy of this portion of the subducted slab and its relation with surface topography, we present a detailed and unified model of the velocity structure of the crust and uppermost mantle of central Mexico. Evidence from seismic anisotropy suggests the existence of a tear in S Cocos slabThe rollback process of the slab is a key element for the abrupt end of the TMVBThe slab window may create a corner flow for the isolated volcanoes in S Mexico

Seismic anisotropy of the lithosphere and asthenosphere was investigated with a dense broadband seismic transect nearly orthogonal to the central San Andreas fault (SAF). A contrast in SK(K)S splitting was found across the SAF, with a clockwise rotation of the fast orientation ~26° closer to the strike of the SAF and greater delay times for stations located within 35 km to the east. Dense seismograph spacing requires heterogeneous anisotropy east of the SAF in the uppermost mantle or crust. Based on existing station coverage, such a contrast in splitting orientations across the SAF may be unusual along strike and its location coincides with the high‐velocity Isabella anomaly in the upper mantle. If the Isabella anomaly is a fossil slab fragment translating with the Pacific plate, the anomalous splitting east of the SAF could indicate a zone of margin‐parallel shear beneath the western edge of North America. Directional dependence of seismic wave speeds, referred to as anisotropy, can illuminate preferred orientations or fabrics in the Earth organized by deformation. Seismic anisotropy near the sharply defined central segment of the San Andreas fault was investigated with a new dense temporary seismic transect. A contrast in uppermost mantle anisotropy across the fault was identified, with nearly fault parallel orientations only on the east side of the fault. We suggest that development of asymmetric anisotropy about the central San Andreas may arise due to fault‐parallel movement of a fossil slab beneath the western edge of North America. SK(K)S splitting parameters were measured along a dense broadband array across the central San Andreas faultAnomalous fault‐parallel splitting was found for stations located within 35 km east of the San Andreas faultThe area of fault‐parallel splitting may be explained by margin‐parallel shear due to a fossil slab translating with the Pacific plate

We construct a new shear velocity model for the San Gabriel, Chino and San Bernardino basins located in the northern Los Angeles area using ambient noise correlation between dense linear nodal arrays, broadband stations, and accelerometers. We observe Rayleigh and Love waves in the correlation of vertical (Z) and transverse (T) components, respectively. By combining Hilbert and Wavelet transforms, we obtain the separated fundamental and first higher mode of the Rayleigh wave dispersion curves based on their distinct particle motion polarization. Basin depths constrained by receiver functions, gravity, and borehole data are incorporated into the prior model. Our 3D shear wave velocity model covers the upper 3–5 km of the crust in the San Gabriel, Chino and San Bernardino basin area. The Vs model is in agreement with the geological and geophysical cross‐sections from other studies, but discrepancies exist between our model and a Southern California Earthquake Center community velocity model. Our shear wave velocity model shows good consistency with the CVMS 4.26 in the San Gabriel basin, but predicts a deeper and slower sedimentary basin in the San Bernardino and Chino basins than the community model. Sedimentary basins northeast of Los Angeles can potentially be a low‐velocity channel that focuses earthquake energy from the San Andreas fault into the Los Angeles region. To better understand the focusing effect, we build a new velocity model of this area using a new seismic data set. With the cross‐correlation technique, we extract the travel time information between two stations from the ambient noise, and together with the basin depths based on gravity and receiver functions build a 3D shear wave velocity model. Many geological features, like sedimentary basins and faults, are captured in our velocity model. Compared to the community velocity model, our model predicts a deeper sedimentary basin with lower seismic velocities, indicating the focusing effect of the sedimentary basins northeast of Los Angeles might be underestimated. We construct a 3D Vs model in San Gabriel and San Bernardino basins using ambient noise correlation between dense array nodal stationsWe separated the Rayleigh wave fundamental mode and first higher mode in dispersion analysis based on the Rayleigh wave particle motionOur Vs model predicts deeper and slower sedimentary basins than the Southern California Earthquake Center CVMS model, yet is consistent with geological and drilling data We construct a 3D Vs model in San Gabriel and San Bernardino basins using ambient noise correlation between dense array nodal stations We separated the Rayleigh wave fundamental mode and first higher mode in dispersion analysis based on the Rayleigh wave particle motion Our Vs model predicts deeper and slower sedimentary basins than the Southern California Earthquake Center CVMS model, yet is consistent with geological and drilling data

The San Gabriel, Chino, and San Bernardino sedimentary basins in Southern California amplify earthquake ground motions and prolong the duration of shaking due to the basins' shape and low seismic velocities. In the event of a major earthquake rupture along the southern segment of the San Andreas fault, their connection and physical proximity to Los Angeles (LA) can produce a waveguide effect and amplify strong ground motions. Improved estimates of the shape and depth of the sediment‐basement interface are needed for more accurate ground‐shaking models. We obtain a three‐dimensional basement map of the basins by integrating gravity and seismic measurements. The travel time of the sediment‐basement P‐to‐Sconversion, and the Bouguer gravity along 10 seismic lines, are combined to produce a linear relationship that is used to extend the 2D profiles to a 3D basin map. Basement depth is calculated using the predicted travel time constrained by gravity with an S‐wave velocity model of the area. The model is further constrained by the basement depths from 17 boreholes. The basement map shows the south‐central part of the San Gabriel basin is the deepest part and a significant gravity signature is associated with our interpretation of the Raymond fault. The Chino basin deepens toward the south and shallows northeastward. The San Bernardino basin deepens eastward along the edge of the San Jacinto Fault Zone. In addition, we demonstrate the benefit of using gravity data to aid in the interpretation of the sediment‐basement interface in receiver functions. The shaking levels in the Los Angeles (LA) metropolitan area due to an earthquake on the San Andreas fault are underestimated. Northeast of LA, the San Gabriel, Chino, and San Bernardino basins influence the amount of shaking the LA area will experience. Sedimentary basins like these can amplify and trap seismic waves. Understanding these basins' shapes will improve our Earth model of the area and therefore seismic hazard estimates. The Basin Amplification Seismic Investigation project installed several small seismic instruments across these basins to characterize the structure of the basins. Along with gravity measurements, which capture information about the rock's density variations, we determine the basins’ depth and shape. The depth model is then combined with a new velocity model of the area to produce an improved Earth model. Future studies of ground shaking should take these improved models into account. Passive seismic and gravity measurements are integrated to estimate the 3D depth of the northern Los Angeles basinsThe maximum depth in the San Gabriel basin is ∼4.5 km, and Chino and San Bernardino basins are less than 2 km deepThe use of gravity helps to delineate faults across the basin, which helps our sediment‐basement interpretation in the receiver functions Passive seismic and gravity measurements are integrated to estimate the 3D depth of the northern Los Angeles basins The maximum depth in the San Gabriel basin is ∼4.5 km, and Chino and San Bernardino basins are less than 2 km deep The use of gravity helps to delineate faults across the basin, which helps our sediment‐basement interpretation in the receiver functions

The ExxonMobil Corp. oil refinery in Torrance, California, experienced an explosion on 18 February 2015, causing ground shaking equivalent to a magnitude 2.0 earthquake. The impulse response for the source was computed from Southern California Seismic Network data for a single force system with a value of 2 × 105kN vertically downward. The refinery explosion produced an air pressure wave that was recorded 22.8 km away in a 52-story high-rise building in downtown Los Angeles by a dense accelerometer array that is a component of the Community Seismic Network. The array recorded anomalous waveforms on each floor displaying coherent arrivals that are consistent with the building's elastic response to a pressure wave caused by the refinery explosion. Using a finite-element model of the building, the force on the building on a floor-by-floor scale was found to range up to 1.42 kN, corresponding to a pressure perturbation of 7.7 Pa.

We use a dense seismic array composed of 5200 vertical geophones to monitor microseismicity in Long Beach, California. Poor signal‐to‐noise ratio due to anthropogenic activity is mitigated via downward‐continuation of the recorded wavefield. The downward‐continued data are continuously back projected to search for coherent arrivals from sources beneath the array, which reveals numerous, previously undetected events. The spatial distribution of seismicity is uncorrelated with the mapped fault traces, or with activity in the nearby oil‐fields. Many events are located at depths larger than 20 km, well below the commonly accepted seismogenic depth for that area. The seismicity exhibits temporal clustering consistent with Omori's law, and its size distribution obeys the Gutenberg‐Richter relation above 20 km but falls off exponentially at larger depths. The dense array allows detection of earthquakes two magnitude units smaller than the permanent seismic network in the area. Because the event size distribution above 20 km depth obeys a power law whose exponent is near one, this improvement yields a hundred‐fold decrease in the time needed for effective characterization of seismicity in Long Beach. Imaging of active faults beneath Long Beach, CaliforniaNew approach for seismic monitoring in a noisy environmentDepth‐dependent earthquake size distribution

Rayleigh wave tomography provides images of the shallow mantle shear wave velocity structure beneath the Gulf of California. Low‐velocity zones (LVZs) are found on axis between 26 and 50 km depth beneath the Guaymas Basin but mostly off axis under the other rift basins, with the largest feature underlying the Ballenas Transform Fault. We interpret the broadly distributed LVZs as regions of partial melting in a solid mantle matrix. The pathway for melt migration and focusing is more complex than an axis‐centered source aligned above a deeper region of mantle melt and likely reflects the magmatic evolution of rift segments. We also consider the existence of solid lower continental crust in the Gulf north of the Guaymas Basin, where the association of the LVZs with asthenospheric upwelling suggests lateral flow assisted by a heat source. These results provide key constraints for numerical models of mantle upwelling and melt focusing in this young oblique rift. Low velocities in the Gulf upper mantle are interpreted as partial meltingPartial melting under the Guaymas Basin and off axis of the other rift basinsLower crustal flow assisted by heat source in N Gulf near mantle upwelling

The morphology of the transition from flat to normal subduction in eastern central Mexico is explored using intraslab earthquakes recorded by temporary and permanent regional seismic arrays. Observations of a sharp transition in slab dip near the abrupt end of the Trans‐Mexican Volcanic Belt (TMVB) suggest a possible slab tear located within the subducted South Cocos plate. The eastern lateral extent of a thin ultra‐slow velocity layer (USL) imaged atop the Cocos slab in recent studies along the Meso America Subduction Experiment array is examined here using additional data. We find an end to this USL which is coincident with the western boundary of a zone of decreased seismicity and the end of the TMVB near the sharp transition in slab dip. Waveform modeling of the 2‐D structure in this region using a finite difference algorithm provides constraints on the velocity and geometry of the slab's seismic structure and confirms the location of the USL. Analysis of intraslab seismicity patterns reveals clustering, sudden increase in depth, variable focal mechanism orientations and faulting types, and alignment of source mechanisms along the sharp transition in slab dip. The seismicity and structural evidence suggests a possible tear in the South Cocos slab. This potential tear, together with the tear along the Orozco Fracture Zone to the northwest, indicates a slab rollback mechanism in which separate slab segments move independently, allowing for mantle flow between the segments. Sharp transition in slab dip observed near abrupt end of volcanic beltSeismicity and structural evidence suggests possible tear in S Cocos slabSlab tears may play an important role in the rollback process

We investigate the role that upper mantle buoyancy anomalies play in determining the behavior of the crust. Recently, Castellanos et al. (2020; https://doi.org/10.1126/sciadv.abb0476) observed that the anisotropy of the Pacific Northwest (PNW) crust correlates with the upper‐mantle velocity structure and suggested that vertical loads in the upper mantle can displace the Moho and drive crustal flow on a regional scale. To provide further insight into this relation, we resolve the crustal anisotropy in regions where near‐surface mantle‐based deformation might have occurred or is presently occurring. Specifically, we focus on the crust around the Rocky Mountains and around California since high‐resolution tomographic images reveal the presence of mantle structures similar to the ones that are thought to be driving the crust in the PNW. Our results reveal crustal flow driven by mantle vertical loading in both regions and suggest that this mechanism may be key in maintaining crustal isostasy during an orogeny. While it has long been hypothesized that the main forces driving tectonic deformation result from the flow in the convecting mantle, there is new evidence suggesting that mantle vertical loading also plays an important role in crustal tectonics. Here, we illuminate the deformation field of the lower crust in different regions of the United States to search for evidence of mantle forcing of crustal flow. Our findings reveal that mantle gravitational loads can uplift or downwarp the crust‐mantle boundary and create lateral pressure gradients that drive the lower crustal away from upwellings and toward downwellings, causing thinning and thickening of the crust. This mechanism differs from conventional models in which crustal thickness variations are associated with topographic variations and suggests that mantle vertical loading might be key to maintaining isostatic equilibrium and a main driver of crustal deformation. Our results bring us closer to understanding how crustal motions respond to mantle‐derived forces and how these influence tectonic evolution processes. Crustal seismic anisotropy reveals the coupling between the crust and the mantleMantle vertical loads drive crustal flow in much of the western United StatesMantle loads can depress the Moho, thicken the crust, and cause isostatic compensation without any topographic expression Crustal seismic anisotropy reveals the coupling between the crust and the mantle Mantle vertical loads drive crustal flow in much of the western United States Mantle loads can depress the Moho, thicken the crust, and cause isostatic compensation without any topographic expression

The proliferation of dense arrays promises to improve our ability to image geological structures at the scales necessary for accurate assessment of seismic hazard. However, combining the resulting local high‐resolution tomography with existing regional models presents an ongoing challenge. We developed a framework based on the level‐set method that infers where local data provide meaningful constraints beyond those found in regional models ‐ for example the Community Velocity Models (CVMs) of southern California. This technique defines a volume within which updates are made to a reference CVM, with the boundary of the volume being part of the inversion rather than explicitly defined. By penalizing the complexity of the boundary, a minimal update that sufficiently explains the data is achieved. To test this framework, we use data from the Community Seismic Network, a dense permanent urban deployment. We inverted Love wave dispersion and amplification data, from the Mw 6.4 and 7.1 2019 Ridgecrest earthquakes. We invert for an update to CVM‐S4.26 using the Tikhonov Ensemble Sampling scheme, a highly efficient derivative‐free approximate Bayesian method. We find the data are best explained by a deepening of the Los Angeles Basin with its deepest part south of downtown Los Angeles, along with a steeper northeastern basin wall. This result offers new progress toward the parsimonious incorporation of detailed local basin models within regional reference models utilizing an objective framework and highlights the importance of accurate basin models when accounting for the amplification of surface waves in the high‐rise building response band. Los Angeles is a major city of the United States that is at high risk of damage due to earthquakes, due to the large number of nearby active faults and its location on a deep bowl of weak rock, which tends to amplify earthquake damage. We use a large number of instruments located in Los Angeles district schools to make measurements of earthquakes that occurred near Ridgecrest, California in July 2019. These earthquakes generated a type of energy that is particularly useful for studying the structures responsible for amplification of earthquakes. Using this data, we applied a new imaging technique to create a local model of the northeast Los Angeles basin at higher resolution than had been previously available. Our imaging technique appropriately balances information from previous, lower resolution inversions with the new data obtained in this study. We generate a new velocity model of the northeastern Los Angeles Basin using data from the Community Seismic NetworkUsing a level‐set framework, we parsimoniously balance the existing Community Velocity Models with new data constraintsThe new model indicates a steeper and deeper basin underneath downtown Los Angeles, significantly amplifying 4–6 s Love waves We generate a new velocity model of the northeastern Los Angeles Basin using data from the Community Seismic Network Using a level‐set framework, we parsimoniously balance the existing Community Velocity Models with new data constraints The new model indicates a steeper and deeper basin underneath downtown Los Angeles, significantly amplifying 4–6 s Love waves

The metropolitan Los Angeles region represents a zone of high‐seismic risk due to its proximity to several fault systems, including the San Andreas fault. Adding to this problem is the fact that Los Angeles and its surrounding cities are built on top of soft sediments that tend to trap and amplify seismic waves generated by earthquakes. In this study, we use three dense petroleum industry surveys deployed in a 16 × 16‐km area at Long Beach, California, to produce a high‐resolution model of the top kilometer of the crust and investigate the influence of its structural variations on the amplification of seismic waves. Our velocity estimates reveal substantial lateral contrasts and correlate remarkably well with the geological background of the area, illuminating features such as the Newport‐Inglewood fault, the Silverado aquifer, and the San Gabriel river. We then use computational modeling to show that the presence of these small‐scale structures have a clear impact on the intensity of the expected shaking, and can cause ground‐motion motion acceleration to change by several factors over a subkilometer horizontal scale. These results shed light onto the scale of variations that can be expected in this type of tectonic settings and highlight the importance of resolution in modern‐day seismic hazard estimates. With a population of almost 20 million, the metropolitan Los Angeles region represents a zone of high‐seismic risk due to its proximity to several fault systems. This unique setting demands extensive seismic hazard analyses, which, in turn, depend on our knowledge of the elastic properties of the subsurface. With the use of three dense petroleum industry surveys, we image the shallow crust beneath Long Beach, CA, and construct a high‐resolution velocity model of the region. We then use these estimates to investigate the amount of earthquake wave amplification that we can expect in the area. Our results reveal that small‐scale structural heterogeneities have a significant impact on the propagation of seismic waves, and can even cause earthquake shaking to vary by several factors over a subkilometer horizontal scale. We extract surface waves from the ambient noise recorded at three dense petroleum industry surveys that were deployed in Long Beach, CaliforniaWe make phase velocity measurements across multiple frequencies and invert for a 3‐D shear wave velocity model of the regionWe propagate synthetic wavefields through our velocity model to investigate the variability in the expected ground shaking intensity We extract surface waves from the ambient noise recorded at three dense petroleum industry surveys that were deployed in Long Beach, California We make phase velocity measurements across multiple frequencies and invert for a 3‐D shear wave velocity model of the region We propagate synthetic wavefields through our velocity model to investigate the variability in the expected ground shaking intensity

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

Density contrasts in the lower mantle, inferred using seismic tomography, drive viscous flow; this results in kilometres of dynamically maintained topography at the core–mantle boundary and at the Earth's surface. The total gravity field due to interior density contrasts and dynamic boundary topography predicts the longest-wavelength components of the geoid remarkably well. Neglecting dynamic surface deformation leads to geoid anomalies of opposite sign to those observed.

In this study, we investigate the shallow shear wave velocity structure of the Los Angeles Basin in southern California, using ambient noise correlations between 5 dense arrays and 21 broadband stations from the Southern California Seismic Network (SCSN). We observe clear fundamental mode and first overtone Rayleigh waves in the frequency band 0.25–2.0 Hz, and obtain group velocity maps through tomography. We further derive a 3D shear wave velocity model, covering a large portion of the central LA Basin for the depths shallower than 3 km. We found that the small scale shallow velocity structure heterogeneities are better resolved compared with the SCEC Community velocity models. Our model captures the presence of the Newport‐Inglewood fault by a NW–SE trending high velocity belt. Our model provides more accurate constraints on local ground motion predictions with detailed mapping of structural heterogeneities. We derive top 3 km Vs structure of the central Los Angeles Basin using dense industrial arrays correlated with broadband seismic stationsOur model better resolves small scale heterogeneities and better predicts dispersion observations compared with the SCEC CVM modelsOur model illuminates the Newport‐Inglewood fault zone by a NW–SE trending high shear wave velocity belt We derive top 3 km Vs structure of the central Los Angeles Basin using dense industrial arrays correlated with broadband seismic stations Our model better resolves small scale heterogeneities and better predicts dispersion observations compared with the SCEC CVM models Our model illuminates the Newport‐Inglewood fault zone by a NW–SE trending high shear wave velocity belt

The reconstruction of body waves from the cross‐correlation of random wavefields has recently emerged as a promising approach to probe the fine‐scale structure of the Earth. However, because of the nature of the ambient noise field, the retrieval of body waves from seismic noise recordings is highly challenging and has only been successful in a few cases. Here, we use seismic noise data from a 5,200‐node oil‐company survey to reconstruct body waves and determine the velocity structure beneath Long Beach, California. To isolate the body wave energy from the ambient noise field, we divide the entire survey into small‐aperture subarrays and apply a modified double‐beamforming scheme to enhance coherent arrivals within the cross‐correlated waveforms. The resulting beamed traces allow us to identify clear refracted Pwaves traveling between different subarray pairs, which we then use to construct a high‐resolution 3D velocity model of the region. The inverted velocity model reveals velocity variations of the order of 3% and strong lateral discontinuities caused by the presence of sharp geologic structures such as the Newport‐Inglewood fault (NIF). Additionally, we show that the resolution that is achieved through the use of high‐frequency body waves allows us to illuminate small geometric variations of the NIF that were previously unresolved with traditional passive imaging methods. Body waves are identified within the ambient noise recorded by a dense seismic array in Long Beach, CaliforniaArray processing tools are used to retrieve refracted Pwaves propagating through different sections of the arrayTraveltime measurements are made to estimate 3D Pwave velocities beneath the array

A 2-D synthetic test study of global traveltime inversion for deep seated earth structure has been undertaken. This was done by generating traveltime residual data using reasonable models for earth structure. The data were then inverted by similar methods to those applied in global studies of earth structure. Our results indicate that models of the aspherical structure in the lower mantle based on traveltime data are only partially successful and only at the largest scales (harmonic degree ≤3) and that maps of the core—mantle boundary based on traveltimes are unsuccessful.

The character of multi-offset reflections from the deep crust in the Mojave Desert are examined to reveal the physical nature of the reflecting structures. We focus on distinguishing classical abrupt discontinuities, such as traditional models of the Conrad and Moho boundaries, from more unusual structures. Finite-difference modeling and simple interference relations show that pre-critical reflections exhibiting an increase in peak frequency with offset arise from thinly-layered horizontal structures, while reflections from step discontinuities show no change in frequency with offset. In the deep crust thin layers may result from sill intrusion or fault motion. The sense of changes in Poisson's ratio and the relative strength of density changes determine whether reflection amplitudes will increase or decrease with offset. A simple linear regression on pre-critical reflection amplitudes against offset is adequate to separate reflections arising from increases in Poisson's ratio from those arising from decreases in Poisson's ratio and/or density changes. The latter condition may be the result of strong anisotropy or the presence of pore fluid. Comparisons of the properties of major deep reflectors across the Mojave Desert suggest that the effects of tectonic motion and fluid injection have penetrated all levels of the crust.