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We propose a new approach capable of measuring local seismic anisotropy from 6C (three‐component translation and three‐component rotation) amplitude observations of ambient seismic noise data. Our recent theory demonstrates that the amplitude ratio of 6C cross‐correlation functions (CCFs) enables retrieving the local phase velocity. This differs from conventional velocity extraction methods based on the travel time. Its local sensitivity kernel beneath the 6C seismometer allows us to study anisotropy from azimuth‐dependent CCFs, avoiding path effects. Such point measurements have great potential in planetary exploration, ocean bottom observations, or volcanology. We apply this approach to a small seismic array at Pin˜ $\widetilde{n}$on Flat Observatory (PFO) in southern California, array‐deriving retrieves rotational ground motions from microseismic noise data. The stress‐induced anisotropy is well resolved and compatible with other tomography results, providing constraints on the origin of depth‐dependent seismic anisotropy. Plain Language Summary: In contrast to the well‐known translational displacement of seismic waves, rotation measures the angle of rotation of a point in a medium as it deforms. Traditionally, the travel time of cross‐correlation functions of translational displacement of ambient seismic noise and its corresponding propagation distance is used to calculate the velocity of the subsurface medium. However, our recent theory shows that the amplitude ratio of translational displacement and rotation allows us to reveal the local velocity of the medium beneath the receiver from ambient seismic noise data and to study anisotropy from earthquake events. This paper points out for the first time that the amplitude observations of 6C cross‐correlation functions of ambient seismic noise provide a new approach to measure the azimuth‐dependent velocity in anisotropic media. Key Points: We show a new approach enabling the extraction of local phase velocity from 6C cross‐correlation functions of ambient noise dataAzimuth‐dependent 6C cross‐correlation functions allow us to study local seismic anisotropy and its depth dependenceLocal anisotropy at Pin˜ $\widetilde{n}$on Flat Observatory is compatible with compression stress, providing constraints on stress‐induced anisotropy [ABSTRACT FROM AUTHOR]

Template matching has proven to be an effective method for seismic event detection, but is biased toward identifying events similar to previously known events, and thus is ineffective at discovering events with non‐matching waveforms (e.g., those dissimilar to existing catalog events). In principle, this limitation can be overcome by cross‐correlating every segment (possible template) of a seismogram with every other segment to identify all similar event pairs, but doing so has been previously considered computationally infeasible for long time series. Here we describe a method, called the 'Matrix Profile' (MP), a "correlate everything with everything" calculation that can be efficiently and scalably computed. The MP returns the maximum value of the correlation coefficient of every sub‐window of continuous data with every other sub‐window, as well as the best‐correlated sub‐window location. Here we show how MP methods can obtain valuable results when applied to months and years of continuous seismic data in both local and global case studies. We find that the MP can identify many new events in Parkfield, California seismicity that are not contained in existing event catalogs and that it can efficiently find clusters of similar earthquakes in global seismic data. Either used by itself, or as a starting point for subsequent template matching calculations, the MP is likely to provide a useful new tool for seismology research. Plain Language Summary: Detecting and cataloging earthquakes through analysis of seismic data—the shapes of seismic waves, recorded by seismometers—is foundational to our understanding of Earth's interior structure and processes, as well as geological hazards such as earthquakes. Methods to improve the efficiency and sensitivity of earthquake detection while maintaining accuracy are critical, as seismic data volumes have grown exponentially in recent decades. Recently, methods that use recorded earthquakes as template patterns to identify in seismic data, have proven capable of detecting several times more earthquakes than traditional methods. However, such methods require knowledge of the template earthquakes ahead of time, and are best at identifying earthquakes whose waveforms are similar to the templates. We present here a new method, called the 'Matrix Profile' (MP), that takes short windows of data from a seismic data stream, and compares them to all other parts of that data stream, identifying parts of the data that are highly similar. The MP effectively identifies earthquakes, even those which are hidden within noise, does not require any templates to be provided upfront, and can be efficiently calculated. We demonstrate its success at detecting earthquakes in different types of seismic data, and provide practical guidelines for applying the method and interpreting the results. Key Points: The matrix profile finds the maximum autocorrelation of every subsequence in a time seriesPeaks of high similarity in the matrix profile can be used to identify seismic events and similar event pairsThe matrix profile method has similar sensitivity to template matching but does not require a priori templates [ABSTRACT FROM AUTHOR]

We use sparse dictionary learning to develop transformations between seismic velocity models of different resolution and spatial extent. Starting with data in the common region of both models, the method can enhance a regional lower‐resolution model to match the style and resolution of local higher‐resolution results while preserving its regional coverage. The method is demonstrated by applying it to two‐dimensional VS and three‐dimensional VP and VS regional and local velocity models in southern California. The enhanced reconstructed regional results exhibit clear visual improvements, especially in the reconstructed VP/VS ratios, and better correlations with geological features. Moreover, the reconstructed regional VP, VS models outperform the original ones in comparison of simulated earthquake waveforms to observations. The improved fitting to observed waveforms extends beyond the domain of the overlapping region. The developed dictionary learning approach provides physically interpretable results and offers a powerful tool for additional applications of data enhancement in earth sciences. Plain Language Summary: Seismic velocity models are essential for many applications including derivation of earthquake properties, studies of tectonic processes, and analysis of earthquake ground motions. In various places there are velocity models of different spatial coverage and resolution. To benefit from the complementary strengths of such models, we develop a new method for enhancing regional velocity models of relatively low‐resolution with information contained in local higher‐resolution models. The method builds a transformation between different models in their common local region based on a sparse dictionary learning and then uses the transformation outside the common region. We apply the method to seismic velocity models in southern California and show that the enhanced regional P and S velocity models outperform the original models in fitting earthquake waveforms and also have better consistency with geological features. The method has a potential for enhancing other data sets in earth sciences. Key Points: We present a new method for merging multi‐scale velocity models with different resolutions and spatial coveragesWe reconstruct multi‐scale VP, VS regional seismic velocity models for southern CaliforniaValidation with waveform simulations shows that the reconstructed models outperform the original ones [ABSTRACT FROM AUTHOR]

The Southern San Andreas Fault (SSAF) in California is one of the most thoroughly studied faults in the world, but its configuration at seismogenic depths remains enigmatic in the Coachella Valley. We use a combination of space geodetic and seismic observations to demonstrate that the relatively straight southernmost section of the SSAF, between Thousand Palms and Bombay Beach, is dipping to the northeast at 60–80° throughout the upper crust (<10 km), including the shallow aseismic layer. We constrain the fault attitude in the top 2–3 km using inversions of surface displacements associated with shallow creep, and seismic data from a dense nodal array crossing the fault trace near Thousand Palms. The data inversions show that the shallow dipping structure connects with clusters of seismicity at depth, indicating a continuous throughgoing fault surface. The dipping fault geometry has important implications for the long‐term fault slip rate, the intensity of ground shaking during future large earthquakes, and the effective strength of the southern SAF. Plain Language Summary: The San Andreas Fault (SAF) is capable of large, destructive earthquakes and poses significant seismic hazard in California. In the Coachella Valley, the geometry of the SAF beneath the Earth's surface has been the subject of much debate. Here, we present new constraints on the fault's structure in the uppermost 2–3 km of the crust from several new sets of observations. We measure slow movement of the fault at the surface using satellite‐based radar and perform simulations of what fault geometry is most likely to produce the observed motion. We also installed an array of sensors across the SAF to measure seismic waves that travel through or reflect off the fault. Analysis of both data sets indicates that the shallow portion of the SAF dips to the northeast in the Coachella Valley. Precise locations of small earthquakes at greater depths exhibit a similar trend—we thus suggest the fault dips throughout the Earth's crust. Better understanding the geometry of this section of the SAF has implications for earthquake hazards in Southern California, as well as the fault's geologic history. Key Points: InSAR measurements and models of shallow creep on the southernmost San Andreas Fault indicate a moderate‐to‐steep northeast fault dipSeismic imaging using data from a dense array deployed near Thousand Palms reveal northeast dipping fault damage zonesA joint interpretation of available data suggests that the northeast dip persists throughout the upper 10 km of the crust [ABSTRACT FROM AUTHOR]

Earthquakes can be dynamically triggered by the passing waves of other distant events. The frequent occurrence of dynamic triggering offers tangible hope in revealing earthquake nucleation processes. However, the physical mechanisms behind earthquake dynamic triggering have remained unclear, and contributions of competing hypotheses are challenging to isolate with individual case studies. To gain a systematic understanding of the spatiotemporal patterns of dynamic triggering, we investigate the phenomenon in southern California from 2008 to 2017. We use the Quake Template Matching catalog and an approach that does not assume an earthquake occurrence distribution. We develop a new set of statistics to examine the significance of seismicity‐rate changes as well as moment‐release changes. Our results show that up to 70% of 1,388 global M ≥ 6 events may have triggered earthquakes in southern California. The triggered seismicity often occurred several hours after the passing seismic waves. The Salton Sea Geothermal Field, San Jacinto fault, and Coso Geothermal Field are particularly prone to triggering. Although adjacent fault segments can be triggered by the same earthquakes, the majority of triggered earthquakes seem to be uncorrelated, suggesting that the process is primarily governed by local conditions. Further, the occurrence of dynamic triggering does not seem to correlate with ground motion (e.g., peak ground velocity) at the triggered sites. These observations indicate that nonlinear processes may have primarily regulated the dynamic triggering cases. Plain Language Summary: Earthquakes interact with each other, such as mainshocks triggering nearby aftershocks. Earthquake dynamic triggering is a type of interaction where seismic waves from an earthquake trigger other earthquakes beyond several fault lengths, and sometimes, up to thousands of kilometers away. Triggered earthquakes may occur upon the arrival of the seismic waves but may also be delayed hours after the wave passage, suggesting the involvement of time‐dependent processes. Identifying delayed cases relies on robust measures of seismicity‐rate changes. Here we present a new method that can identify triggering cases without many assumptions. We find that earthquakes in southern California are frequently triggered by distant earthquakes around the globe, and the triggered earthquakes tend to cluster in space and time. We also find that the triggering incidences do not seem to correlate with the seismic wave characteristics of the distant earthquakes. Our findings suggest that dynamically triggered earthquakes in southern California are likely caused by time‐dependent, local complex processes. Key Points: Earthquake dynamic triggering is ubiquitous in southern CaliforniaTriggered earthquakes are frequently associated with significant moment‐release anomalies and are likely controlled by local processesOur procedure does not assume that seismicity follows a Poissonian distribution when identifying dynamic triggering [ABSTRACT FROM AUTHOR]

In this study, we perform a 2‐frequency sequential receiver function stacking investigation in Southern California. The resulting Moho depths exhibit similar patterns to previous studies while the crystalline crustal Vp/Vs values show more regional variations. Most Vp/Vs variations can be explained by compositional differences. We observe a dichotomy in Moho depth, Vp/Vs, and crustal strain rates between the Peninsular Ranges and Southern San Andreas Fault system. Comparisons between strain rates, Vp/Vs, and temperature suggest that crustal compositional variations may have played a more critical role in influencing the crustal strain rate variations in the Peninsular Ranges and Southern San Andreas than temperature. The structural and compositional variations provide a new insight into the causes of the migration of the Southern San Andreas Fault system and the formation of the "Big Bend." Plain Language Summary: This study estimates the crystalline crustal seismic wave speed ratio (Vp/Vs) in Southern California after removing the possible sedimentary layer effects. We find different crystalline crust Vp/Vs in different geological regions and relate the differences to the quartz abundance. Particularly for southernmost California, we find that crust with lower Vp/Vs mostly possesses higher strain rates, and this crust is located within the Southern San Andreas Fault system. Additionally, we find crustal strain rates in southernmost California are not correlated with temperature except for the Salton trough. These findings suggest that the primary influence on the strain rate variations between the Peninsular Ranges (PR) and the Southern San Andreas region is associated with chemical compositional differences between these two regions. We propose that the high‐Vp/Vs, low strain rates, and strong block‐behavior of the Peninsular Ranges diverted the Pacific‐North American plate boundary eastward into the interior of Southern California where the low Vp/Vs and higher quartz content in the crust is present. This eastward jump in the plate boundary system from west of the rheologically strong PR to east of the PR is responsible for the "Big Bend" geometry of the San Andreas fault. Key Points: Thickness and Vp/Vs of the sedimentary and crystalline crust layers are revealed by a sequential H‐κ stacking in Southern CaliforniaCrustal Vp/Vs, an indicator of composition, correlates with the crustal strain rates in the Peninsular Ranges and Southern San AndreasThe compositional contrasts between the Peninsular Ranges and Southern San Andreas may have facilitated the formation of the "Big Bend" [ABSTRACT FROM AUTHOR]

Establishing a baseline of ongoing secular velocity variations at the subsurface can improve the accuracy of detecting and interpreting short‐term velocity changes, and advance the understanding of observed seismic motions and the behavior of subsurface materials. Toward these goals, we develop and apply a deconvolved autocorrelation (DA) method to estimate regional daily and seasonal changes of seismic velocities in southern California. The DA method combines advantages of traditional autocorrelation and Horizontal‐to‐Vertical Spectral Ratio, and is used to analyze over 10 years of data recorded by 50 stations. The results indicate widespread daily and seasonal changes of up to 10% and 4%, respectively, in the top tens of meters of the crust. The thickness of the surface layer, distance from the coast, and topographic variations are important factors controlling the amplitudes of the resolved velocity variations. The results suggest that changes of soil moisture and thermoelastic strain are likely dominant factors affecting the daily and seasonal variations, respectively. The developed DA method can improve the accuracy and robustness of estimated changes of subsurface materials at other locations. Plain Language Summary: Temporal changes of subsurface seismic velocities can be important for many topics including understanding observed seismic motions and interpretation of crustal processes and structures. To accurately estimate temporal changes of subsurface velocities, we develop a new method and apply it to data recorded at 50 sites in southern California. The results reveal widespread daily and seasonal velocity variations with amplitudes up to 10% and 4% in the top tens of meters. We compare the amplitudes of the velocity variations with various attributes including thickness of the surface layer, surface geology, topography, geographical location, and vegetation. The amplitudes are generally larger in areas with reduced thickness of the surface layer. In addition, the amplitudes of daily variations increase with proximity to the coast, while the amplitudes of seasonal variations increase in areas with larger local topographic variations. The results suggest that changes of soil moisture and thermoelastic strain may govern the amplitudes of the daily and seasonal velocity variations at many sites. Key Points: A newly developed deconvolved autocorrelation method can estimate accurately and robustly subsurface variations of seismic velocitiesAnalysis of seismic data in southern California indicates widespread daily and seasonal velocity variations in the top tens of metersChanges of soil moisture and thermoelastic strain are likely key factors affecting the daily and seasonal variations, respectively [ABSTRACT FROM AUTHOR]

We utilized relative polarity measurements and machine learning techniques to better resolve focal mechanisms and stress orientations considering a catalog of ∼29,000 relocated earthquakes that occurred during 1984–2021 in the southeastern San Francisco Bay Area. Earthquake focal mechanisms are commonly produced using P wave first motion polarities, which traditionally requires events to be well‐recorded across a seismic network with good focal sphere coverage. We adapted recently developed approaches that are less dependent on high signal‐to‐noise records and exploit similar waveforms to produce relative polarity and amplitude measurements between earthquake pairs. These techniques were previously only applied on localized earthquake sequences, and we further developed these approaches so that they can be utilized for regional catalogs. We validated or corrected manually identified polarities by performing polarity consensuses using earthquake pairs. Missing and unreliable polarity measurements were imputed using iterative random forests, an unsupervised ensemble machine learning method. Relative P and S wave amplitude measurements were made between earthquakes, constraining S/P ratios for low signal‐to‐noise waveforms. Using these techniques, we were able to reduce focal mechanism uncertainties by an average of ∼13° and produced well‐constrained focal mechanisms for ∼6 times as many earthquakes than those produced using only the traditionally derived polarities. We performed stress inversions using the focal mechanisms by grouping the focal mechanism results into a quadtree structure. Our stress results are consistent with previous work, albeit at a higher spatial resolution, and demonstrate these techniques can aid our understanding of fault structures and kinematics in more detail than was previously possible. Plain Language Summary: We determined whether earthquake compressional waves recorded on seismic stations caused either an initial upwards or downwards motion by comparing hundreds of thousands of waveforms against one another in a portion of the southeastern San Francisco Bay Area. If the waveforms were similar, the polarities for two events were likely the same. If the waveforms were anti‐similar (one waveform was "flipped"), then the polarities for the two events were likely different. Using these comparisons, we were able to correct polarities that had been manually selected in addition to increasing the number of polarity measurements. Some polarities for earthquakes recorded at a seismic station could still not be determined, for example, a station was installed after an earthquake occurred. To further increase the number of polarity measurements, we used a machine learning technique to estimate these missing polarities. All of these polarity measurements were used to determine how the faults hosting the cataloged earthquakes slipped, which allowed us to better understand stresses in the area. Key Points: We developed a new approach using relative polarity measurements between earthquakes to constrain focal mechanisms in regional catalogsConsidering a catalog of 29,179 earthquakes, we determined quality mechanisms for 18,226 events and used them to constrain stress fieldFocal mechanism stress inversion results are consistent with previous work, albeit at a higher resolution [ABSTRACT FROM AUTHOR]

Earthquake focal mechanisms, determined with P‐wave polarities and S/P amplitude ratios, are primary data for analyzing fault zone geometry, sense of slip, and the crustal stress field. Solving for the focal mechanisms of small earthquakes is often challenging because phase arrivals and first‐motion polarities are hard to be separated from noise. To overcome this challenge, we implement convolutional‐neural‐network algorithms (Ross, Meier, & Hauksson, 2018, Ross, Meier, Hauksson, & Heaton, 2018, https://doi.org/10.1029/2017jb015251, https://doi.org/10.1785/0120180080) to detect additional phases and polarities. Using both existing and these new data, we build a high‐quality focal mechanism catalog of 297,478 events that occurred from 1981 to 2021 in southern California with the HASH method of Hardebeck and Shearer (2002), https://doi.org/10.1785/0120010200, Hardebeck and Shearer (2003), https://doi.org/10.1785/0120020236. The new focal mechanism catalog is overall consistent with the standard catalog (Yang et al., 2012, https://doi.org/10.1785/0120110311) but includes 40% more focal mechanisms, and is more consistent with moment tensor solutions derived using waveform‐fitting methods. We apply the new catalog to identify changes in focal mechanism properties caused by the occurrences of large mainshocks such as the 2010 Mw7.2 El Mayor‐Cucapah and 2019 Mw7.1 Ridgecrest earthquakes. Such changes may be associated with co‐seismic stress drops, post‐seismic deformation processes, and static stress changes on a regional scale. The new high‐resolution catalog will contribute to improved understanding of the crustal stress field, earthquake triggering mechanisms, fault zone geometry, and sense of slip on the faults in southern California. Plain Language Summary: Fault geometry and stress field within the Earth's crust strongly affect crustal deformation and seismic hazard but are difficult to be captured due to limited direct observations at depth. Fortunately, the earthquake seismic waves recorded at the Earth's surface are strongly affected by the fault orientations and slip directions of earthquake ruptures (focal mechanism), which can be used for crustal monitoring. However, because small‐amplitude seismic waves attenuate along propagation paths, and are often masked by near‐surface seismic noise, many small (M < 3) earthquakes do not have high‐quality seismic records and focal mechanism solutions. To overcome this difficulty, we implement convolutional‐neural‐network algorithms to extract additional phase arrivals and P‐wave first‐motion polarities from noisy seismic records. We utilize both existing and these new data to solve focal mechanisms and build a new focal mechanism catalog for earthquakes occurring from 1981 to 2021 in southern California. The new catalog has 40% more solutions and illuminate strong spatiotemporal correlation of focal mechanism variations with the local and regional stress perturbations caused by major earthquake ruptures. The new catalog can help to capture small‐scale and small‐amplitude crustal stress perturbations, differentiate earthquake triggering mechanisms, and better understand crustal dynamics and seismic hazards. Key Points: We present a new focal mechanism catalog for southern California based on the added phases and polarities picked by convolutional‐neural‐network algorithmsThe new catalog has 40% more solutions with smaller uncertainties and higher consistency with moment tensor solutionsAfter major ruptures, focal mechanism variations show spatial correlation with co‐seismic slip locally and static stress changes regionally [ABSTRACT FROM AUTHOR]

Nowcasting is a term originating from economics, finance, and meteorology. It refers to the process of determining the uncertain state of the economy, markets or the weather at the current time by indirect means. In this paper, we describe a simple two‐parameter data analysis that reveals hidden order in otherwise seemingly chaotic earthquake seismicity. One of these parameters relates to a mechanism of seismic quiescence arising from the physics of strain‐hardening of the crust prior to major events. We observe an earthquake cycle associated with major earthquakes in California, similar to what has long been postulated. An estimate of the earthquake hazard revealed by this state variable time series can be optimized by the use of machine learning in the form of the Receiver Operating Characteristic skill score. The ROC skill is used here as a loss function in a supervised learning mode. Our analysis is conducted in the region of 5° × 5° in latitude‐longitude centered on Los Angeles, a region which we used in previous papers to build similar time series using more involved methods (Rundle & Donnellan, 2020, https://doi.org/10.1029/2020EA001097; Rundle, Donnellan et al., 2021, https://doi.org/10.1029/2021EA001757; Rundle, Stein et al., 2021, https://doi.org/10.1088/1361-6633/abf893). Here we show that not only does the state variable time series have forecast skill, the associated spatial probability densities have skill as well. In addition, use of the standard ROC and Precision (PPV) metrics allow probabilities of current earthquake hazard to be defined in a simple, straightforward, and rigorous way. Plain Language Summary: Earthquake nowcasting refers to the determination of hazard for major earthquakes at the present time, the recent past, and the near future. Nowcasting is an idea borrowed from economics, markets, and meteorology, where it has been frequently used. In this paper, we show that there is order hidden within chaotic earthquake seismicity using a very simple transformation of the data. Small earthquakes appear to transition from unstable stick‐slip events that produce seismic waves, to stable sliding where no seismic waves are produced. Our hypothesis is that this transition is due to a material phenomenon called strain‐hardening, that is frequently observed in laboratory rock mechanics experiments. The result is a state variable time series, computed over the last 51 years in California, that strongly resembles the long‐anticipated cycle of stress accumulation and release. Using supervised machine learning techniques, we can optimize the two‐parameter model. From that optimized model, we can rigorously calculate the probability of current hazard from major earthquakes. Extending these methods, we can also compute spatial hazard as well. The result is a new method for assessing earthquake hazard that may be useful for a variety of applications. Key Points: "Chaotic" seismicity contains hidden structure in the form of state variable time seriesStandard data science methods can be used to convert the time series to probabilitiesBoth temporal and spatial probabilities can be computed [ABSTRACT FROM AUTHOR]

We present a high‐resolution P‐wave azimuthally anisotropic velocity model for the upper and middle crust beneath southern California by a novel adjoint‐state traveltime tomography technique. Our model reveals significant anisotropy variations between tectonic blocks that clearly reflect both past and current plate boundary deformation. In the shallow crust, seismic anisotropy is mostly controlled by the preferred alignment of microcracks related to the present N‐S compressive stress; while at deeper depths (>∼6 km), seismic anisotropy mainly records paleofabrics formed during the long‐lived Farallon subduction and later extension that have not been fully reset by the present transform motion. Interestingly, our model demonstrates distinct fast axes beneath the western Transverse Ranges from its neighboring blocks, probably reflecting the large‐scale vertical axis clockwise rotation of the block. In addition, we identify layered structures with distinct anisotropy features beneath the Salton Trough, which could be a result of the current transtension. Plain Language Summary: Seismic anisotropy, which is defined as the directional dependence of the speed of seismic waves in a rock matrix, provides important constraints on the dynamic processes of the Earth. Here, we investigate this parameter in the upper to middle crust beneath southern California where the Pacific Plate slides northwestward with respect to the North American Plate by a novel seismic imaging technique. We find that present crustal anisotropy in southern California reflects both the past and current plate boundary deformation. While the shallow crustal anisotropy is dominantly controlled by the present stress, seismic anisotropy at larger depths (>∼6 km) mainly reflects paleofabrics associated with Mesozoic subduction and later extension. The clockwise rotation of the western Transverse Ranges is clearly resolved by its distinct anisotropy as compared to its neighboring blocks. On the other hand, we show that seismic anisotropy beneath the Salton Trough may be fully controlled by the current deformation, forming layered structures with distinct anisotropy features. Key Points: We build a 3‐D high‐resolution upper to middle crust azimuthally anisotropic model of southern California using local P‐wave arrival timesPresent anisotropy mostly reflects shape preferred orientation of stress‐related microcracks in the shallow crust and crystallographic preferred orientation of paleo mineral fabrics at larger depthsDistinct anisotropy beneath the western Transverse Ranges from its adjacent blocks reflects its history with rotation up to 110° [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]

The Parkfield segment of the San Andreas Fault (SAF) sits at the transition between the locked Cholame segment to the South and the SAF creeping segment to the North. The Parkfield segment hosts regular ∼Mw $\sim {\boldsymbol{M}}_{\boldsymbol{w}}$6 earthquakes followed by postseismic deformation. Recent studies based on geodetic data have highlighted spatial and temporal variations of aseismic slip rate in addition to postseismic slip along this section of the fault. We combine Global Navigation Satellite Systems (GNSS) and seismicity data over the 2006–2018 period to detail a comprehensive picture of transient slip events. We produce a catalog of relocated seismicity and repeating earthquakes. We use a variational Bayesian independent component analysis decomposition on GNSS data to separate geodetic deformation due to non‐tectonic sources from signals of potential tectonic origin. We then reconstruct the temporal evolution of fault slip and detect potential slip transients. Those events, determined as mostly aseismic with the exception of one related to a Mw ${\boldsymbol{M}}_{\boldsymbol{w}}$4.8 earthquake, occur more frequently during the 2004 Mw ${\boldsymbol{M}}_{\boldsymbol{w}}$6 post‐seismic period than during the subsequent inter‐seismic phase. Our study illustrates the rich dynamics of seismic and aseismic slip during both post‐ and inter‐seismic periods along active faults. Plain Language Summary: Faults can slip abruptly, generating earthquakes and seismic waves, or slowly and aseismically. Such a slow motion has now been observed along multiple active faults and is recognized as one of the important processes influencing the Earth's crust stress field. Aseismic slip interacts with earthquakes, and monitoring its time evolution on faults help us in better modeling the seismic cycle. Here we focus our attention on the Parkfield fault segment, sitting along the San Andreas Fault in California in between a section that generated a large earthquake (M7.9) in 1857 and a 150‐km‐long central creeping section that slips aseismically. We dig in noisy time series of surface displacements measured by GNSS (Global Navigation Satellite System) to detect and model small variations of aseismic slip rate (10–100 mm/year) along the fault. We find multiple candidate slow slip events during the period following the 2004 Parkfield earthquake, confirming and expanding previous independent observations. While aseismic slip following earthquakes was first thought to be a steady release of tectonic stress, we show that, in Parkfield, it is evolving with sudden jumps and slowdown. Key Points: Independent Component Analysis of GPS time series highlights slip transients along the San Andreas Fault after the 2004 M6 Parkfield eventWe document the spatio‐temporal distribution of seismic and aseismic slip historySlow Slip Events are more frequent during the 2004 M6 post‐seismic period than the subsequent interseismic period [ABSTRACT FROM AUTHOR]

Bridges in earthquake-prone states like California have been studied for near-field and far-field loadings. However, there is a research gap in terms of how an earthquake of a certain strike, dip, and rake is going to influence the bridge response. Here, we focus on multispan continuous concrete single-frame box girder bridges. In this study, we simulate earthquakes of varying focal mechanisms and perform nonlinear dynamic analysis of bridges with the synthesized ground motion. Furthermore, the influence of azimuth orientation with respect to the source is studied by placing the bridge model a few kilometers from the source at certain azimuths motivated by the radiation pattern of seismic waves. Fixing a magnitude, depth, and reference focal mechanism angles determining a focal mechanism is varied to study the effect of strike, dip, and rake on the bridges. In each of these cases, the potential risk is identified, and the study underscores the need to include these focal mechanism parameters into bridge fragility computations in risk assessment platforms such as HAZUS. [ABSTRACT FROM AUTHOR]

Recent progresses in artificial intelligence and machine learning make it possible to automatically identify seismic phases from exponentially growing seismic data. Despite some exciting successes in automatic picking of the first P‐ and S‐wave arrivals, auto‐identification of later seismic phases such as the Moho‐reflected PmP waves remains a significant challenge in matching the performance of experienced analysts. The main difficulty of machine‐identifying PmP waves is that the identifiable PmP waves are rare, making the problem of identifying the PmP waves from a massive seismic database inherently unbalanced. In this work, by utilizing a high‐quality PmP data set (10,192 manual picks) in southern California, we develop PmPNet, a deep‐neural‐network‐based algorithm to automatically identify PmP waves efficiently; by doing so, we accelerate the process of identifying the PmP waves. PmPNet applies similar techniques in the machine learning community to address the unbalancement of PmP datasets. The architecture of PmPNet is a residual neural network (ResNet)‐autoencoder with additional predictor block, where encoder, decoder, and predictor are equipped with ResNet connection. We conduct systematic research with field data, concluding that PmPNet can efficiently achieve high precision and high recall simultaneously to automatically identify PmP waves from a massive seismic database. Applying the pre‐trained PmPNet to the seismic database from January 1990 to December 1999 in southern California, we obtain nearly twice more PmP picks than the original PmP data set, providing valuable data for other studies such as mapping the topography of the Moho discontinuity and imaging the lower crust structures of southern California. Plain Language Summary: The success of machine learning in computer sciences, medical sciences, and many other fields has accelerated the implementation and development of machine learning techniques in seismology, making it possible to automatically identify seismic phases from the exponentially growing seismic data. At present, the auto‐identification of later seismic phases, such as the Moho‐reflected PmP waves, remains a significant challenge in matching the performance of experienced analysts. The main difficulty lies in the rare identifiable PmP waves, which makes the identification problem inherently unbalanced. In this work, by utilizing a high‐quality PmP data set in southern California, we develop a deep‐neural‐network‐based algorithm, PmPNet, to accelerate the process of identifying the PmP waves. We conduct systematic research with field data, and conclude that the PmPNet can efficiently achieve high precision and high recall simultaneously for automatically identifying the PmP waves from a massive seismic database. Applying the pre‐trained PmPNet to the seismic database from January 1990 to December 1999, we have tripled the PmP data set in southern California. Key Points: A deep‐neural‐network‐based algorithm, PmPNet, is developed to automatically pick the Moho‐reflected seismic waves PmPPmPNet efficiently achieves high precision and recall simultaneously to automatically identify PmP waves from a massive seismic databasePmPNet creates a large PmP database with a total of 28,093 high‐quality PmP picks in southern California [ABSTRACT FROM AUTHOR]

The theory of Green's function retrieval essentially requires homogeneously distributed noise sources. Even though these conditions are not fulfilled in nature, low-frequency (<1 Hz) surface waves generated by ocean–crust interactions have been used successfully to image the crust with unprecedented spatial resolution. In contrast to low-frequency surface waves, high-frequency (>1 Hz) body waves have a sharper, more localized sensitivity to velocity contrasts and temporal changes at depth. In general, their retrieval using seismic interferometry is challenging, and recent studies focus on powerful, localized noise sources. They have proven to be a promising alternative but break the assumptions of Green's function retrieval. In this study, we present an approach to model correlations between P waves for these scenarios and analyse their sensitivity to 3-D Earth structure. We perform a series of numerical experiments to advance our understanding of these signals and prepare for an application to fault monitoring. In the considered cases, the character of the signals strongly diverges from Green's function retrieval, and the sensitivity to structure has significant contributions in the source direction. An accurate description of the underlying physics allows us to reproduce observations made in the context of monitoring the San Jacinto Fault in California using train-generated seismic waves. This approach provides new perspectives for detecting and localizing temporal velocity changes previously unnoticed by commonly exploited surface-wave reconstructions. [ABSTRACT FROM AUTHOR]

In this paper, the performance of two 18-story steel moment frame buildings (UBC 1982 and 1997 designs) in southern California under plausible San Andreas fault earthquakes in the next 30 years is quantified. The approach integrates rupture-to-rafters simulations into the PEER performance-based earthquake engineering (PBEE) framework. Using stochastic sources and computational seismic wave propagation, three-component ground motion histories at 636 sites in southern California are generated for 60 scenario earthquakes on the San Andreas fault. The ruptures, with moment magnitudes in the range of 6.0–8.0, are assumed to occur at five locations on the southern section of the fault. Two unilateral rupture propagation directions are considered. The 30-year probabilities of all plausible ruptures in this magnitude range and in that section of the fault, as forecast by the United States Geological Survey, are distributed among these 60 earthquakes based on proximity and moment release. The response of the two 18-story buildings hypothetically located at each of the 636 sites under three-component shaking from all 60 events is computed using 3-D nonlinear time-history analysis. Using these results, the probability of the structural response exceeding Immediate Occupancy (IO), Life-Safety (LS), and Collapse Prevention (CP) performance levels under San Andreas fault earthquakes over the next 30 years is evaluated. [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]

Reflection seismic imaging usually suffers from a loss of resolution and contrast because of the fluctuations of the wave velocities in the Earth's crust. In the literature, phase distortion issues are generally circumvented by means of a background wave velocity model. However, it requires a prior tomography of the wave velocity distribution in the medium, which is often not possible, especially in depth. In this paper, a matrix approach of seismic imaging is developed to retrieve a 3-D image of the subsoil, despite a rough knowledge of the background wave velocity. To do so, passive noise cross-correlations between geophones of a seismic array are investigated under a matrix formalism. They form a reflection matrix that contains all the information available on the medium. A set of matrix operations can then be applied in order to extract the relevant information as a function of the problem considered. On the one hand, the background seismic wave velocity can be estimated and its fluctuations quantified by projecting the reflection matrix in a focused basis. It consists in investigating the response between virtual sources and detectors synthesized at any point in the medium. The minimization of their cross-talk can then be used as a guide star for approaching the actual wave velocity distribution. On the other hand, the detrimental effect of wave velocity fluctuations on imaging is overcome by introducing a novel mathematical object: The distortion matrix. This operator essentially connects any virtual source inside the medium with the distortion that a wavefront, emitted from that point, experiences due to heterogeneities. A time reversal analysis of the distortion matrix enables the estimation of the transmission matrix that links each real geophone at the surface and each virtual geophone in depth. Phase distortions can then be compensated for any point of the underground. Applied to passive seismic data recorded along the Clark branch of the San Jacinto fault zone (SJFZ), the present method is shown to provide an image of the fault until a depth of 4 km over the frequency range 10–20Hz with an horizontal resolution of 80 m. Strikingly, this resolution is almost one eighth below the diffraction limit imposed by the geophone array aperture. The heterogeneities of the subsoil play the role of a scattering lens and of a transverse waveguide which increase drastically the array aperture. The contrast is also optimized since most of the incoherent noise is eliminated by the iterative time reversal process. Beyond the specific case of the SJFZ, the reported approach can be applied to any scales and areas for which a reflection matrix is available at a spatial sampling satisfying the Nyquist criterion. [ABSTRACT FROM AUTHOR]

Rocks are heterogeneous materials that exhibit nonlinear elastic (anelastic) behavior at scales ranging from the laboratory to Earth. In the laboratory, typical, complex relationships exist between stress and strain that include hysteresis, finite relaxation times, strain rate, and history dependence. These behaviors are linked to important characteristics such as stress, porosity, permeability, material integrity, and material failure. We adopted a "pump‐probe" type experiment common in laboratory studies, using solid earth tides as the low‐frequency pump and empirical Green's function as the high‐frequency probe. By probing the velocity at different points in the pump cycle, we constrained important information about the strain‐modulus relationship. Near the San Andreas Fault, we observed strongly nonlinear elastic behavior that characterizes the damage zone. We also constrained important aspects of hysteretic behavior that are related to damage properties and possibly pore pressure. Away from the fault, the nonlinear behavior is diminished. Plain Language Summary: When intact materials are slightly squeezed or stretched, the amount of the material compresses or extends is typically some multiple of the applied force. Damaged materials have internal fractures, which makes this relationship more complicated. This is because fractures can be open or closed, each contributing differently to the overall material behavior. The more internal fractures exist, the more complicated the material behavior will be. By measuring complexity in how a material deforms when a force is applied, we can learn something about how damaged it is. We apply this analysis to rocks in the subsurface using seismic waves and solid earth tides. Solid earth tides are the stretching and compressing of the Earth due to the gravitational pull of the sun and moon. As the Earth is stretched and compressed, fractures in the Earth can open and close. We use seismic waves to measure how the Earth becomes softer or stiffer as these fractures open and close. The amount of this softening or stiffening tells us something about the material damage along the San Andreas Fault, and may also tell us something about the earthquake cycle. Key Points: The subsurface along the San Andreas Fault near Parkfield, California exhibits inhomogeneous nonlinear elastic behaviorThe magnitude of nonlinear response increases with decreasing distance to the San Andreas FaultWe constrain key properties of modulus‐strain hysteresis in the damage zone of the San Andreas Fault [ABSTRACT FROM AUTHOR]

Knowledge of attenuation structure is important for understanding subsurface material properties. We have developed a double-difference seismic attenuation (DDQ) tomography method for high-resolution imaging of 3-D attenuation structure. Our method includes two main elements, the inversion of event-pair differential |${t^*}$| (⁠|$d{t^*}$|⁠) data and 3-D attenuation tomography with the |$d{t^*}$| data. We developed a new spectral ratio method that jointly inverts spectral ratio data from pairs of events observed at a common set of stations to determine the |$d{t^*}$| data. The spectral ratio method cancels out instrument and site response terms, resulting in more accurate |$d{t^*}$| data compared to absolute |${t^*}$| from traditional methods using individual spectra. Synthetic tests show that the inversion of |$d{t^*}$| data using our spectral ratio method is robust to the choice of source model and a moderate degree of noise. We modified an existing velocity tomography code so that it can invert |$d{t^*}$| data for 3-D attenuation structure. We applied the new method to The Geyser geothermal field, California, which has vapour-dominated reservoirs and a long history of water injection. A new Qp model at The Geysers is determined using P -wave data of earthquakes in 2011, using our updated earthquake locations and Vp model. By taking advantage of more accurate |$d{t^*}$| data and the cancellation of model uncertainties along the common paths outside of the source region, the DDQ tomography method achieves higher resolution, especially in the earthquake source regions, compared to the standard tomography method using |${t^*}$| data. This is validated by both the real and synthetic data tests. Our Qp and Vp models show consistent variations in a normal temperature reservoir that can be explained by variations in fracturing, permeability and fluid saturation and/or steam pressure. A prominent low- Qp and Vp zone associated with very active seismicity is imaged within a high temperature reservoir at depths below 2 km. This anomalous zone is likely partially saturated with injected fluids. [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]

This paper describes the methods used to estimate positions, velocities, breaks, and seasonal terms from daily Global Navigation Satellite System (GNSS) measurements. Break detection and outlier removal have been automated so that decades of daily measurements from thousands of stations can be processed in a few hours. New measurements are added, and parameters are updated every week. Model parameters allow separation of interseismic, annual, coseismic, and postseismic signals. Tools available through GeoGateway (http://geo-gateway.org) allow rapid visualization and analysis of these terms for results that can be subsetted in time or space. Results show highly variable and nonlinear motion for GPS stations in southern California. The variable motion is related to seasonal motions, distributed tectonic motion, earthquakes, and postseismic motions that can continue for years. In some areas results suggest that additional processes are responsible for the observed motions. In general, following earthquakes, stations return to their long‐term motions after 2–3 years, though some exceptions occur. The use of the tools shows nonlinear motion in the Salton Trough of southern California related to the 2010 M7.2 El Mayor‐Cucapah earthquake, 2012 Brawley earthquake swarm, and a creep event on the Superstition Hills fault in 2017. Key Points: Positions, velocities, breaks, and seasonal terms for thousands of GNSS stations are updated every weekThese results can be used to study interseismic plate motion, coseismic deformation, postseismic deformation, and seasonal variationsThe use of these tools shows highly variable nonlinear motion of GPS stations in southern California [ABSTRACT FROM AUTHOR]

We present a new method for estimating time-series of relative seismic velocity changes (dv/v) within the Earth. Our approach is a Markov chain Monte Carlo (MCMC) technique that seeks to construct the full posterior probability distribution of the dv/v variations. Our method provides a robust, computationally efficient way to compute dv/v time-series that can incorporate information about measurement uncertainty, and any prior constraints that may be available. We demonstrate the method with a synthetic experiment, and then apply the MCMC algorithm to three data examples. In the first two examples we reproduce dv/v time-series associated with the response to the 2010 M 7.2 El Mayor-Cucapah earthquake at two sites in southern California, that have been studied in previous literature. In the San Jacinto fault zone environment we reproduce the dv/v signature of a deep creep slip sequence triggered by the El Mayor-Cucapah event, that is superimposed on a strong seasonal signal. At the Salton Sea Geothermal Field we corroborate the previously observed drop-and-recovery in seismic velocity caused by ground shaking related to the El Mayor-Cucapah event. In a third, new example we compute a month long velocity change time-series at hourly resolution at Piñon Flat, California. We observe a low amplitude variation in seismic velocity with a dominant frequency of 1 cycle per day, as well as a second transient signal with a frequency of 1.93 cycles per day. We attribute the 1-d periodicity in the dv/v variation to the combined effects of the diurnal tide and solar heating. The frequency of the signal at 1.93 cycles per day matches that of the lunar (semi-diurnal) tide. Analysis of the uncertainties in the Piñon Flat time-series shows that the error contains a signal with a frequency of 1 cycle per day. We attribute this variation to seismic noise produced by freight trains operating within the Coachella Valley. By demonstrating the applicability of the MCMC method in these examples, we show that it is well suited to tackle problems involving large data volumes that are typically associated with modern seismic experiments. [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]

In present work we aimed to analyze regularity of seismic process based on all its spatial, temporal and energetic characteristics. Increments of cumulative times, increments of cumulative distances and increments of cumulative seismic energies, have been calculated from southern California earthquake catalogue, 1975 to 2017. Used method of analysis represented combination of multivariate Mahalanobis distance calculation with the surrogate data testing. Prior to proceed to the analysis of dynamical features of seismic process we have tested used approach for two different 3 dimensional models in which dynamical features were changed from more regular to the more randomized conditions by adding some extent of noises. Analysis of variability in the extent of regularity of seismic process have been accomplished for different representative threshold values. According to results of our analysis about third part of considered 50 data windows, the original seismic process is indistinguishable from random process by its features of temporal, spatial and energetic variability. It was shown that prior to strong earthquake occurrences, in periods of relatively small earthquakes generation, percentage of windows in which seismic process is indistinguishable from random process essentially increases (to 60-80 %). At the same time, in periods of aftershock activity in all considered windows the process of small earthquake generation become regular and thus is strongly different from randomized catalogues. In some periods of catalogue time span seismic process looks closer to randomness while in other cases it becomes closer to regular behavior. Exactly, in periods of relatively decreased earthquake generation activity (at smaller energy release), seismic process looks random-like while in periods of occurrence of strong events, followed by series of aftershocks, it reveal significant deviation from randomness - the extent of regularity essentially increases. The period, for which such deviation from the random behavior can last, depends on the amount of seismic energy released by the strong earthquake. [ABSTRACT FROM AUTHOR]

We image the internal structure of the San Jacinto fault zone (SJFZ) in the trifurcation area southeast of Anza, California, with seismic records from dense linear and rectangular arrays. The examined data include recordings from more than 20 000 local earthquakes and nine teleseismic events. Automatic detection algorithms and visual inspection are used to identify P and S body waves, along with P- and S-types fault zone trapped waves (FZTW). The location at depth of the main branch of the SJFZ, the Clark fault, is identified from systematic waveform changes across lines of sensors within the dense rectangular array. Delay times of P arrivals from teleseismic and local events indicate damage asymmetry across the fault, with higher damage to the NE, producing a local reversal of the velocity contrast in the shallow crust with respect to the large-scale structure. A portion of the damage zone between the main fault and a second mapped surface trace to the NE generates P- and S-types FZTW. Inversions of high-quality S-type FZTW indicate that the most likely parameters of the trapping structure are width of ~70 m, S-wave velocity reduction of 60 per cent, Q value of 60 and depth of ~2 km. The local reversal of the shallow velocity contrast across the fault with respect to large-scale structure is consistent with preferred propagation of earthquake ruptures in the area to the NW. [ABSTRACT FROM AUTHOR]

Local and teleseismic earthquake waveforms recorded by a 180-m-long linear array (BB) with seven seismometers crossing the Clark fault of the San Jacinto fault zone northwest of Anza are used to image a deep bimaterial interface and core damage structure of the fault. Delay times of P waves across the array indicate an increase in slowness from the southwest most (BB01) to the northeast most (BB07) station. Automatic algorithms combined with visual inspection and additional analyses are used to identify local events generating fault zone head and trapped waves. The observed fault zone head waves imply that the Clark fault in the area is a sharp bimaterial interface, with lower seismic velocity on the southwest side. The moveout between the head and direct P arrivals for events within ~40 km epicentral distance indicates an average velocity contrast across the fault over that section and the top 20 km of 3.2 per cent. A constant moveout for events beyond ~40 km to the southeast is due to off-fault locations of these events or because the imaged deep bimaterial interface is discontinuous or ends at that distance. The lack of head waves from events beyond ~20 km to the northwest is associated with structural complexity near the Hemet stepover. Events located in a broad region generate fault zone trapped waves at stations BB04-BB07. Waveform inversions indicate that the most likely parameters of the trapping structure are width of ~200 m, S velocity reduction of 30-40 per cent with respect to the bounding blocks, Q value of 10-20 and depth of ~3.5 km. The trapping structure and zone with largest slowness are on the northeast side of the fault. The observed sense of velocity contrast and asymmetric damage across the fault suggest preferred rupture direction of earthquakes to the northwest. This inference is consistent with results of other geological and seismological studies. [ABSTRACT FROM AUTHOR]

Near-vertical faults can be imaged using reflected refractions identified in controlled-source seismic data. Often theses phases are observed on a few neighboring shot or receiver gathers, resulting in a low-fold data set. Imaging can be carried out with Kirchhoff prestack depth migration in which migration noise is suppressed by construc-tive stacking of large amounts of multifold data. Fresnel volume migration can be used for low-fold data without severe migration noise, as the smearing along isochrones is limited to the first Fresnel zone around the reflection point. We developed a modified Fresnel volume migration technique to enhance imaging of steep faults and to suppress noise and undesired coherent phases. The modifications include target-oriented filters to separate reflected refractions from steep-dipping faults and reflections with hyperbolic moveout. Undesired phases like multiple reflections, mode conversions, direct P and S waves, and surface waves are suppressed by these filters. As an alternative approach, we developed a new prestack line-drawing migration method, which can be considered as a proxy to an infinite frequency approximation of the Fresnel volume migration. The line-drawing migration is not considering waveform information but requires significantly shorter computational time. Target-oriented filters were extended by dip filters in the line-drawing migration method. The migration methods were tested with synthetic data and applied to real data from the Waltham Canyon fault, California. The two techniques are applied best in combination, to design filters and to generate complementary images of steep faults. [ABSTRACT FROM AUTHOR]

The Sacramento-San Joaquin Delta is an inland delta at the western extent of the Central Valley. Levees were built around swampy islands starting after the Civil War to reclaim these lands for farming. Various studies show that these levees could fail in concert from shaking from a major local or regional earthquake resulting in salty water from the San Francisco Bay contaminating the water in the Delta. We installed seismographs around the Delta and on levees to assess the contribution of site response to the seismic hazard of the levees. Cone penetrometer testing shows that the upper 10 s of meters of soil in the Delta have shear-wave velocities of about 200 m/s, which would give a strong site response. Seismographs were sited following two strat-egies: pairs of stations to compare the response of the levees to nearby sites, and a more regional deployment in the Delta. Site response was determined in two different ways: a traditional spectral ratio (TSR) approach of S waves using station BDM of the Berkeley Digital Seismic Net as a reference site, and using SH/SV ratios of noise (or Nakamura's method). Both estimates usually agree in spectral character for stations whose response is dominated by a resonant peak, but the most obvious peaks in the SH/SV ratios usually are about two-thirds as large as the main peaks in the TSRs. Levee sites typically have large narrow resonances in the site response function com-pared to sites in the farmland of the Delta. These resonances, at a frequency of about 1-3 Hz, have amplitudes of about 15 with TSR and 10-12 with Nakamura's method. Sites on farmland in the Delta also have amplifications, but these are typically broader and not as resonant in appearance. Late (slow) Rayleigh waves were recorded at sta-tions in the Delta, have a dominant period of about one second, and are highly mono-chromatic. Results from a three-station array at the Holland Marina suggest that they have a phase velocity of about 600 m/s and arrive at about the same azimuth as the straight-line back azimuth to the source. A dispersion curve determined for the basin or valley waves yields a shallow velocity profile that increases from about 350 m/s in the upper 0.2 km to about 1.1 km/s at a depth of about 2 km. [ABSTRACT FROM AUTHOR]

Abstract: An autonomous underwater vehicle (AUV) carrying a multibeam sonar and a chirp profiler was used to map sections of the seafloor within the La Jolla Canyon, offshore southern California, at sub-meter scales. Close-up observations and sampling were conducted during remotely operated vehicle (ROV) dives. Minisparker seismic-reflection profiles from a surface ship help to define the overall geometry of the La Jolla Canyon especially with respect to the pre-canyon host sediments. The floor of the axial channel is covered with unconsolidated sand similar to the sand on the shelf near the canyon head, lacks outcrops of the pre-canyon host strata, has an almost constant slope of 1.0° and is covered with trains of crescent shaped bedforms. The presence of modern plant material entombed within these sands confirms that the axial channel is presently active. The sand on the canyon floor liquefied during vibracore collection and flowed downslope, illustrating that the sediment filling the channel can easily fail even on this gentle slope. Data from the canyon walls help constrain the age of the canyon and extent of incision. Horizontal beds of moderately cohesive fine-grained sediments exposed on the steep canyon walls are consistently less than 1.232million years old. The lateral continuity of seismic reflectors in minisparker profiles indicate that pre-canyon host strata extend uninterrupted from outside the canyon underneath some terraces within the canyon. Evidence of abandoned channels and point bar-like deposits are noticeably absent on the inside bend of channel meanders and in the subsurface of the terraces. While vibracores from the surface of terraces contain thin (<10cm) turbidites, they are inferred to be part of a veneer of recent sediment covering pre-canyon host sediments that underpin the terraces. The combined use of state of the art seafloor mapping and exploration tools provides a uniquely detailed view of the morphology within an active submarine canyon. [Copyright &y& Elsevier]

Abstract: Pulverized rocks have been found in the damage zone around the San Andreas Fault, at distances greater than 100 m from the fault core. This damage is atypical in that it is pervasive and strain is not localized along main fractures as expected at these distances from the fault core. With high strain rate experiments, the authors have previously shown that above a strain rate threshold, the localization of strain along a few fractures is inhibited. Pulverized rocks may be generated by seismic waves at high frequency. Here we generalize these conclusions by discussing the effect of the initial fracture network in the sample on the transition from strain localization along a few fractures to diffuse damage throughout the sample. Experimental data are compared with statistical theory for fracture propagation. This analysis shows that the threshold in strain rate is a power law of initial fracture density and that a pre-damaged rock is easier to pulverize. This implies that pulverized rocks observed on the field may result from successive loadings. [Copyright &y& Elsevier]

SUMMARY We performed forward modelling of seismic anisotropy beneath a migrating strike-slip plate boundary to: (1) test if such a geodynamic context might explain teleseismic shear wave splitting data in the vicinity of the central part of the San Andreas Fault and (2) constrain the power of such data to unravel vertical and lateral variations in deformation patterns in the upper mantle. The modelling involves five steps: (1) thermomechanical modelling, using a finite-element code, of the deformation field, (2) viscoplastic self-consistent modelling of the resulting olivine and pyroxene crystal preferred orientations, (3) calculation of the elasticity tensors for different domains of the finite elements (FEs) model, (4) forward modelling of seismic wave propagation through the model using ray theory, finite-frequency theory and a full-wave approach and (5) performing splitting measurements on the synthetic seismograms. SKS splitting data in central California are best fitted by a model with a hotter geotherm within 60 km of the plate boundary accounting for the opening of an asthenospheric window due to the northward migration of the Mendocino Triple Junction. The westward motion of the plate boundary cannot however explain the rotation of fast polarization directions east of the San Andreas Fault in central California. Comparison between modelled and measured individual shear wave splitting also implies that the homogeneity of the two-layer models accounting for the observations in the vicinity of the San Andreas Fault indicates a sharp transition between lithospheric and asthenospheric deformations beneath this plate boundary. The ability of different synthetic approaches to localize horizontally and vertically the plate boundary-related deformation differs significantly. Splitting in ray theory synthetics closely follow variations in olivine crystallographic preferred orientations in the model. In contrast, splitting analysis on full-wave synthetics, which should be more representative of actual long-period SKS waves, results in smooth apparent lateral variations of anisotropy; the exact location and width of the plate boundary may only be retrieved by comparing fast-polarization profiles obtained using a multichannel analysis on waves with different periods. [ABSTRACT FROM AUTHOR]

SUMMARY This study examines the effect of the angle of seismic incidence θ on the fragility curves of bridges. Although currently, fragility curves of bridges are usually expressed only as a function of intensity measure of ground motion ( IM) such as peak ground acceleration, peak ground velocity, or Sa( ω1), in this study they are expressed as a function of IM with θ as a parameter. Lognormal distribution function is used for this purpose with fragility parameters, median cm and standard deviation ζ to be estimated for each value of θ chosen from 0 < θ < 360°. A nonlinear 3D finite element dynamic analysis is performed, and key response values are calculated as demand on the bridge under a set of acceleration time histories with different IM values representing the seismic hazard in Los Angeles area. This method is applied to typical straight reinforced concrete bridges located in California. The results are validated with existing empirical damage data from the 1994 Northridge earthquake. Even though the sample bridges are regular and symmetric with respect to the longitudinal axis, the results indicate that the weakest direction is neither longitudinal nor transverse. Therefore, if the angle of seismic incidence is not considered, the damageability of a bridge can be underestimated depending on the incidence angle of seismic wave. Because a regional highway transportation network is composed of hundreds or even thousands of bridges, its vulnerability can also be underestimated. Hence, it is prudent to use fragility curves taking the incident angle of seismic waves into consideration as developed here when the seismic performance of a highway network is to be analyzed. Copyright © 2012 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Analog station LUC near Lucerne, California, recorded strong motions from the nearby (1.25 km) fault rupture during the 28 June 1992 Landers mainshock. The records illustrate general issues that can arise at near-field stations. In the area of the station, weathered granite regolith with 400 m/s S-wave velocity overlies intact granite with 3000 m/s S-wave velocity. A strongly reverberating signal persisted for several seconds after near-field velocity pulse passed. Recovery of the vertical record in digital form provided calibration of resonant site properties and qualitative separation of site and source effects. The dominant signal on the vertical spectrum arises from vertically reverberating P waves in an ∼13.5-m-thick layer. Horizontal spectra marginally resolve the analogous resonance for vertical S waves and coupling of vertical P waves into horizontal motion. Here, resonant amplification of a broad high-frequency band ∼5-40 Hz by a factor of a few over lower frequencies sufficed to dominate acceleration records and to make velocity records jittery. Conversely, the amplitude damping times of these resonances are much less than 1 s, indicating that the time-domain decay of the acceleration signal over ∼8 s is an incident-wave effect. Overall, the raw seismograms represent incident incoming high-frequency 5-40 Hz signals, but poorly resolved directional effects preclude straightforward determination of the incident body waves. [ABSTRACT FROM AUTHOR]

Abstract: This study examines one-dimensional wave propagation in a multi-story building with seismic excitation. In particular, the building is modeled as a series of shear beams for columns/walls and lumped masses for floors. Wave response at one location of the building is then derived to an impulsive motion such as displacement and acceleration at another location in time and frequency domains, termed here as wave-based or generalized impulse and frequency response function (GIRF and GFRF), which is dependent upon the building characteristics above the impulse location. Not only does this study illustrate features of GIRF and GFRF in terms of building properties, it also shows broad-based applications of the modeling. Two examples are presented with the use of the modeling. One is wave-based characterization of ten-story Millikan Library in Pasadena, California with the recordings of Yorba Linda earthquake of September 3, 2002. The other is analysis for influence of stochastic floor-to-column mass ratio, story-height and seismic input in seismic wave responses. [Copyright &y& Elsevier]

SUMMARY Direct detection of Rayleigh-wave azimuthal anisotropy is reported by applying an array analysis to broad-band seismic data in Southern California, USA. Our approach has excellent resolution for frequencies between 30 and 60 mHz and good resolution between 10 and 30 mHz. Limitation from array size limits accuracy below 10 mHz and complicated wave propagation effects lead to difficulty above 60 mHz. Between 30 and 60 mHz, azimuthal anisotropy of Rayleigh-wave phase velocity is detected cleanly. Phase velocity results show that the 2θ components dominate the 4θ components. Strength of anisotropy hovers around 1 per cent for the 2θ components with standard error ( 1 σ) of about 0.4 per cent. Fast axes orientations show stable orientation in the direction 110° clockwise from north and its opposite direction 290°. Depth sensitivity kernels for P-wave and S-wave anisotropy indicate that anisotropy is confined to depths in the upper 100 km and may even be to shallower depths. We speculate that the fast axes alignment may be associated with a strong shearing process over the thickness of lithosphere due to obliquely collisional motions between the Pacific Plate and the North America Plate in the region. [ABSTRACT FROM AUTHOR]

Long duration noisy-looking waveforms such as those obtained in randomly multiple scattering and reverberant media are complex; they resist direct interpretation. Nevertheless, such waveforms are sensitive to small changes in the source of the waves or in the medium in which they propagate. Monitoring such waveforms, whether obtained directly or obtained indirectly by noise correlation, is emerging as a technique for detecting changes in media. Interpretation of changes is in principle problematic; it is not always clear whether a change is due to sources or to the medium. Of particular interest is the detection of small changes in propagation speeds. An expression is derived here for the apparent, but illusory, waveform dilation due to a change of source. The expression permits changes in waveforms due to changes in wave speed to be distinguished with high precision from changes due to other reasons. The theory is successfully compared with analysis of a laboratory ultrasonic data set and a seismic data set from Parkfield California. [ABSTRACT FROM AUTHOR]

We examine the relations between spatial symmetry properties of earthquake patterns along faults in California (CA) and local velocity structure images to test the hypothesis that ruptures on bimaterial faults have statistically preferred propagation directions. The analysis employs seismic catalogues for 25 fault zones in CA. We distinguish between clustered and homogeneous parts of each catalogue, using a recently introduced earthquake cluster analysis, and examine asymmetry of offspring with respect to parent events within the clustered portion of each catalogue. The results indicate strong asymmetric patterns along large faults with prominent bimaterial interfaces (e.g. sections of the San Andreas Fault), with enhanced activities in the directions predicted for the local velocity contrasts, and absence of significant asymmetry along most other faults. Assuming the observed asymmetric properties of seismicity reflect the properties of the parent earthquake ruptures, the discussed methodology and results can be used to develop refined estimates of seismic shaking hazard associated with individual fault zones. [ABSTRACT FROM AUTHOR]

We used borehole seismic records of four repeating-earthquake clusters and two explosions to investigate the coseismic changes of the scattering wave field of the 28 September 2004 M 6 Parkfield earthquake. We found systematic changes in P-wave and S-wave coda recorded from repeating events that occurred before and immediately after the earthquake. Applying the coda wave interferometry technique allowed us to determine that the observed changes are caused by a localized change in the scattering field. We further developed a technique based on decorrelation indexes calculated from running time windows to locate migrating scatterers. Synthetic tests showed that the technique is relatively insensitive to changes in background velocity of the medium and source location and thus can be applied to records of loosely colocated clusters. We found a localized change of material property within the fault zone at ~3 km depth beneath the Middle Mountain area. The change is shown most clearly in the P-to-S scattering mode of the active source data and both the P-to-S and S-to-S scattering modes of the repeating earthquakes data, suggesting that the observed scattering property change is a result of charge or discharge of fluids in fractures caused by the 2004 Parkfield earthquake. The same scatterer(s) was found to respond to the 1993 aseismic stress/strain transient event, acting like an in situ stress-strain meter at seismogenic depth. Four-dimensional time-lapse imaging of the scattering wave field thus provides an effective way to monitor the subsurface stress-strain field. [ABSTRACT FROM AUTHOR]

We developed a method to determine the depth extent of low-velocity zone (LVZ) associated with a fault zone (FZ) using S-wave precursors from local earthquakes. The precursors are diffracted S waves around the edges of LVZ and their relative amplitudes to the direct S waves are sensitive to the LVZ depth. We applied the method to data recorded by three temporary arrays across three branches of the San Jacinto FZ. The FZ dip was constrained by differential traveltimes of P waves between stations at two side of the FZ. Other FZ parameters (width and velocity contrast) were determined by modelling waveforms of direct and FZ-reflected P and S waves. We found that the LVZ of the Buck Ridge fault branch has a width of ∼150 m with a 30–40 per cent reduction in and a 50–60 per cent reduction in . The fault dips 70 ± 5° to southwest and its LVZ extends only to 2 ± 1 km in depth. The LVZ of the Clark Valley fault branch has a width of ∼200 m with 40 per cent reduction in and 50 per cent reduction in . The Coyote Creek branch is nearly vertical and has a LVZ of ∼150 m in width and of 25 per cent reduction in and 50 per cent reduction in . The LVZs of these three branches are not centred at the surface fault trace but are located to their northeast, indicating asymmetric damage during earthquakes. [ABSTRACT FROM AUTHOR]

Source processes commonly posed to explain instances of remote dynamic triggering of tectonic (nonvolcanic) tremor by surface waves include frictional failure and various modes of fluid activation. The relative potential for Love- and Rayleigh-wave dynamic stresses to trigger tectonic tremor through failure on critically stressed thrust and vertical strike-slip faults under the Coulomb--Griffith failure criteria as a function of incidence angle is anti-correlated over the 15- to 30-km-depth range that hosts tectonic tremor. Love-wave potential is high for strike-parallel incidence on low-angle reverse faults and null for strike-normal incidence; the opposite holds for Rayleigh waves. Love-wave potential is high for both strike-parallel and strike-normal incidence on vertical, strike-slip faults and minimal for ~45° incidence angles. The opposite holds for Rayleigh waves. This pattern is consistent with documented instances of tremor triggered by Love waves incident on the Cascadia megathrust and the San Andreas fault (SAF) in central California resulting from shear failure on weak faults (apparent friction, μ* ≤ 0.2). However, documented instances of tremor triggered by surface waves with strike-parallel incidence along the Nankai megathrust beneath Shikoku, Japan, is associated primarily with Rayleigh waves. This is consistent with the tremor bursts resulting from mixed-mode failure (crack opening and shear failure) facilitated by near-lithostatic ambient pore pressure, low differential stress, with a moderate friction coefficient (μ ~ 0.6) on the Nankai subduction interface. Rayleigh-wave dilatational stress is relatively weak at tectonic tremor source depths and seems unlikely to contribute significantly to the triggering process, except perhaps for an indirect role on the SAF in sustaining tremor into the Rayleigh-wave coda that was initially triggered by Love waves. [ABSTRACT FROM AUTHOR]

We investigate temporal variations in teleseismic receiver functions using 11 yr of data at station PKD near Parkfield, California, by stacking power spectral density (PSD) functions within 12-month windows. We find that PSD levels for both radial and transverse components drop by ~5 dB following the 2003 San Simeon (M 6.5) earthquake, with a persistent reduction in background levels of ~2 dB, relative to the pre-2003 levels, after the 2004 Parkfield (M 6) earthquake, corresponding to an estimated decrease in shear-wave velocity of ~0.12 and ~0.06 km/sec, respectively, or equivalent negative changes in Poisson's ratio of ~0.02 and ~0.01. Our results suggest that the perturbation originates at middle to lower crustal levels, possibly caused by the redistribution of crustal pore fluids, consistent with increased and sustained tremor activity near Parkfield following both earthquakes. This study shows that we can resolve temporal variations in crustal scattering structure near a major seismogenic fault using the receiver function method. [ABSTRACT FROM AUTHOR]

The five-year experiment of the Regional Earthquake Likelihood Models (RELM) working group was designed to compare several prospective forecasts of earthquake rates in latitude-longitude-magnitude bins in and around California. This forecast format is being used as a blueprint for many other earthquake predictability experiments around the world, and therefore it is important to consider how to evaluate the performance of such forecasts. Two tests that are currently used are based on the likelihood of the observed distribution of earthquakes given a forecast; one test compares the binned space-rate-magnitude observation and forecast, and the other compares only the rate forecast and the number of observed earthquakes. In this article, we discuss a subtle flaw in the current test of rate forecasts, and we propose two new tests that isolate the spatial and magnitude component, respectively, of a space-rate-magnitude forecast. For illustration, we consider the RELM forecasts and the distribution of earthquakes observed during the first half of the ongoing RELM experiment. We show that a space-rate-magnitude forecast may appear to be consistent with the distribution of observed earthquakes despite the spatial forecast being inconsistent with the spatial distribution of observed earthquakes, and we suggest that these new tests should be used to provide increased detail in earthquake forecast evaluation. We also discuss the statistical power of each of the likelihood-based tests and the stability (with respect to earthquake catalog uncertainties) of results from the likelihood-based tests. [ABSTRACT FROM AUTHOR]

Receiver functions (RFs) from station PKD located ∼3 km SW of the San Andreas Fault (SAF) samples the Salinian terrane near Parkfield. Crustal multiples indicate a 26-km-thick crust with a VP/ VS of 1.88, which is slightly lower (1.83) for the upper and middle crust in the west. For the mid-crust, arrivals are observed at times corresponding to recently imaged seismic reflectors and may correspond to a layer of metasedimentary rocks below the base of the granitic batholith exposed at the surface. For the lower crust, RFs display strong polarity reversals with backazimuth and a change in Moho amplitude that require strong seismic anisotropy (>15 per cent) in a low velocity, high VP/ VS, possibly serpentinite or fluid filled schist layer that has a ENE dipping (∼35°) rock fabric. Similar patterns of amplitude variations and polarity reversal observed in RFs for some southern California stations located west of the SAF support the hypothesis that the cause of these data characteristics is a regionally prevalent lower crustal anisotropy. The orientation of this anisotropic fabric is inconsistent with the recent San Andreas sense of shear and is most likely a fossilized fabric of past eastward-directed (Farallon Plate) subduction. [ABSTRACT FROM AUTHOR]

Abstract: Multi-scale frequency-domain acoustic waveform inversion was applied to refraction and wide-angle reflection seismic surveys of the upper crust across the plate-bounding San Andreas Fault, California, and the Chesapeake Bay impact structure, Virginia. This paper presents and compares the data, the preconditioning, and different inversion strategies. The inversions start at 3 Hz, and they are stopped at 15 Hz due to increasing non-linear artifacts. The resulting waveform models show a significant improvement in resolution compared to the traveltime starting models, but they do not achieve the resolution demonstrated in synthetic studies. Possible reasons for this limitation are discussed, especially the possibility of insufficient bandwidth, and the effect of normalizing amplitudes. The comparison of results from normalized-amplitude and true-amplitude waveform inversions indicates that amplitudes do not strongly constrain the velocity structure. Phase coherency weights are presented that mitigate the impact of noise. Maps of phase misfits provide a quantitative overview of the data residuals and some insight into the spatial distribution of errors. They are helpful for monitoring the progress of the inversion, and for determining if and when it must be stopped. [Copyright &y& Elsevier]

The consideration of spatially correlated seismic hazard could be of importance for seismic risk assessment. The estimation of this correlation for the peak ground acceleration and the pseudospectral acceleration has been reported in the literature, although it is not presented as a necessary ingredient for developing ground-motion prediction equations (GMFEs). In the present study, we show that spatial correlation can be incorporated in the existing regression algorithms given by Joyner and Boore for assessing a GMPE. The modified algorithms can be used to estimate both GMPE coefficients and spatial correlation model parameters simultaneously. In particular, they are used to investigate the influence of spatial correlation on GMPEs and to assess parameters of an empirical spatial correlation model by considering a set of 592 California records. Analysis results indicate that the effects of incorporating spatial correlation on the estimated GMPEs are insignificant, and that spatial correlation parameters obtained using the modified algorithm are similar to those estimated based on statistical analysis of regression residuals. [ABSTRACT FROM AUTHOR]

The short-term earthquake probability (STEP) forecasting model applies the Omori-Utsu aftershock-decay relation and the Gutenberg-Richter frequency-magnitude relation to clusters of earthquakes. It is mainly intended to forecast aftershock activity and depends on a time-invariant background model to forecast most of the major earthquakes. On the other hand, the long-range earthquake forecasting model EEPAS (every earthquake a precursor according to scale) exploits the precursory scale increase phenomenon and associated predictive scaling relations to forecast the major earthquakes months, years, or decades in advance, depending on magnitude. Both models are shown to be more informative than time-invariant models of seismicity. By forming a mixture of the two, we aim to create an even more informative short-term forecasting model. Using the Advanced National Seismic System catalog of California over the period 1984-2004, the optimal mixture model for forecasting earthquakes with M ≥5.0 is a convex linear combination consisting of 0.42 of the EEPAS forecast and 0.58 of the STEP forecast. This mixture gives an average probability gain of more than 2 compared to each of the individual models. Several different mixture models will be submitted to the CSEP Testing Center at the Southern California Earthquake Center to ascertain whether or not this result is borne out by real-time tests of the models against future earthquakes. [ABSTRACT FROM AUTHOR]