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USGS2019

Length-scale-dependent relationships between VS30 and topographic slopes in southern California

In engineering seismology, the time‐averaged shear‐wave velocity ( ⁠ V S ⁠ ) of the upper 30 m of the crust ( ⁠V S30⁠ ) is the primary parameter used in ground‐motion models to predict seismic site effects. ⁠V S30⁠ is typically derived from in situ recordings of V S ⁠ , although proxy‐based approaches (using geologic and/or geomorphometric classifications) are provisionally adopted when measurement‐based ⁠V S30⁠ are sparse or not readily available. Despite the acceptance of proxy approaches, there are limited studies that examine the empirical relationships between ⁠V S30⁠ and topographic attributes measured from various length scales and different resolutions of the digital elevation model. In this study, we examine the relationships between compiled ⁠V S30⁠ measurements from 218 sites in southern California and topographic metrics of slope and relief measured over various length scales. We find that the correlations between topographic metrics and ⁠V S30⁠ are weak but statistically significant. The correlations are improved when topographic slopes and relief are measured over length scales longer than typical hillslopes and ⁠V S30⁠ sites are separated by different geologic groups. This is likely because ⁠V S30⁠ ⁠ , especially on the rock sites, is better reflected in topographic metrics that capture large‐scale topographic relief, as well as landscape positions such as hilltops and valley bottoms.

California↗
USGS2019

Earthquake arrival association with backprojection and graph theory

The association of seismic‐wave arrivals with causative earthquakes becomes progressively more challenging as arrival detection methods become more sensitive, and particularly when earthquake rates are high. For instance, seismic waves arriving across a monitoring network from several sources may overlap in time, false arrivals may be detected, and some arrivals may be of unknown phase (e.g., P or S waves). We propose an automated method to associate arrivals with earthquake sources and obtain source locations applicable to such situations. To do so, we use a pattern detection metric based on the principle of backprojection to reveal candidate sources followed by graph‐theory‐based clustering and an integer linear optimization routine to associate arrivals with the minimum number of sources necessary to explain the data. This method solves for all sources and phase assignments simultaneously, rather than in a sequential greedy procedure as is common in other association routines. We demonstrate our method on both synthetic and real data from the Integrated Plate Boundary Observatory Chile seismic network of northern Chile. For the synthetic tests, we report results for cases with varying complexity, including rates of 500 earthquakes/day and 500 false arrivals/station/day, for which we measure true positive detection accuracy of >95% . For the real data, we develop a new catalog between 1 January 2010 and 31 December 2017 containing 817,548 earthquakes, with detection rates on average 279 earthquakes/day and a magnitude‐of‐completion of M∼1.8 ⁠ . A subset of detections are identified as sources related to quarry and industrial site activity, and we also detect thousands of foreshocks and aftershocks of the 1 April 2014 M w 8.2 Iquique earthquake. During the highest rate of aftershock activity, >600 earthquakes/day are detected in the vicinity of the Iquique earthquake rupture zone.

Bulletin of the Seismological Society of America↗
USGS2019

Comment on “Interpretation of Kappa and fmax filters as source effect”, by Igor A. Beresnev

Beresnev (2019) advocates the use of an earthquake slip function that produces an ω-2.5 high-frequency falloff of Fourier displacement spectra in the far field, where ω denotes the angular frequency. He argues that the observed high-frequency decay of earthquake spectra can be adequately modeled by this ω-2.5 falloff, without needing to include high frequency attenuation at shallow depth under the site, commonly characterized as fmax or kappa. In his abstract, Beresnev (2019) describes source models with falloffs intermediate between ω-2 and ω-3 as “providing natural high-cut filtering exclusively as a source effect.” In many studies to date, observed spectra are modeled using an ω-2 source spectrum combined with attenuation along the propagation path, including strong attenuation at shallow depths (< 1 km) beneath a site. It is not clear whether Beresnev (2019) is claiming that path effects (including site attenuation) are unimportant to ground motions or if he is proposing a simple, pragmatic method to fit the high-frequency decay of earthquake spectra.

Bulletin of the Seismological Society of America↗
USGS2019

Relaxing segmentation on the Wasatch Fault Zone: Impact on seismic hazard

The multisegment Wasatch fault zone is a well-studied normal fault in the western United States that has paleoseismic evidence of recurrent Holocene surface-faulting earthquakes. Along the 270-km-long central part of the fault, four primary structural complexities provide possible along-strike limits to these ruptures and form the basis for models of fault segmentation. Here, we assess the impact that the Wasatch fault segmentation model has on seismic hazard by evaluating the time-independent long-term rate of ruptures on the fault that satisfy fault slip rates and paleoseismic event rates, adapting standard inverse theory used in the Uniform California Earthquake Rupture Forecast 3 (UCERF3), and implementing a segmentation constraint where ruptures across primary structural complexities are penalized. We define three models with varying degrees of rupture penalization: (1) segmented (ruptures confined to individual segments), (2) penalized (multi-segment ruptures allowed, but penalized), and (3) unsegmented (all ruptures allowed). Seismic-hazard results show that on average, hazard is highest for the segmented model, where seismic moment is accommodated by frequent moderate (moment magnitude, M w 6.2–6.8) earthquakes. The unsegmented model yields the lowest average seismic hazard because part of the seismic moment is accommodated by large (M w 6.9–7.9), but infrequent ruptures. We compare these results to model differences derived from other inputs such as slip rate and magnitude scaling relationships and conclude that segmentation exerts a primary control on seismic hazard. This study demonstrates the need for additional geologic constraints on rupture extent and methods by which these observations can be included in hazard-modeling efforts.

Utah↗
USGS2019

On the portability of ML-MC as a depth discriminant for small seismic events recorded at local distances

In this paper we show that M L -M C is a viable and regionally portable depth discriminant and therefore may contribute in nuclear test ban treaty verification. A recent study found that the difference between local magnitude (M L ) and coda duration magnitude (M C ) discriminates shallow seismic events (mining blasts, mining-induced earthquakes, and shallow tectonic earthquakes) from deeper tectonic earthquakes in the Utah region. The shallow seismic events had anomalously high M C values, with increasingly negative M L -M C values as depth decreased. Here we evaluate the performance of M L -M C as a depth discriminant in three new regions, finding that M L -M C increases between 0–9 km depth in all cases. Initially, we investigated M L -M C as a function of depth for naturally occurring earthquakes in the region around Yellowstone National Park, as recorded by the University of Utah Seismograph Stations. For 3,358 Yellowstone earthquakes with well-constrained depths, we found M L -M C increased 0.030 ± 0.007 magnitude units (m.u.) for each 1 km increase in depth up to 10 km depth. Next, we examined M L -M C values for anthropogenic seismicity in northern Oklahoma and southern Kansas, as recorded by the National Earthquake Information Center. For 1,628 events with well-constrained depths, we computed a slope for M L -M C of 0.022 ± 0.010 m.u./km. Finally, we analyzed M L -M C for 28,722 well-located earthquakes in Italy, as recorded by the National Institute of Geophysics and Volcanology, and found an M L -M C slope of 0.018 ± 0.001 m.u./km. In each case, the quoted error bounds represent 95% confidence regions which exclude zero, implying that the depth-dependence of M L -M C is statistically significant. We performed several robustness tests in which we varied the criterion used to define a well-constrained depth and the depth range used in the linear fit. In nearly all cases, we found a positive slope for M L -M C vs. depth at a confidence level above 95%.

Utah, Montana, Idaho, Wyoming, Kansas, Oklahoma↗
USGS2020

Estimating rupture dimensions of three major earthquakes in Sichuan, China, for early warning and rapid loss estimates

Large earthquakes like in Wenchuan in 2008, MW 7.9, Sichuan, China, provide opportunity for earthquake early warning (EEW) as many heavily shaken areas are far (~50 km) from the epicenter and warning time could be long enough (≥ 5 s) to take effective preventative action. On the other hand, earthquakes with magnitudes larger than ~M 6.5 are challenging for EEW since source dimensions need to be defined in order to adequately estimate shaking. The Finite-Fault Rupture Detector (FinDer) is an approach to identify fault rupture extents from real-time strong motion and/or broadband records. In this study, we playback local and regional on-scale strong motion waveforms recorded during the 2008 MW 7.9 Wenchuan, 2013 MW 6.6 Lushan, and 2017 MW 6.5 Jiuzhaigou earthquakes to study the performance of FinDer for the current layout of the China Strong Motion Network. Overall, the FinDer line-source models agree well with the observed spatial distribution of aftershocks and fault models determined from waveform inversion. However, since FinDer models are constructed to characterize seismic ground motions (as needed for EEW) instead of source parameters, the rupture length can be overestimated for events radiating high levels of high-frequency motions, as is the case in the Lushan earthquake. If the set of strong motion data used had been available in real-time, 50% to 80% of sites experiencing shaking of intensity MMI IV-VII (light to very strong) and 30% experiencing VIII-IX (severe to violent) could have been issued a warning with 10 s and 5 s, respectively, before the arrival of the destructive S-wave. We also show that loss estimates after devastating earthquakes based on the FinDer line-source are more accurate compared to a point-source model. For the Wenchuan earthquake, for example, they predict a four to six times larger number of fatalities and injured, which is consistent with official reports. At the same time, these losses could be provided 1/2~3 hours faster than if based on more complex inversion rupture models.

Sichuan↗
USGS2019

Establishing high-frequency noise baselines to 100 Hz based on millions of power spectra from IRIS MUSTANG

Advances in seismic instrumentation have enabled data to be recorded at increasing sample rates. This has in turn created a need to establish higher-frequency baselines for assessing data quality, as the widely-used New High (NHNM) and Low Noise Models (NLNM) of Peterson (1993) do not extend to frequencies above 10 Hz. To provide a baseline for higher frequencies (10-100 Hz), we examine power spectral density probability density functions (PSDPDFs) for high-sample-rate stations available from the Incorporated Research Institutions for Seismology Data Services (IRIS DS) MUSTANG quality control system. We compute high-frequency high and low noise baselines by matching the appropriate composite PSDPDF percentile points to NHNM and NLNM power levels at overlapping frequencies (1-10 Hz) and then extending to higher frequencies (10-100 Hz) with piecewise linear fits to the matching PSDPDF percentile. We find that the Peterson NLNM remains an accurate representation of the lower bound of global ambient Earth noise since it is matched by only 0.1% of Global Seismographic Network (GSN) PSDs. We present high-frequency high and low noise baselines intended primarily for use by temporary networks targeting high-frequency signals (e.g. monitoring of aftershocks or induced seismicity) based on statistics of PSDPDFs from all publicly available high-sample-rate data. Most publicly-available high-sample-rate data is recorded by temporary deployments, and the experiment design and scientific targets of these deployments strongly influence the observed statistical distribution of high-frequency noise. We anticipate that the noise baselines presented here will be useful in automated quality control of high-sample-rate seismic data. However, we note that establishing a low noise model that accurately represents the lowest possible ambient Earth noise at frequencies up to 100 Hz will require additional continuous high-sample-rate data from high-quality permanent stations in low-noise environments.

Bulletin of the Seismological Society of America↗
USGS2019

Holocene rupture history of the central Teton fault at Leigh Lake; Grand Teton National Park, Wyoming

Prominent scarps on Pinedale glacial surfaces along the eastern base of the Teton Range confirm latest Pleistocene to Holocene surface‐faulting earthquakes on the Teton fault, but the timing of these events is only broadly constrained by a single previous paleoseismic study. We excavated two trenches at the Leigh Lake site near the center of the Teton fault to address open questions about earthquake timing and rupture length. Structural and stratigraphic evidence indicates two surface‐faulting earthquakes at the site that postdate deglacial sediments dated by radiocarbon and optically stimulated luminescence to ∼10–11 ka⁠. Earthquake LL2 occurred at ∼10.0 ka (9.7–10.4 ka; 95% confidence range) and LL1 at ∼5.9 ka (4.8–7.1 ka; 95%). LL2 predates an earthquake at ∼8ka identified in the previous paleoseismic investigation at Granite Canyon. LL1 corresponds to the most recent Granite Canyon earthquake at ∼4.7–7.9 ka (95% confidence range). Our results are consistent with the previously documented long‐elapsed time since the most recent Teton fault rupture and expand the fault’s earthquake history into the early Holocene.

Wyoming↗
USGS2019

Directivity of M 3.1 earthquake near Anza, California and the effect on peak ground motion

We show the effect of rupture directivity on peak ground‐motion values for a moderate magnitude event at Anza, California, and neighboring stations at the Imperial Valley. The event was located near Borrego Springs on the west side of the Salton Sea and was well recorded at broadband stations near Anza, California, and at stations on the west side of the Imperial Valley. After correcting for regional attenuation, an anomalously large residual in peak motion was observed at station ERR just to the southeast of the epicenter. Using the algorithm from Boatwright (2007) , peak motions from the regional seismic networks in southern California were inverted to determine directivity, which was to the southeast along the trend of the San Jacinto fault toward station ERR. This algorithm uses peak values compiled for the ShakeMap system mostly at regional distances. It does not capture the main features of the source time function (STF) predicted by directivity. Consequently, we determined the second‐degree moments for this earthquake, which confirmed that station ERR has a shorter and higher STF compared to stations to the northwest suggesting rupture propagated to the southeast. The azimuthal distribution of local stations is sparse, but nevertheless the largest amplitudes (such as at station ERR) correlate well with the maximum in the radiation pattern and smaller values with the minima, which is the radiation pattern for SH plus the effect of directivity. Using the data from the analysis of the second‐degree moments, the characteristic length of the fault is 0.58 km, assuming an idealized unilateral extended rupture with a rupture time of 0.09 s. This yields an apparent rupture velocity of 6.4 &#x2009;&#x2009; km / s "> 6.4 km / s for an idealized model, which is super shear. This value is model dependent and would change if, for example, the rupture was bilateral. Although this value is even greater than the P ‐wave velocity, it supports the idea that the rupture velocity is super shear and would enhance the correlation between the peak motions and the radiation pattern.

California↗
USGS2020

Earthquake magnitude and Lg Q variations between the Grenville and northern Appalachian geologic provinces of eastern Canada

This article assesses the ability of regionally specific, frequency‐dependent crustal attenuation ( ⁠ 1 / Q "> 1 / Q ⁠ ) to reduce mean magnitude discrepancies between seismic stations in the northern Appalachian and Grenville provinces (NAP and GP) of Canada. Lg Q ( f ) "> Q ( f ) is an important parameter in ground‐motion models used in probabilistic seismic hazard analysis. Discrepancies in regional magnitude estimates have long been noted to exist between stations in the two provinces for common event origins. Such discrepancies could arise from systematic site condition variations between the geologic provinces or from varying crustal attenuative properties. To evaluate the effect of frequency‐dependent anelastic attenuation, Lg Q ( f ) "> Q ( f ) on estimated magnitudes, we analyze Lg amplitudes from &gt; 6000 "> > 6000 waveforms recorded by Grenville and northern Appalachian receivers from 420 natural earthquakes of M N "> M N magnitude 3–5.6. Waveform analysis is strictly limited to analyst‐reviewed, vertical‐component waveforms in which Lg is clearly identified, ensuring that the datasets exhibit dominant, high‐frequency energy in the Lg velocity window. Lg Q ( f ) "> Q ( f ) is found to be higher in the GP than in the northern Appalachians. In the Grenville, Q ( f ) = 761 ( &#xB1; 145 ) f 0.25 ( &#xB1; 0.014 ) "> Q ( f ) = 761 ( ± 145 ) f 0.25 ( ± 0.014 ) ⁠ , and in the northern Appalachians, attenuation is higher: Q ( f ) = 506 ( &#xB1; 172 ) f 0.33 ( &#xB1; 0.310 ) "> Q ( f ) = 506 ( ± 172 ) f 0.33 ( ± 0.310 ) . Earthquake magnitude determined using the peak amplitude of the Lg phase ( ⁠ m b L g "> m b L g ) for eastern Canada is corrected to incorporate the frequency‐dependent, regionally specific Lg Q ( f ) "> Q ( f ) determined in this study. Using the new Lg Q ( f ) "> Q ( f ) values diminishes and nearly resolves magnitude discrepancies between the provinces. Correcting regional magnitude discrepancies between provinces is critical for reliable regional seismic hazard estimates because magnitude error in a particular region could lead to increased uncertainty in seismic hazard models.

Grenville Geologic Province, Northern Appalachian ↗
USGS2020

Basin amplification effects in the Puget Lowland, Washington from strong motion recordings and 3D simulations

Sedimentary basins in the Puget Sound region, Washington State, increase ground‐motion intensity and duration of shaking during local earthquakes. We analyze Pacific Northwest Seismic Network and U.S. Geological Survey strong‐motion recordings of five local earthquakes ( M 3.9–6.8), including the 2001 Nisqually earthquake, to characterize sedimentary basin effects within the Seattle and Tacoma basins. We observe basin‐edge generated surface waves at sites within the Seattle basin for most ray paths that cross the Seattle fault zone. We also note previously undocumented basin‐edge surface waves in the Tacoma basin during one of the local earthquakes. To place quantitative constraints on basin amplification, we determine amplification factors by computing the spectral ratios of inside‐basin sites to outside‐basin sites at 1, 2, 3, and 5 s periods. Ground shaking is amplified in the Seattle basin for all the earthquakes analyzed and for a subset of events in the Tacoma basin. We find that the largest amplification factors in the Seattle basin are produced by a shallow earthquake located to the southwest of the basin. Our observation suggests that future shallow crustal and megathrust earthquakes rupturing west of the Puget Lowland will produce greater amplification within the Seattle basin than has been seen for intraslab events. We also perform ground‐motion simulations using a finite‐difference method to validate a 3D Cascadia velocity model (CVM) by comparing properties of observed and synthetic waveforms up to a frequency of 1 Hz. Basin‐edge effects are well reproduced in the Seattle basin, but are less well resolved in the Tacoma basin. Continued study of basin effects in the Tacoma basin would improve the CVM.

Washington↗
USGS2020

Regionally Optimized Background Earthquake Rates from ETAS (ROBERE) for probabilistic seismic hazard assessment

We use an epidemic‐type aftershock sequence (ETAS) based approach to develop a regionally optimized background earthquake rates from ETAS (ROBERE) method for probabilistic seismic hazard assessment. ROBERE fits parameters to the full seismicity catalog for a region with maximum‐likelihood estimation, including uncertainty. It then averages the earthquake rates over a suite of catalogs from which foreshocks and aftershocks have been removed using stochastic declustering while maintaining the same Gaussian smoothing currently used for the U.S. Geological Survey National Seismic Hazard Model (NSHM). The NSHM currently determines these rates by smoothing a single catalog from which foreshocks and aftershocks have been removed using the method of Gardner and Knopoff (1974 ; hereafter, GK74). The parameters used in GK74 were determined from subjectively identified aftershock sequences, unlike ROBERE, in which both background rate and aftershock triggering parameters are objectively fitted. A major difference between the impacts of the two methods is GK74 significantly reduces the b "> b ‐value, a critical value for seismic hazard analysis, whereas ROBERE maintains the original b "> b ‐value from the full catalog. We apply these methods to the induced seismicity in Oklahoma and Kansas and tectonic activity in the San Francisco Bay Region. Using GK74 gives lower overall earthquake rates but estimates higher hazard due to the reduction in the b "> b ‐value. ROBERE provides higher earthquake rates, at the magnitude of completeness, but lower hazard because it does not alter the b "> b ‐value. We test two other declustering methods that produce results closer to ROBERE but do not use objectively fit parameters, include uncertainty, and may not work as well in other areas. We suggest adopting ROBERE for the NSHM so that our hazard estimates are based on an objective analysis, including uncertainty, and do not depend strongly on potentially biased b "> b ‐values, which was never the goal of the existing methodology.

Bulletin of the Seismological Society of America↗
USGS2020

Evolution of faulting induced by deep fluid injection, Paradox Valley, Colorado

High‐pressure fluid injection into a subhorizontal confined aquifer at 4.3–4.6 km depth induced &gt; 7000 "> > 7000 >7000 earthquakes between 1991 and 2012 within once seismically quiescent Paradox Valley in Colorado, with magnitudes up to M w "> M w Mw 3.9. Earthquake hypocenters expanded laterally away from the well with time, defining the margins of the aquifer pressurized by injection at the well. Within 5 km of the well, alignment of earthquake hypocenters defines strikes of nine vertical fault zones. Previous studies show that these fault zones predate injection, producing left‐stepping offsets in the normal faults of the Wray‐Mesa fault system that cradles Paradox Valley. Hypocenters, rakes, and strikes of 2041 well‐constrained focal mechanisms show that most injection‐related earthquakes occur where these vertical faults intersect the pressurized aquifer. Well‐defined focal mechanisms show that this induced seismicity consists of Riedel shear faults at acute angles to the strikes of these fault zones. These small faults develop an anastomosing fault structure of focal planes along each planar fault zone, as fluid injection continues, even as their hypocenters define a single planar fault zone. Failure conditions at each hypocenter are found using a fully coupled poroelastic analysis of stress induced by fluid injection, and this analysis indicates a minimum Coulomb failure condition of 0.1 MPa. This failure condition is primarily a result of aquifer pore‐fluid pressurization, as almost all well‐located seismicity is within the pressurized aquifer. Reducing the rate of injection and frequent well shutdowns in the second decade nearly eliminated induced seismicity, except very near the well where gradients in pressurization are the largest. Despite these decreases in failure conditions and seismicity, some fault zones continued to produce earthquakes larger than M 3 as injection continued.

California↗
USGS2020

Types and areal distribution of ground failure associated with the 2019 Ridgecrest, California, earthquake sequence

T he July 2019 Ridgecrest, California, earthquake sequence included the largest earthquake ( M 7.1) to strike the conterminous United States in the past 20 yr. To characterize the types, numbers, and areal distributions of different types of ground failure (landslides, liquefaction, and ground cracking), I conducted a field investigation of ground failure triggered by the sequence around the periphery of the epicentral area (which had limited access). The earthquake sequence triggered sparse and widely scattered landslides over an area of &#x223C; 22 , 000 &#x2009;&#x2009; km 2 "> ∼ 22 , 000 km 2 ∼22,000 km2 and at a maximum epicentral distance of 114 km; these metrics are within the upper bounds as compared with global averages for earthquakes of similar size. Some rock falls blocked primary and secondary roads, but no other landslide damage was reported. Almost all of the landslides in the peripheral area were small rock falls ( ⁠ &#x223C; 1 &#x2013; 10 &#x2009;&#x2009; m 3 "> ∼ 1 – 10 m 3 ∼1–10 m3 ⁠ ), but a few larger ( ⁠ &#x223C; 100 &#x2009;&#x2009; m 3 "> ∼ 100 m 3 ∼100 m3 ⁠ ) rock slides also occurred. Though there are only informal reports about ground failure in the immediate epicentral area and we lack a detailed survey there, the small number (hundreds) and size of the landslides still seems to be far below global averages for M 7.1. This could be a result of the arid landscape and lack of a deeply weathered zone of soil and regolith. Liquefaction occurred along part of the western margin of Searles Valley. One large ( ⁠ &#x223C; 0.4 &#x2009;&#x2009; km 2 "> ∼ 0.4 km 2 ∼0.4 km2 ⁠ ) lateral spread caused by liquefaction severely damaged parts of Trona. Minor liquefaction also occurred in a &#x223C; 100 &#x2010; m &#x2010; wide "> ∼ 100 ‐ m ‐ wide ∼100‐m‐wide band along the fault‐rupture zone in some places.

California↗
USGS2020

Repeatable source, path, and site effects from the 2019 Ridgecrest M7.1 earthquake sequence

We use a large instrumental dataset from the 2019 Ridgecrest earthquake sequence ( Rekoske et al. , 2019 , 2020 ) to examine repeatable source‐, path‐, and site‐specific ground motions. A mixed‐effects analysis is used to partition total residuals relative to the Boore et al. (2014 ; hereafter, BSSA14) ground‐motion model. We calculate the Arias intensity stress drop for the earthquakes and find strong correlation with our event terms, indicating that they are consistent with source processes. We look for physically meaningful trends in the partitioned residuals and test the ability of BSSA14 to capture the behavior we observe in the data. We find that BSSA14 is a good match to the median observations for M &gt; 4 "> M > 4 M>4 ⁠ . However, we find bias for individual events, especially those with small magnitude and hypocentral depth &#x2265; 7 &#x2009;&#x2009; km "> depth ≥ 7 km depth≥7 km ⁠ , for which peak ground acceleration is underpredicted by a factor of 2.5. Although the site amplification term captures the median site response when all sites are considered together, it does not capture variations at individual stations across a range of site conditions. We find strong basin amplification in the Los Angeles, Ventura, and San Gabriel basins. We find weak amplification in the San Bernardino basin, which is contrary to simulation‐based findings showing a channeling effect from an event with a north–south azimuth. This and an additional set of ground motions from earthquakes southwest of Los Angeles suggest that there is an azimuth‐dependent southern California basin response related to the orientation of regional structures when ground motion from waves traveling south–north are compared with those in the east–west direction. These findings exhibit the power of large, spatially dense ground‐motion datasets and make clear that nonergodic models are a way to reduce bias and uncertainty in ground‐motion estimation for applications like the U.S. Geological Survey National Seismic Hazard Model and the ShakeAlert earthquake early warning System.

California↗
USGS2020

A stress-similarity triggering model for aftershocks of the MW6.4 and MW7.1 Ridgecrest earthquakes

The July 2019 M w "> M w 6.4 and 7.1 Ridgecrest earthquakes triggered numerous aftershocks, including clusters of off‐fault aftershocks in an extensional stepover of the Garlock fault, near the town of Olancha, and near Panamint Valley. The locations of the off‐fault aftershocks are consistent with the stress‐similarity model of triggering, which hypothesizes that aftershocks preferentially occur in areas where the mainshock static stress change tensor is similar in orientation to the background stress tensor. The background stress field is determined from the inversion of earthquake focal mechanisms, with the spatial resolution adapted to the local density of earthquakes. The mainshock static stress change is computed using finite‐source models for the M w "> M w 6.4 foreshock and M w "> M w 7.1 mainshock. I quantify the similarity between these two stress fields using the tensor dot product of the normalized deviatoric stress tensors. The off‐fault aftershocks in the Garlock stepover and the Olancha area fall within lobes of positive stress similarity, whereas the aftershocks near Panamint Valley are partially within a lobe. The cluster in the Garlock fault stepover and the smaller of two clusters near Olancha occur in regions of locally anomalous background stress that results in higher stress similarity. I compute the spatial density of M &#x2265; 2.0 "> M≥2.0 aftershocks and find that the aftershock density increases as a function of stress similarity, with a factor of &#x223C; 15 "> ∼15 difference between high stress‐similarity and low stress‐similarity areas. This result is robust with respect to the choice of mainshock model and the uncertainty of the background stress field. The aftershock density varies substantially inside the high stress‐similarity lobes, however, indicating that other variable background conditions, such as material properties, temperature, and fluid pressure, may also be playing a role. Specifically, temperature and fluid pressure conditions might help explain the low rate of aftershocks in the Coso geothermal field.

California↗
USGS2020

Seismicity rate change at the Coso Geothermal Field following the July 2019 Ridgecrest M7.1 earthquake

Many geothermal and volcanic regions experience remote and regional triggering following large earthquakes. The transient or permanent changes in stresses acting on faults and fractures can lead to changes in seismicity rates following either the passage of teleseismic waves or the permanent change in stresses following regional events. One such region of prevalent triggering is the Coso Geothermal Field (CGF) in eastern California, which is located roughly 30km to the north of the 2019 M7.1 Ridgecrest epicenter. Previous regional earthquakes have seemingly caused increases in seismicity rate surrounding the CGF, but also showed an absence of such rate increases in the CGF itself. To test whether seismicity rates in the CGF were dissimilar to the surrounding area following the M7.1 Ridgecrest earthquake, I carry out seismicity rate change calculations using a catalog of seismicity complied using a local seismic network and find that the CGF behaves identical to the surrounding area. Comparisons of seismicity rate changes calculated using a regional network derived catalog and the local network derived catalog show that for a moderate, regional earthquake, the local network catalog reveals a change in seismicity rate while the regionally network catalog shows no significant changes. The differences are likely related to incomplete sampling of seismicity using the regional network due to the existence of a very shallow brittle ductile transition centered on the CGF. The CGF thus is prone to triggering from both teleseismic and regional earthquakes.

California↗
USGS2020

Kinematics of fault slip associated with the July 4-6 2019 Ridgecrest, Californai earthquakes sequence

The 2019 Ridgecrest, California, earthquake sequence produced observable crustal deformation over much of central and southern California, as well as surface rupture over several tens of kilometers. To obtain a detailed picture of the fault slip involved in the 4 July M 6.4 foreshock and 6 July M 7.1 mainshock, we combine strong‐motion seismic waveforms with crustal deformation observations to obtain kinematic and static slip models of both events. We sample the regional seismic wavefield for both the foreshock and mainshock with three‐component records from 31 stations of the California Integrated Seismic Network. The deformation observations include Global Positioning System (GPS), Interferometric Synthetic Aperture Radar (InSAR), and borehole strainmeter recordings of the dynamic strain field. These data collectively constrain the kinematic coseismic slip distributions of the events, with measurements variously observing coseismic slip from one event (e.g., seismic waveforms, kinematic solutions from continuous GPS, and strainmeter time series) or coseismic slip from both events combined (InSAR). We find that the foreshock ruptured two separate faults, one with left‐lateral strike slip on a northeast–southwest‐trending fault and the other with right‐lateral strike slip on an orthogonal fault, with unilateral rupture propagation along both. The mainshock ruptured a series of northwest–southeast‐trending faults with right‐lateral strike slip concentrated in the uppermost 6 km with exceptionally low‐rupture velocity averaging 1.0 &#x2013; 1.5 &#x2009;&#x2009; km / s ">

California↗