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Abstract Earthquake focal mechanisms provide crucial information about subsurface fault geometry and stress orientations. Focal mechanisms are typically inferred through analysis of seismic radiation patterns, for example, using P-wave first-motion polarities, potentially in combination with S/P amplitude ratios, to identify nodal planes. The motivation for this procedure is well-founded, as P- and S-wave radiation patterns depend fundamentally on the fault orientation. However, in practice, S/P amplitude ratio measurements can be strongly influenced by factors that are unrelated to the source mechanism. In this study, I characterize several underappreciated issues with S/P amplitude ratio data that are relevant to focal mechanism inversion. The analysis combines synthetic tests with new waveform measurements from ∼64,000 ML≥1.0 earthquakes in Nevada and California. Key findings include that (1) the statistical distribution of S/P amplitude ratio data differs markedly in shape and width from the theoretical expectation, (2) S/P amplitude ratios decay systematically with source-station distance beyond ∼60 km or so, (3) this distance effect is more severe for smaller earthquakes than for larger ones, and (4) modifying the frequency band in which amplitudes are measured can shift the observed amplitude ratio distribution but does not significantly mitigate issues (1)–(3). Taken together, these findings indicate that S/P amplitude ratio measurements are influenced by differential path attenuation and signal-to-noise effects that are not accounted for with existing workflows. Using independent moment tensor solutions, I systematically test various strategies to incorporate S/P amplitude ratios into focal mechanism solutions. The best-performing strategies transform S/P amplitude data to better match the theoretical expectation. Overall, S/P amplitude ratio data appear helpful in improving a typical mechanism solution, but even with the best-performing strategies considered here, the inclusion of S/P amplitude ratio data is expected to hinder rather than improve the solution for a subset of events. 

Abstract The Rock Valley fault zone in southern Nevada has a notable history of seismic activity and is the site of a future direct comparison experiment of explosion and earthquake sources. This study aims to gain insight into regional tectonic processes by leveraging recent advances in seismic monitoring capabilities to elucidate the local stress regime. A crucial step in this investigation is the accurate determination of P-wave first-motion polarities, which play a vital role in resolving earthquake focal mechanisms of small earthquakes. We deploy a deep learning-based method for automatic determination of first-motion polarities to vastly expand the polarity dataset beyond what has been reviewed by human analysts. By the integrating P-wave polarities with new measurements of S/P amplitude ratios, we obtain robust focal mechanism estimates for 1306 earthquakes with a local magnitude of 1 and above occurring between 2010 and 2023 in southern Nevada. We then use the focal mechanism catalog to examine the regional stress orientation, confirming an overall trans-tensional stress regime with smaller scale complexities illuminated by individual earthquake sequences. These findings demonstrate how detailed analyses of small earthquakes can provide fundamental information for understanding earthquake processes in the region and inform future experiments at the Nevada National Security Site. 

Abstract Understanding the generation of damaging, high‐frequency ground motions during earthquakes is essential both for fundamental science and for effective hazard preparation. Various theories exist regarding the origin of high‐frequency ground motions, including the standard paradigm linked to slip heterogeneity on the rupture plane, and alternative perspectives associated with fault complexity. To assess these competing hypotheses, we measure ground motion amplitudes in different frequency bands for 3 ≤ M ≤ 5.8 earthquakes in Southern California and compare them to empirical ground motion models. We utilize a Bayesian inference technique called the Integrated Nested Laplace Approximation (INLA) to identify earthquake source regions that produce higher or lower ground motions than expected. Our analysis reveals a strong correlation between fault complexity measurements and the high‐frequency ground motion event terms identified by INLA. These findings suggest that earthquakes on complex faults (or fault networks) lead to stronger‐than‐expected ground motions at high frequencies. 

Abstract Earthquakes are clustered in space and time, with individual sequences composed of events linked by stress transfer and triggering mechanisms. On a global scale, variations in the productivity of earthquake sequences—a normalized measure of the number of triggered events—have been observed and associated with regional variations in tectonic setting. Here, we focus on resolving systematic variations in the productivity of crustal earthquake sequences in California and Nevada—the two most seismically active states in the western United States. We apply a well-tested nearest-neighbor algorithm to automatically extract earthquake sequence statistics from a unified 40 yr compilation of regional earthquake catalogs that is complete to M ∼ 2.5. We then compare earthquake sequence productivity to geophysical parameters that may influence earthquake processes, including heat flow, temperature at seismogenic depth, complexity of quaternary faulting, geodetic strain rates, depth to crystalline basement, and faulting style. We observe coherent spatial variations in sequence productivity, with higher values in the Walker Lane of eastern California and Nevada than along the San Andreas fault system in western California. The results illuminate significant correlations between productivity and heat flow, temperature, and faulting that contribute to the understanding and ability to forecast crustal earthquake sequences in the area. 

Abstract Accurate precipitation monitoring is crucial for understanding climate change and rainfall-driven hazards at a local scale. However, the current suite of monitoring approaches, including weather radar and rain gauges, have different insufficiencies such as low spatial and temporal resolution and difficulty in accurately detecting potentially destructive precipitation events such as hailstorms. In this study, we develop an array-based method to monitor rainfall with seismic nodal stations, offering both high spatial and temporal resolution. We analyze seismic records from 1825 densely spaced, high-frequency seismometers in Oklahoma, and identify signals from nine precipitation events that occurred during the one-month station deployment in 2016. After removing anthropogenic noise and Earth structure response, the obtained precipitation spatial pattern mimics the one from a nearby operational weather radar, while offering higher spatial (~ 300 m) and temporal (< 10 s) resolution. We further show the potential of this approach to monitor hail with joint analysis of seismic intensity and independent precipitation rate measurements, and advocate for coordinated seismological-meteorological field campaign design. 

Abstract Analysis of earthquake spectra can aid in understanding source characteristics like stress drop and rupture complexity. There is growing interest in probing the similarities and differences of fault rupture for natural and human-induced seismic events. Here, we analyze waveform data from a shallow, buried geophone array that recorded seismicity during a hydraulic fracturing operation near Fox Creek, Alberta. Starting from a quality-controlled catalog of 4000 events between magnitude 0 and 3.2, we estimate source-spectral corner frequencies using methods that account for the band-limited nature of the sensor response. The stress-drop values are found to be approximately self-similar, but with a slight magnitude dependence in which larger events have higher stress drop (∼10 MPa). Careful analysis of the relative corner frequencies shows that individual fault and fracture segments experienced systematic variations in relative corner frequency over time, indicating a possible change in the stress state. Clustering analysis of source spectra based on the relative proportion of high- and low-frequency content relative to the Brune model further shows that event complexity evolves over time. In addition, the faults produce earthquakes with systematically larger stress-drop values than the fractures. Combined, these results indicate that the features activated by hydraulic fracturing experience observable changes in source behavior over time and exhibit different properties depending on the orientation, scale, and fabric of the structural feature on which they occur. 

Abstract On 8 July 2021 a M6.0 normal faulting earthquake rocked the community of Walker and the surrounding region near the California‐Nevada border. In the 1990s, field surveys of nearby Meadowcliff Canyon identified numerous precarious rocks deemed likely to topple in the event of strong shaking. Despite their proximity (∼6 km) to the 2021 earthquake, the precarious rocks still remain standing. In this work, we combine advanced source and ground motion characterization techniques to help unravel this mystery. High‐precision hypocentral locations reveal a clear north/south‐striking, east‐dipping rupture plane along the southern extension of the Slinkard Valley fault. The mainshock nucleated near the base of the fault, triggering thousands of aftershocks. Bayesian source spectral analyses indicate that the mainshock had a moderately‐high stress drop (∼17 MPa), and that aftershocks with deeper hypocenters have higher stress drops. Peak Ground Acceleration (PGA) recordings at regional stations agree well with existing ground motion models, predicting PGA of ∼0.3 g in Meadowcliff Canyon, a level sufficient to topple precarious rocks based on PGA‐derived stability criteria. We demonstrate that despite these large ground accelerations, the pulse duration in Meadowcliff Canyon is too short to supply the impulse necessary to damage these features, observations which support the application of dynamic toppling models that account for the joint effects of pulse amplitude and duration when assessing rock fragility. This study provides a unique vantage point from which to interpret rarely‐observed strong‐motion recordings from close to an active normal fault, one of many that dominate hazard along the eastern Sierra. 

Foreshocks are the most obvious signature of the earthquake nucleation stage and could, in principle, forewarn of an impending earthquake. However, foreshocks are only sometimes observed, and we have a limited understanding of the physics that controls their occurrence. In this work, we use high-resolution earthquake catalogs and estimates of source properties to understand the spatiotemporal evolution of a sequence of 11 foreshocks that occurred ~ 6.5 hours before the 2020 Mw 4.8 Mentone earthquake in west Texas.  Elevated pore-pressure and poroelastic stressing from subsurface fluid injection from oil-gas operations is often invoked to explain seismicity in west Texas and the surrounding region. However, here we show that static stresses induced from the initial ML 4.0 foreshock significantly perturbed the local shear stress along the fault and could have triggered the Mentone mainshock. The majority (9/11) of the earthquakes leading up to the Mentone mainshock nucleated in areas where the static shear stresses were increased from the initial ML 4.0 foreshock. The spatiotemporal properties of the 11 earthquakes that preceded the mainshock cannot easily be explained in the context of a preslip or cascade nucleation model. We show that at least 6/11 events are better classified as aftershocks of the initial ML 4.0.  Together, our results suggest that a combination of physical mechanisms contributed to the occurrence of the 11 earthquakes that preceded the mainshock, including static-stressing from earthquake-earthquake interactions, aseismic creep, and stress perturbations induced from fluid injection.  Our work highlights the role of earthquake-earthquake triggering in induced earthquake sequences, and suggests that such triggering could help sustain seismic activity following initial stressing perturbations from fluid injection. 

The state of Nevada is home to one of the most seismically active regions in the world, with crustal deformation associated with the Walker Lane transitioning into Basin and Range tectonics as one traverses from west to east across the state. Despite hosting numerous prominent earthquake sequences over the past century and beyond, at present, there exists no unified research-quality earthquake catalog for the state and its surrounding region. Here, we present a newly compiled, high-precision catalog of more than 180,000 earthquakes occurring around Nevada from 2008 to 2023. The data processing workflow to create this catalog includes an absolute location step that accounts for topography and 3D variations in subsurface wavespeed, and a relative relocation step that refines event positions using differential times measured from waveform cross-correlation. We also provide an update to the local magnitude scale that better accounts for the observed distance attenuation of waveform amplitudes as well as local site effects. We describe some fundamental insights that can be derived from the new catalog, including regional variations in event depth distributions and sequence clustering statistics, and publish the catalog to the wider community to facilitate future research efforts.