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1. Comparison of EGF Methods for Ridgecrest Sequence: Can EGFs Help Resolve Ambiguity in Isolating Source Spectra?

   https://doi.org/10.1785/0120240161  

   Abercrombie, Rachel E; Chen, Xiaowei; Huang, Yihe; Chu, Shanna (March 2025, Bulletin of the Seismological Society of America)

   ABSTRACT The spectral stress drop is a popular parameter for the simple quantification and characterization of an earthquake source and its expected seismic radiation, enabling investigation of earthquake spatial and temporal variability for larger numbers of events. In addition, spectral measurements are one of the few possible for earthquake characterization and hazard prediction in regions of low seismicity. However, spectral stress-drop estimates are uncertain, especially as recorded earthquakes may be too complex to characterize ideally with a single parameter. Empirical Green’s function (EGF) approaches to isolate the earthquake source are widely regarded as one of the best for individual analysis of well-recorded earthquakes. However, analysis decisions related to the selection of stations, EGFs, time windows, frequency bandwidth, and source models can cause discrepancies in resulting estimates of the source spectrum, source time function, and source parameters. We present results following one well-developed EGF approach, and compare it with those from three other independent methods applied to earthquakes in the 2019 Ridgecrest, California, earthquake, sequence selected for the Southern California Earthquake Center /U.S. Geological Survey Community Stress Drop Validation Study. The common data set consists of two weeks of earthquakes from the 2019 Ridgecrest earthquake sequence, including nearly 13,000 events of M 1 and greater, recorded on stations within 100 km. We obtain estimates of corner frequency and spectral stress drop for 75 earthquakes (M 2.2–4.6) and find varying degrees of similarity among studies. We investigate four events in detail (M 2.7–4.1) and find that we obtain consistent results when the sources are relatively simple. Multiple EGFs produce good ratios and source time functions at stations with good azimuthal distribution. This suggests that there is a role for such approaches to resolve the inherent ambiguity in larger scale inversions between source scaling and attenuation and site effects. 

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2. Insights on earthquake source processes from the 2019 Ridgecrest earthquake source spectra and its azimuthal variation

   Liu, Meichen; Neo, Jing Ci; Huang, Yihe (January 2022, SSA Annual Meeting)

   The 2019 Ridgecrest, CA earthquake sequence has provided a unique opportunity and a rich dataset to understand earthquake source properties and near-fault structure. Using the high-quality seismic data provided by the SCEC Stress Drop Validation group, we first estimate the corner frequency of M2.0-4.5 earthquakes by applying the spectral ratio method based on empirical Green’s function (Liu et al., 2020). We relate corner frequency estimates to stress drops assuming the Brune source model and circular cracks. Our preliminary results show increasing median stress drops with magnitude for both P and S waves, from 1 MPa for M2.0 events to 10 MPa for M4.0 events, though the limited frequency bandwidth may cause underestimation for small events. The estimated moment magnitude is proportional to the catalog magnitude by a factor of 0.72, which is close to 0.74 estimated by Trugman (2020) for the Ridgecrest earthquake sequence. In the second part of the study, we examine the impact of fault zone structure on the azimuthal variation of the source spectra. Using kinematic simulations and observations of the 2003 Big Bear earthquake sequence, Huang et al. (2016) showed that fault damage zones can act as an effective wave guide and cause high-frequency wave amplification along directions close to fault strike. We use clusters of M1.5-3 earthquakes in the Ridgecrest region to further examine the azimuthal variation of the stacked source spectra and investigate if the near-source structure can affect our corner frequency estimates. We aim to develop robust methods that utilize high-quality seismic data to illuminate earthquake source processes and fault zone properties. 

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3. Investigating Near-Fault Ground Motions Using Data Recorded by Dense Arrays Around the 2019 Mw 7.1 Ridgecrest, California, Earthquake Rupture

   https://doi.org/10.1785/0120250092  

   Meng, Xiaofeng; Pinilla-Ramos, Camilo; Kottke, Albert; Ben-Zion, Yehuda (September 2025, Bulletin of the Seismological Society of America)

   ABSTRACT Following the 2019 Mw 7.1 Ridgecrest, California, earthquake, 15 dense 1D and 2D arrays (461 sites) were deployed around the main ruptures, including four 1D arrays across the surface ruptures of the mainshock. The dense arrays captured numerous aftershocks and provided an unprecedented dataset for studying near-fault ground motions. We combine the near-fault ground motions measured from the dense arrays with a regional ground-motion dataset and develop a near-fault partially nonergodic ground-motion model (GMM). The partially nonergodic GMM results are used to investigate several aspects of the near-fault ground motions that are related to source and fault zone properties. We find that shallow events tend to produce weaker short-period energy due to smaller stress drop or stronger attenuation but stronger long-period energy from surface-wave excitation. Significant ground-motion variations across the fault zone are observed, which include amplifications within the damage zones due to both reduced seismic velocities and trapped waves at certain periods. We also observe double-couple radiation pattern effects only in long-period ground motions, agreeing with a period-dependent earthquake rupture process. Between 3 and 15 days following the mainshock, the ground motions of aftershocks decrease with time, likely reflecting decreasing stress drops. No temporal evolution of site response and wave propagation effects within the fault zone are observed between 10 and 35 days following the mainshock. Our study demonstrates the capability of dense array data to improve our understanding of earthquake processes, fault zone properties, and near-fault seismic hazard. 

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4. Characterizing Directivity in Small (M 2.4–5) Aftershocks of the Ridgecrest Sequence

   https://doi.org/10.1785/0120240146  

   Chu, Shanna; Baltay, Annemarie; Abercrombie, Rachel (December 2024, Bulletin of the Seismological Society of America)

   ABSTRACT Directivity, or the focusing of energy along the direction of an earthquake rupture, is a common property of earthquakes of all sizes and can cause increased hazard due to azimuthally dependent ground-motion amplification. For small earthquakes, the effects of directivity are generally less pronounced due to reduced rupture size, yet the directivity in small events can bias source property estimates and provide important insights into general regional faulting patterns. However, due to observational limitations, directivity is usually only measured and modeled for large events. As such, many studies of small earthquakes either ignore directivity altogether or assume a constant rupture direction for all events in a cluster. In our study, we apply a refined directivity fitting method constrained with two separate methods of source deconvolution to the dataset of aftershocks of the 2019 Ridgecrest earthquakes, which contain a large number of well-recorded small-to-mid sized earthquakes occurring in close proximity to each other. The revealed directivity of 100+ small (M 2.4–5) earthquakes is highly heterogeneous and primarily oblique to and away from the main fault strike, suggesting a complex postseismic stress redistribution. In addition, the energy focusing effect of directivity appears to bias the selection of high-quality data from stations in the direction of rupture, leading to average stress-drop increases of 50% if directivity is not accounted for. 

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5. Rupture Directivity From Energy Envelope Deconvolution: Theory and Application to 58 Ridgecrest M 3.5–5.5 Earthquakes

   https://doi.org/10.1029/2024JB029683  

   Jia, Zhe; Shearer, Peter M; Fan, Wenyuan (January 2026, Journal of Geophysical Research: Solid Earth)

   Abstract Earthquake rupture directivity impacts ground motions and provides insights on fault zone properties and earthquake physics. However, measuring directivity of small earthquakes is challenging due to their compact rupture sizes and complex path and site effects at high frequencies. Here, we develop a new approach that deconvolves energy envelopes of the S wave trains to remove path and site effects and robustly resolve azimuthal variations in durations of apparent source‐time functions. Our method benefits from the coherence of energy envelopes for high‐frequency seismic data, which provides more stable directivity results than conventional waveform deconvolution methods. We validate our method using both synthetic tests and real observations. We apply the algorithm to determine rupture directivities of 58 magnitude 3.5–5.5 earthquakes during the 2019 Ridgecrest earthquake sequence. The rupture directivities suggest an orthogonal interlocking fault system consistent with aftershock locations. Additionally, the rupture directivity pattern appears to correlate with spatial heterogeneity in earthquake stress drops. Our energy envelope deconvolution method enables directivity measurements at smaller magnitudes than traditional approaches and has potential for constraining small earthquake rupture dynamics. 

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