SCEC Award Number 25231 View PDF
Proposal Category Individual Research Project (Single Investigator / Institution)
Proposal Title Identifying seismicity bursts across California to probe fault-zone conditions
Investigator(s)
Name Organization
Wenyuan Fan University of California, San Diego
SCEC Milestones A1-2, B2-2, D1-1, D2-1, D2-2 SCEC Groups Seismology, FARM, SDOT
Report Due Date 03/15/2026 Date Report Submitted 03/27/2026
Project Abstract
 Earthquakes tend to cluster in space and time, developing into sequences driven by stress perturbations and transient fault-zone processes (Omori, 1894; King et al., 1994; Peng et al., 2025). The characteristics of earthquake sequences can vary depending on the driving process, and this variability challenges our ability to observe and distinguish these mechanisms at high resolution. This project systematically identified highly clustered seismicity bursts throughout southern California using new statistical methods (DeSalvio and Fan, 2023; Fan et al., 2021) and examined their causes using independent geophysical observations. Seismicity bursts were defined as statistically significant seismicity rate anomalies relative to the local background rate, an approach that is data-driven and depends on few earthquake occurrence assumptions. We found abundant seismicity bursts across the region, most frequently occurring along the San Jacinto Fault and in the Salton Sea and Coso geothermal fields. These seismicity bursts are highly compact in space and time, often encompassed by a 5 km radius with durations of less than 10 hours. Through analyses of magnitudefrequency distributions, spectral stress drops, spatiotemporal clustering, and stress ratios, we demonstrated that the seismicity bursts have systematically different characteristics from regional seismicity. These differences suggest that seismicity bursts are driven by transient fault-zone processes, including pore fluid pressure changes, aseismic transients, and stress-dependent cascading effects, acting frequently across fault networks.
Intellectual Merit This project systematically investigated seismicity bursts and the associated fault zone evolution in southern California. Through the development and application of a new data-driven statistical approach, we identified abundant and widespread highly clustered seismicity bursts throughout the region. The identified seismicity bursts provide valuable candidates for case studies to further investigate local fault zone processes, offering new opportunities to understand earthquake sequences at kilometer-scale spatial resolution and hour-scale temporal resolution. The methodology developed in this project is readily applicable to other regions with high-quality earthquake catalogs.
Broader Impacts This project supported PhD student Nicolas DeSalvio at SIO/UCSD. The study constitutes a core component of his doctoral thesis, in which he developed and applied the statistical burst identification method, conducted the comprehensive analyses of burst characteristics, and led the interpretation of the results. The project provided DeSalvio with extensive training in statistical seismology, earthquake source physics, and the analysis of large-scale seismological datasets.

DeSalvio, N. D., W. Fan, A. J. Barbour, and J. L. Hardebeck (2025), Compact Seismicity Bursts Have Different Characteristics from Regional Seismicity, Journal of Geophysical Research: Solid Earth, doi:10.1029/2025JB032917.
Project Participants Nicolas DeSalvio and Wenyuan Fan from SIO/UCSD, and Andrew Barbour and Jeanne Hardebeck from USGS.
Exemplary Figure Study area and locations of the identified seismicity bursts. (a) Number of seismicity bursts identified by the β-statistic. Inset shows the magnitude of completeness (Mc) and spatial extent of each grid. (b) Magnitude time series for a seismicity burst identified within the grid adjacent to the (b) label in panel (a), near Brawley, CA, on June 2, 2008. The burst lasts one day (shaded in blue); the seismicity rate change in the subsequent 24-hour period (shaded in purple) is not statistically significant. Seismicity bursts like this are abundant and widespread throughout southern California.
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