SCEC Award Number 25253 View PDF
Proposal Category Collaborative Research Project (Multiple Investigators / Institutions)
Proposal Title Analyzing Key Earthquake Questions with Nodal Array of Arrays around the San Jacinto Fault
Investigator(s)
Name Organization
Peter Shearer University of California, San Diego Wenyuan Fan University of California, San Diego John Vidale University of Southern California
SCEC Milestones A2-3 SCEC Groups Seismology, CEM, RC
Report Due Date 03/15/2026 Date Report Submitted 03/31/2026
Project Abstract
 Clarifying the relationship between frequent small earthquakes and the much rarer large earthquakes is fundamental to understanding fault mechanics and earthquake nucleation, yet this connection remains elusive. More complete catalogs allow us to address key questions such as the prevalence and characteristics of foreshocks, the spatiotemporal evolution of swarms, and the finescale structure of faults. Despite recent advances in event detection using template matching and machine-learning techniques, the detection threshold for tiny (M < 0) earthquakes has not yet been
reached, leaving a hidden frontier of microearthquakes largely unexplored.

Beamforming applied to waveforms from small-scale nodal seismic arrays can substantially improve signal-to-noise ratios relative to single stations, enabling the detection of very small events. However, most previous array studies have used a single array, limiting location accuracy due to wavefront distortion from heterogeneous velocity structures. In this project, we deployed five 81-element nodal arrays around an active section of the San Jacinto Fault for four months through February 2025. Each array consisted of a 9 × 9 grid of sensors with 100 m aperture, which facilitated installation and maintenance while providing high signal-to-noise levels for smallearthquake detection.

By combining stacked array waveforms with nearby SCSN data, we build a seismic catalog by applying machine-learning phase picking and association. We also apply template matching event relocation from waveform cross-correlation results. Using this new catalog, we examine foreshock and aftershock statistics for small earthquakes near the San Jacinto Fault. This multi-array experiment pushes the limits of microseism detectability, offering new insights into fault mechanics and seismicity.
Intellectual Merit Resolving spatial and temporal patterns in seismicity is important for describing the processes that drive earthquake occurrence. Our knowledge of these patterns is limited because earthquake catalogs, even those generated with machine-learning and template-matching methods, are only complete above a threshold magnitude and vast numbers of tiny earthquakes are missed. We are conducting a focused experiment to see whether arrays of nodal instruments in active regions can detect still smaller earthquakes and improve our outstanding of swarms and foreshock occurrence and driving mechanisms.
Broader Impacts The Array of Arrays experiment provided an opportunity for more than 10 students to participate in fieldwork and gain firsthand experience with nodal seismic instrumentation. The project has also enabled one graduate student, Taiga Morioka, to attend the annual meetings of SCEC, AGU, and SSA, contributing to his PhD thesis. In the long run, our research will help improve our understanding of processes that drive earthquake occurrence and may lead to better forecasts of future seismic activity.
Project Participants Peter Shearer, Wenyuan Fan, Dan Hollis, Taiga Morioka, Ian Vandevert, Jeremy Wong (UCSD)
John Vidale, Hao Zhang (USC)
Elizabeth Cochran (USGS)
Exemplary Figure Figure 1. Map of the five arrays (pins) installed in this experiment with M ≥ 1 seismicity in our observation
period as red dots [1]. Pins show the location of the Pinyon Flat Observatory, CVCC, Sky Oaks, Borrego
Valley and Jordan arrays. Pink lines show the fault lines around the SJF. Blue star at the center of the map
shows epicenter of M 3.35 event in Borrego Spring on November 7th, 2024.
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