Probing Seismicity Secrets with Five Nodal Arrays around the San Jacinto Fault

Taiga Morioka, Florent Brenguier, Elizabeth S. Cochran, Wenyuan Fan, Quentin Higueret, Dan Hollis, Peter M. Shearer, Frank L. Vernon, John E. Vidale, Ruoyan Wang, & Hao Zhang

Submitted September 7, 2025, SCEC Contribution #14804, 2025 SCEC Annual Meeting Poster #TBD

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 fine-scale 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 small-earthquake 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.

Key Words
Nodal arrays, Small earthquake detection, San Jacinto Fault

Citation
Morioka, T., Brenguier, F., Cochran, E. S., Fan, W., Higueret, Q., Hollis, D., Shearer, P. M., Vernon, F. L., Vidale, J. E., Wang, R., & Zhang, H. (2025, 09). Probing Seismicity Secrets with Five Nodal Arrays around the San Jacinto Fault. Poster Presentation at 2025 SCEC Annual Meeting.

Related Projects & Working Groups
Seismology