Detecting hidden seismic swarms using a 300 nodal long-term array

Earthquake swarms may be driven by slow processes within fault zones, such as fluid migration, or silent slip events. A prominent example is the 2024 Mw 7.5 Noto earthquake in Japan, which was preceded by a significant seismic swarm, likely triggered by upward fluid migration (Wang et al. 2024). These swarms provide insights into dynamic fault processes, and there is thus a need to detect smaller earthquakes for a clearer understanding of the temporal evolution of seismicity and the mechanisms driving fault activity (Shearer et al., 2022).
To address these challenges, we used a dense array of 300 seismic nodes deployed for three years at the Piñon Flat Observatory (South.Cal.) along the San Jacinto Fault, one of the most seismically active areas in California. Using advanced slant stacking techniques tuned to the fault geometry at crustal P- and S-wave velocities, we significantly enhanced our detection capabilities down to approximately magnitude -2. By detecting four times as many events as the standard USGS catalog, this allowed us to highlight three distinct swarm episodes that were not identified before. These episodes exhibit a characteristic progression: an initial activation phase, a steady state culminating in a peak, followed by a final decay.
We further investigate the magnitude distribution and spatial migration to propose a possible driving mechanism. This approach can be extended to other fault zones to unveil hidden fault activity.