Seeking Waveform Changes in a San Jacinto Fault Aftershock Sequence by Dense-Array Stacking

Mingze Du, John E. Vidale, & Quentin Higueret

Submitted August 30, 2026, SCEC Contribution #15631, 2026 SCEC Annual Meeting Poster #TBD

The trifurcation zone of the San Jacinto Fault is a structurally complex region where multiple fault strands interact, making it an important setting for investigating fault zone structure and how earthquake sources and the surrounding fault zone evolve during a seismic sequence. Small temporal changes in earthquake waveforms may provide clues on short-term evolution of the earthquake source region. Dense seismic arrays provide an opportunity to help us observing small waveform variation by enhancing weak coherent signals and enabling robust comparisons among closely spaced earthquakes. We analyzed data from the FaultScan nodal seismic array deployed near the trifurcation area of the San Jacinto Fault in southern California (Higueret et al., 2026). The deployment comprised approximately 300 three-component stations distributed across an approximately 2 × 2 km area. We focus on an ML 3.4 earthquake on September 30, 2022, one foreshock, and more than 30 aftershocks recorded during the following 12 hours. Our goal is to identify changes in the stacked waveforms that are systematically related to event timing and to assess whether these changes are consistent with short-term evolution of the source region following the mainshock.

We align and stack recordings across the array to improve the signal-to-noise ratio of the small events and reduce incoherent and station-specific contributions. Most of the events have fairly similar waveforms. Waveform-similarity analysis by cross-correlation identifies five clusters of two to nine highly similar earthquakes. Within individual clusters, the array-stacked waveforms reveal subtle but perhaps systematic differences in the S-wave coda. In particular, distinctive features at the onset of the S-wave appear in early aftershocks following the mainshock and weaken or disappear in later events, while additional variations with time are observed in the first few seconds of coda following the S wave, which is only revealed by stacking the very dense array.
Ongoing work is exploring possible mechanisms for these spatial and temporal waveform variations, including in source properties, small undetected aftershocks, event location, and near-source fault-zone structure.

References:
Higueret, Q., Brenguier, F., Mordret, A., Sheng, Y., Vernon, F., Hollis, D., ... & Ben‐Zion, Y. (2026). The FaultScan Long‐Term Dense Nodal Array to Study the San Jacinto Fault (Southern California). Seismological Research Letters

Citation
Du, M., Vidale, J. E., & Higueret, Q. (2026, 08). Seeking Waveform Changes in a San Jacinto Fault Aftershock Sequence by Dense-Array Stacking. Poster Presentation at 2026 SCEC Annual Meeting.

Related Projects & Working Groups
Seismology