The FaultScan Long-Term Dense Nodal Array to Study the San Jacinto Fault (Southern California)

Large earthquakes involve processes that occur primarily at seismogenic depths of 5-10 km or more and that are thus difficult to observe in detail with traditional seismic networks. The FaultScan project establishes the first long-term, dense nodal array experiment by acquiring continuous data at 300 stations along the San Jacinto Fault (SJF, South California) at the Piñon Flat Observatory for 2.5 years (April 2022-Nov. 2024).

We report on the experiment logistics, data quality and describe the characteristics of the High-Frequency (HF, >1 Hz) back-ground noise with a focus on incoherent wind generated noise, train traffic tremors and far distant coherent urban noise. We illustrate how these HF coherent noise sources recorded mostly as body-waves can be used to track velocity changes across the SJF by applying seismic interferometry between the array and nearby permanent seismic stations.

We further describe how much slant-stacking increases the level of signal to noise ratio for the detection of small earthquakes along the San Jacinto Fault and show how newly detected events improve our description of foreshock/aftershock patterns. Our search for tectonic tremors along the San Jacinto Fault turns up empty but we observe tremor-like signals, mostly T-phases coming exclusively from the Tonga subduction and one intriguing T-phase like sequence originating from the October 2023, offshore Japan volcanic crisis.