Internal structure of the San Jacinto fault zone at the Ramona Reservation, north of Anza, California, from dense array seismic data

We image the internal structure of the San Jacinto fault zone (SJFZ) near Anza, California, with seismic data recorded by two dense arrays (RA and RR) from similar to 42 000 local and similar to 180 teleseismic events occurring between 2012 and 2017. The RA linear array has short aperture (similar to 470 m long with 12 strong motion sensors) and recorded for the entire analysed time window, whereas the RR is a large three-component nodal array (97 geophones across a similar to 2.4 km x 1.4 km area) that operated for about a month in September-October 2016. The SJFZ at the site contains three near-parallel surface traces F1, F2 and F3 from SW to NE that have accommodated several M-w > 6 earthquakes in the past 15 000 yr. Waveform changes in the fault normal direction indicate structural discontinuities that arc consistent with the three fault surface traces. Relative slowness from local events and delay time analysis of teleseismic arrivals in the fault normal direction suggest a slower SW side than the NE with a core damage zone between F1 and F2. This core damage zone causes similar to 0.05 s delay at stations RR26 31 in the teleseismic P arrivals compared with the SW-most station, and generates both P- and S-type fault zone trapped waves. Inversion of S trapped waves indicates the core damaged structure is similar to 100 m wide, similar to 4 km deep with a Q value of similar to 20 and 40 per cent S-wave velocity reduction compared with bounding rocks. Fault zone head waves observed at stations SW of F3 indicate a local bimaterial interface that separates the locally faster NE block from the broad damage zone in the SW at shallow depth and merges with a deep interface that separates the regionally faster NE block from rocks to the SW with slower velocities at greater depth. The multiscale structural components observed at the site are related to the geological and earthquake rupture history at the site, and provide important information on the preferred NW propagation of earthquake ruptures on the San Jacinto fault.

Automated summary

Using seismic data, we imaged the internal structure of the San Jacinto fault zone near Anza, California, revealing three near-parallel surface traces that have experienced several Mw > 6 earthquakes. Analysis suggests the presence of a core damage zone in the fault zone, causing waveform changes and delays, as well as generating different types of fault zone trapped waves. The multiscale structural components observed relate to the geological and earthquake rupture history at the site, providing insights into the preferred northwest propagation of earthquake ruptures on the San Jacinto fault.