Internal Structure of the San Jacinto Fault Zone at the Ramona Reservation, North of Anza, California from Data of Dense Seismic Arrays

Session: Seismic Imaging of Fault Zones [Poster]
Type: Poster
Date: 4/29/2020
Time: 08:00 AM
Room: Ballroom
Description: 

We image the internal structure of the San Jacinto fault zone at the Ramona Reservation north of Anza, California, with seismic data recorded by two dense arrays (RA and RR) using ~42,000 local and ~180 teleseismic events occurring during 2012-2017. The RA array with an aperture of ~470 m (12 strong motion sensors) recorded for the entire time window, while the RR array (97 geophones across a ~2.4 km x 1.4 km area) operated for about 1 month in September-October 2016. Three near-parallel surface fault traces F1, F2 and F3 at the site likely accommodated Mw>6 earthquakes in the past. Waveform changes in the fault normal direction indicate structural discontinuities consistent with the three fault surface traces. Delay time analysis of local and tele seismic arrivals normal to the fault suggests a slower SW side and a core damage zone between F1 and F2 causing ~0.05-second delay in the teleseismic P arrivals in this zone compared with the SW-most station. P- and S-type fault zone trapped waves (FZTW) are observed inside the core damage zone; inversion of S-type FZTW indicates a core damaged structure that is ~100 m wide and ~4 km deep, with a Q value of ~20 and 40% S-wave velocity reduction compared with the surrounding rocks. Fault zone head waves observed SW of F3 suggest a shallow bimaterial interface that separates the locally faster NE block from the broad damage zone in the SW and merges with a deep interface that separates the regionally faster block to the NE from slower rocks to the SW. Teleseismic P wave polarization analysis also indicates a slower block to the SW and fault related damage structures between F1 and F2. The overall velocity contrast across the fault and damage structure have important implications for earthquake physics in the examined fault section.

Presenting Author: Pieter-Ewald Share

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