Summary 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 ∼42,000 local and ∼180 teleseismic events occurring between 2012–2017. The RA linear array has short aperture (∼470 m long with 12 strong motion sensors) and recorded for the entire analyzed time window, whereas the RR is a large three-component nodal array (97 geophones across a ∼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 Mw > 6 earthquakes in the past 15,000 years. Waveform changes in the fault normal direction indicate structural discontinuities that are 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 ∼0.05 second 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 ∼100 m wide, ∼4 km deep with a Q value of ∼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 multi-scale 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.
We image the internal structure of the San Jacinto fault zone (SJFZ) in the trifurcation area southeast of Anza, California, with seismic records from dense linear and rectangular arrays. The examined data include recordings from more than 20 000 local earthquakes and nine teleseismic events. Automatic detection algorithms and visual inspection are used to identify P and S body waves, along with P-and S-types fault zone trapped waves (FZTW). The location at depth of the main branch of the SJFZ, the Clark fault, is identified from systematic waveform changes across lines of sensors within the dense rectangular array. Delay times of P arrivals from teleseismic and local events indicate damage asymmetry across the fault, with higher damage to the NE, producing a local reversal of the velocity contrast in the shallow crust with respect to the large-scale structure. A portion of the damage zone between the main fault and a second mapped surface trace to the NE generates P-and S-types FZTW. Inversions of high-quality S-type FZTW indicate that the most likely parameters of the trapping structure are width of ∼70 m, S-wave velocity reduction of 60 per cent, Q value of 60 and depth of ∼2 km. The local reversal of the shallow velocity contrast across the fault with respect to large-scale structure is consistent with preferred propagation of earthquake ruptures in the area to the NW.
Seismograms from~700 local earthquakes recorded at various depths (0, 6, 15, 22, 50, and 150 m) by sensors of the Garner Valley Downhole Array in Southern California are used to analyze the shallow velocity structure and temporal changes of seismic velocities after the 2010 M7.2 El Mayor-Cucapah (EMC) earthquake. The direct P and S wave travel times between surface and borehole stations reveal very low shear wave velocities (178-259 m/s) and very high V p /V s ratios (6.2) in the top 22 m. Temporal changes of seismic velocities after the EMC earthquake are estimated using autocorrelations of data in moving time windows at two borehole stations (22 and 50 m) and seismic interferometry between multiple station pairs of the Garner Valley Downhole Array. The S wave velocity in the top 6 m drops abruptly by 14.3 ± 3.3%, during the passage of surface waves from the EMC event with a peak ground acceleration of 39 Gal, and recovers in~236 s. The average velocity reductions decrease with depth and are 10.9 ± 3.1%, 8.5 ± 2.1%, 6.3 ± 2.1%, and 4.5 ± 2.0% in the top 15, 22, 50, and 150 m, respectively. Comparisons of seismic interferometry results between sensor pairs at 0-22 and 22-150 m indicate that statistically significant velocity changes are limited at the site to the top 22 m. Pore pressure data are in phase with the surface displacement and reach maxima when the highest velocity drop occurs, suggesting fluid effects contribute to the observed velocity reductions. Key Points:• The top~20 m at the site has very low seismic velocities and very high V P /V S ratios • Moderate PGA produces large coseismic S wave velocity reductions in the top~20 m • The coseismic changes are followed by rapid recoveries in less thañ 240 s Supporting Information:• Supporting Information S1
The properties and dynamics of shallow materials are of great importance for a wide range of topics including environmental seismology, seismic ground motion prediction and performance of infrastructure above and below the surface. Near-surface materials have extremely low shear wave velocities (V S ) of 100-400 m/s (e.g.,
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