Internal structure of the San Jacinto fault zone at Blackburn Saddle from seismic data of a linear array

Abstract

S U M M A R YLocal and teleseismic earthquake waveforms recorded by a 180-m-long linear array (BB) with seven seismometers crossing the Clark fault of the San Jacinto fault zone northwest of Anza are used to image a deep bimaterial interface and core damage structure of the fault. Delay times of P waves across the array indicate an increase in slowness from the southwest most (BB01) to the northeast most (BB07) station. Automatic algorithms combined with visual inspection and additional analyses are used to identify local events generating fault zone head and trapped waves. The observed fault zone head waves imply that the Clark fault in the area is a sharp bimaterial interface, with lower seismic velocity on the southwest side. The moveout between the head and direct P arrivals for events within ∼40 km epicentral distance indicates an average velocity contrast across the fault over that section and the top 20 km of 3.2 per cent. A constant moveout for events beyond ∼40 km to the southeast is due to off-fault locations of these events or because the imaged deep bimaterial interface is discontinuous or ends at that distance. The lack of head waves from events beyond ∼20 km to the northwest is associated with structural complexity near the Hemet stepover. Events located in a broad region generate fault zone trapped waves at stations BB04-BB07. Waveform inversions indicate that the most likely parameters of the trapping structure are width of ∼200 m, S velocity reduction of 30-40 per cent with respect to the bounding blocks, Q value of 10-20 and depth of ∼3.5 km. The trapping structure and zone with largest slowness are on the northeast side of the fault. The observed sense of velocity contrast and asymmetric damage across the fault suggest preferred rupture direction of earthquakes to the northwest. This inference is consistent with results of other geological and seismological studies.

Imaging subsurface structures in the San Jacinto fault zone with high-frequency noise recorded by dense linear arrays

“…Ben-Zion & Sammis 2003). These observations provide extensions essentially up to the surface for previous imaging of prominent low-velocity zones around the arrays to depths of about 2-4 km based on analyses of internal fault zone reflections and trapped waves (Yang et al 2014;Qiu et al 2017;Share et al 2017;Qin et al 2018). These studies combined with our results indicate the existence of hierarchical damage flower structures at different sections of the SJFZ and other locations (e.g.…”

“…1; e.g. Yang et al 2014;Qiu et al 2017;Share et al 2017;Qin et al 2018). Double-difference earthquake tomography of P and S traveltimes provided accurate images in the approximate depth range 2-17.5 km (Allam & Ben-Zion 2012;Allam et al 2014), while inversions of Rayleigh and Love wave properties constructed from the ambient noise in the frequency range 0.07-0.33 Hz yielded reliable images in the approximate depth range 0.5-10 km (Zigone et al 2015).…”

“…These regional velocity models were augmented by imaging fault damage zones and bimaterial interfaces based on data recorded by linear arrays (Fig. 1) at different locations (Yang et al 2014;Qiu et al 2017;Share et al 2017;Qin et al 2018).…”

Wave-equation dispersion inversion of surface waves recorded on irregular topography

“…Figure 5b seismometers. With a total aperture of 2.4 km and a mean station spacing of 20 m, the array locally spans the fault zone from a low-velocity crustal block on the South-West (SW) through the damage structure of the fault to a high-velocity crustal block on the North-East (NE) (Allam and Ben-Zion, 2012;Share et al, 2017). The data were continuously recorded for 36 days at a 1000-Hz sampling rate.…”

Nonmonotonicity of the Frictional Bimaterial Effect

“…Bimaterial configurations with two solids of different elastic properties in contact exist widely in earth and man-made systems. One class of examples involves large strike-slip faults that separate different crustal blocks, as reported for the San Andreas, Hayward and San Jacinto faults in California (Allam et al, 2014;Ben-Zion & Malin, 1991;Eberhart-Phillips & Michael, 1993;Share et al, 2017), and the North Anatolian fault in Turkey (Ozakin et al, 2012;Sandvol et al, 2001). Historic records and recent observations indicate that such strike-slip faults are able to produce earthquakes with magnitude up to 8 (Bohnhoff et al, 2016;Scholz, 2002;Rockwell et al, 2015).…”