Low‐velocity zones along the San Jacinto Fault, Southern California, from body waves recorded in dense linear arrays

Abstract

We derive high-resolution information on low-velocity fault zone (FZ) structures along the San Jacinto Fault Zone (SJFZ), Southern California, using waveforms of local earthquakes that are recorded at multiple linear cross-fault arrays. We observe clear across-fault delays of direct P and S waves, indicating damage zones at different segments of the SJFZ. We then compute synthetic traveltimes and waveforms using generalized ray theory and perform forward modeling to constrain the FZ parameters. At the southern section near the trifurcation area, the low-velocity zone (LVZ) of the Clark branch has a width of~200 m, 30-45% reduction in Vp, and~50% reduction in Vs. From array data across the Anza seismic gap, we find a LVZ with~200 m width and~50% reduction in both Vp and Vs, nearly as prominent as that on the southern section. We only find prominent LVZs beneath three out of the five arrays, indicating along-strike variations of the fault damage. FZ-reflected phases are considerably less clear than those observed above the rupture zone of the 1992 Landers earthquake shortly after the event. This may reflect partially healed LVZs with less sharp boundaries at the SJFZ, given the relatively long lapse time from the last large surface-rupturing event. Alternatively, the lack of observed FZ-reflected phases could be partially due to the relatively small aperture of the arrays. Nevertheless, the clear signatures of damage zones at Anza and other locations indicate very slow healing process, at least in the top few kilometers of the crust.

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

“…13 and geological mapping near Hog Lake southeast of the array (Dor et al 2006). Stations at larger distance from the fault may record, in addition to the phases analysed in this work, also P and S body waves reflected within a low velocity fault zone layer (Yang et al 2014) and waves reflected from bimaterial fault interfaces to off-fault stations (Najdahmadi et al 2016). A recent deployment of a longer aperture array across the Clark fault at the same location of the BB array (Lin et al 2016) provides opportunities for analysing these and other signals indicative of the inner structure of the fault.…”

“…Internal structural components of the SJFZ have been studied using various seismic arrays that cross the fault at different locations (e.g. Li & Vernon 2001;Lewis et al 2005;Yang et al 2014;Ben-Zion et al 2015;Li et al 2015;Hillers et al 2016), along with geological mapping of rock damage and analysis of geomorphologic signals (Dor et al 2006;Wechsler et al 2009).…”

“…These and other less coherent parts of the fault damage zone also produce delay time of direct P and S waves propagating through the fault zone structure (e.g. Lewis & Ben-Zion 2010;Yang et al 2014;Qiu et al 2017).…”

Recent advances in imaging crustal fault zones: a review

“…2) (Li et al 2007;Yang and Zhu 2010a;Yang et al 2011Yang et al , 2014). This technique not only uses measurements of travel-time delays of direct P and S waves, but also takes into account of the differential times between the direct and FZ-reflected P and S waves.…”

The effect of gradational velocities and anisotropy on fault-zone trapped waves

“…Given these apparent limitations of FZTWs, other methods have potentially been more successful in determining the depth of fault zones. Refracted and reflected P-and S-waves can be used to infer the depth extent and properties of fault zones (Yang & Zhu 2010;Yang et al 2011Yang et al , 2014. These studies have demonstrated that the Calico and San Jacinto fault zones are shallow.…”