Geodetic Evidence for a Blind Fault Segment at the Southern End of the San Jacinto Fault Zone

Abstract

The San Jacinto Fault (SJF) splits into several active branches southeast of Anza, including the Clark fault and the Coyote Creek fault. The Clark fault, originally believed to terminate at the southern tip of the Santa Rosa Mountains, was suggested to extend further to the southeast to a junction with the Superstition Hills fault based on space geodetic observations and geologic mapping. We present new interferometric synthetic aperture radar and GPS data that confirm high deformation rates along the southeastern extent of the Clark fault. We derive maps of horizontal and vertical average velocities by combining data from the ascending and descending satellite orbits with an additional constraint provided by the azimuth of the horizontal component of secular velocities from GPS data. The resulting high‐resolution surface velocities are differentiated to obtain a map of maximum shear strain rate. Joint inversions of InSAR and GPS data suggest that the hypothesized blind segment of the Clark fault and the Coyote Creek fault have slip rates of 13 ± 3 mm/yr and 5 ± 4 mm/yr, respectively. The blind southern segment of the Clark fault thus appears to be the main active strand of the SJF, posing a currently unrecognized seismic hazard.

Slow Slip Event On the Southern San Andreas Fault Triggered by the 2017 Mw8.2 Chiapas (Mexico) Earthquake

“…The slip depths suggested by our inversions are comparable to the range of thicknesses of the sedimentary layer and the depth extent of creep reported by Sieh and Williams (1990). The inversions also suggest systematic increases in the fault dip angle from the Coachella Valley toward the Bombay Beach, consistent with models of Tymofyeyeva and Fialko (2018). At Box Canyon, inversions suggest a dip angle as small as ∼50 • and depth of slip of 0.5 km (Figure 10a).…”

“…The inversions also suggest systematic increases in the fault dip angle from the Coachella Valley toward the Bombay Beach, consistent with models of Tymofyeyeva and Fialko (2018). At Box Canyon, inversions suggest a dip angle as small as ∼50 • and depth of slip of 0.5 km (Figure 10a).…”

“…This is in good agreement with the average surface creep velocities obtained from the Envisat Interferometric Synthetic Aperture Radar (InSAR) observations spanning 2004–2010 (Lindsey, Fialko, et al, ), suggesting that the average rate of shallow creep on the SSAF may be essentially constant over time scales on the order of 10 years or greater. The average value of the surface creep rate of a few millimeters per year is consistent with the estimated slip rate on the SSAF of ∼18 mm/year (Behr et al, ; van der Woerd et al, ; Lindsey & Fialko, ), provided the depth extent of shallow creep is on the order of a few kilometers (Kaneko et al, ; Li & Rice, ; Lindsey & Fialko, ; Savage & Lisowski, ; Tymofyeyeva & Fialko, ; Wei et al, ).…”

Characteristics of Frequent Dynamic Triggering of Microearthquakes in Southern California

“…On the deeper part of the faults, these depth distributions suggest a possible correlation with triggered, transient aseismic creep events that have been measured with seismic and geodetic data (Inbal et al, 2017;Lohman & McGuire, 2007;Tymofyeyeva & Fialko, 2018;Wdowinski, 2009). Transient creeping events can cause an instantaneous or delayed-dynamic triggering of earthquakes or tremor (e.g., Inbal et al, 2017;Shelly et al, 2011), and such creep events might also temporarily alter the effective loading rates, reduc-FAN ET AL.…”

Spatial Variations of Upper Crustal Anisotropy Along the San Jacinto Fault Zone in Southern California: Constraints From Shear Wave Splitting Analysis

“…SWS technique to investigate the spatial distribution and possible temporal variation of crustal anisotropy in the seismogenic zone associated with active strike slip faults (Mizuno et al, 2005). Both the BRF and CF are right-lateral strike-slip faults dipping toward the NNE (Figure 1b; Ross et al, 2017;Sharp, 1967), with a strike of about 115° (WNW-ESE) counted clockwise from the North and a GPS-determined slipping rate of 10-16 mm/year for the CF (Tymofyeyeva & Fialko, 2018) and 3.4-4 mm/year for the BRF (Onderdonk et al, 2015). The direction of SHmax determined by earthquake focal mechanisms is N-S (Heidbach et al, 2018).…”