Triggered and Spontaneous Slow Slip Transients on the Anza Segment of the San Jacinto Fault Zone, Southern California

Heather R. Shaddox, Susan Y. Schwartz, & Noel M. Bartlow

Submitted June 27, 2020, SCEC Contribution #10150

Understanding the interplay of seismic and aseismic slip is key in seismic hazard evaluation. It is particularly important to know if the same fault can host different slip modes and understand the transition between modes of slip. We investigate this in the trifurcation area of the Anza segment of the San Jacinto fault where deep creep driving seismicity below the geodetic locking depth has been proposed. We focus on local moderate-sized earthquakes triggering slow slip transients by combining the occurrence of burst-type repeating earthquakes, seismicity, and borehole strainmeter data. We find that all Mw > 4.5 earthquakes from 2010-2020 triggered slow slip transients with coplanar burst-type repeating earthquakes and off-fault microseismicity. These observations include newly detected triggered slow slip transients following the June 10, 2016 Mw 5.2 Borrego Springs earthquake and the April 4, 2020 Mw 4.9 earthquake. Slow slip geometries defined by burst-type repeating earthquakes are consistent with strain change observations and reveal multiple active planes both on and off of the triggering earthquake fault. We conclude that burst-type repeating earthquakes, similar to low-frequency earthquakes, are useful indicators of slow slip transients and can reveal faulting complexities during slow slip. Further, we identify the first evidence of spontaneous slow slip in this region from burst-type repeating families on two minor faults in 2015. Taken together, our observations support a model where deep microseismicity is located in a transitional region at the bottom of the seismogenic zone with spatially heterogeneous frictional properties that produces frequent slow slip transients.

Shaddox, H. R., Schwartz, S. Y., & Bartlow, N. M. (2020). Triggered and Spontaneous Slow Slip Transients on the Anza Segment of the San Jacinto Fault Zone, Southern California. Journal of Geophysical Research, (submitted).