Newly Detected Strain Transient on the Anza Segment of the San Jacinto Fault Zone, Southern California

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

Published July 31, 2019, SCEC Contribution #9303, 2019 SCEC Annual Meeting Poster #193

The Anza segment of the San Jacinto fault zone (SJFZ) in southern California contains a ~20 km long seismic gap that presents a future earthquake hazard. Inbal et al. (2017) identified two episodes of deep (>10 km locking depth) aseismic slip on the SJFZ near Anza following the April 4, 2010 regional Mw 7.2 El Mayor-Cucapah earthquake and a July 7, 2010 local Mw 5.4 earthquake. Agnew et al. (2013) also identified a deep strain transient in this region following a March 11, 2013 local Mw 4.7 earthquake. Each of these transients were identified on borehole strainmeters (BSMs) and appear to increase with log(time). Cataloging and quantifying these deep creep events is important for understanding the interaction of slow and fast slip. Combining seismic and BSM data we have identified a new strain transient in June 2016. We perform cross-correlations between log(time) template functions of varying length and each of the strain components of 5 BSMs to detect potential strain transients. A peak in absolute cross-correlation is identified on four BSMs around June 10, 2016 that may extend for several weeks. A significant increase in seismicity (mostly 8-9 km depth) occurs in the region starting on June 10, 2016. We perform template matching using 938 earthquakes from the Southern California Seismic Network (SCSN) catalog in this region on SCSN and Anza Seismic Network seismic stations. Using a minimum average correlation coefficient of 0.90 at a minimum of 3 stations and template lengths of 5 seconds we find 17 families of “burst-type” repeating earthquakes from June 10 – June 21, 2016. “Burst-type” repeating earthquakes have nearly identical waveforms and occur closely in time with irregular repeat intervals and can be an indicator of aseismic slip. Depths of these "burst-type" repeating earthquakes range from 9.5-13 km, with an average depth of 11.5 km. The log(time) BSM detection, seismicity increase, and "burst-type" repeating earthquakes are strong evidence of an aseismic transient on the Anza segment of the SJFZ starting on June 10, 2016 at greater than 10 km depth. Based on timing and proximity, it is likely that the local June 10, 2016 Mw 5.2 earthquake triggered the strain transient. Dynamic triggering of slow slip events from large regional and teleseismic events has been documented. Our results suggest that moderate-sized local earthquakes may also be an important trigger of slow slip.

Key Words
Aseismic slip, repeating earthquakes

Citation
Shaddox, H., Schwartz, S. Y., & Bartlow, N. M. (2019, 07). Newly Detected Strain Transient on the Anza Segment of the San Jacinto Fault Zone, Southern California . Poster Presentation at 2019 SCEC Annual Meeting.

Related Projects & Working Groups
Stress and Deformation Over Time (SDOT)