Diverse volumetric seismicity in the Trifurcation area of the San Jacinto fault zone

Yifang Cheng, Zachary E. Ross, & Yehuda Ben-Zion

Submitted August 9, 2017, SCEC Contribution #7462, 2017 SCEC Annual Meeting Poster #201

The trifurcation area of San Jacinto fault zone is highly active, producing over 10% of all Southern California earthquakes during 2000-2016 and frequent M > 3.5 earthquakes with an apparently increasing rate. To understand the seismicity structures in the area, we examine variations of event sizes and faulting types using a relocated catalog and focal mechanism data. Most M > 3.5 events have strike-slip mechanisms, hypocenters within 1 km from the main (Clark, Buck Ridge and Coyote Creek) faults, and depth of 10-13 km. In contrast, many smaller events have normal mechanisms and hypocenters at intra-fault areas (between and with distance > 1 km from the main faults) and depth > 13 km. Off-fault regions (northeast of the Buck Ridge and southwest of the Coyote Creek faults with distance > 0.5 km from the faults) have numerous additional small events with hypocenter depth < 13 km, and they depict complex seismicity structures including lineations normal to the main faults. Five events with M4.7 to M5.4 have similarly high aftershock rate (~100 M > 1.5 events within 1 day following the mainshock). To obtain additional details on aftershock sequences of five M > 4.5 events, we detect and locate additional events with the matched filter method. The augmented aftershock sequences exhibit complex and informative structures. There are almost no aftershocks within 1 km from the mainshocks, consistent with large mainshock stress drops (and low residual stress). The five aftershock sequences have little overlap with each other. Most aftershocks are at intra-fault and off-fault regions; their locations and shapes reflect the mainshock rupture directions and many also follow structures normal to the main faults. The diverse seismicity structures highlight the volumetric and complex nature of faulting in the Trifurcation area. The increasing rate of moderate events, highly productive aftershock sequences and large lapse time since the last major San Jacinto fault zone earthquake, suggest that the region is approaching the next major event.

Cheng, Y., Ross, Z. E., & Ben-Zion, Y. (2017, 08). Diverse volumetric seismicity in the Trifurcation area of the San Jacinto fault zone . Poster Presentation at 2017 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)