title: Abundant off-fault seismicity and orthogonal structures in the San Jacinto fault zone.
abstract: The trifurcation area of the San Jacinto fault zone has produced more than 10% of all earthquakes in southern California since 2000, including the June 2016Mw(moment magnitude) 5.2 Borrego Springs earthquake. In this area, the fault splits into three subparallel strands and is associated with broadVP/VSanomalies. We synthesize spatiotemporal properties of historical background seismicity and aftershocks of the June 2016 event. A template matching technique is used to detect and locate more than 23,000 aftershocks, which illuminate highly complex active fault structures in conjunction with a high-resolution regional catalog. The hypocenters form dipping seismicity lineations both along strike and nearly orthogonal to the main fault, and are composed of interlaced strike-slip and normal faults. The primary faults change dip with depth and become listric by transitioning to a dip of ~70° near a depth of 10 km. TheMw5.2 Borrego Springs earthquake and past events withM> 4.0 occurred on the main faults, whereas most of the low-magnitude events are located in a damage zone (several kilometers wide) at seismogenic depths. The lack of significant low-magnitude seismicity on the main fault traces suggests that they do not creep. The very high rate of aftershocks likely reflects the large geometrical fault complexity and perhaps a relatively high stress due to a significant length of time elapsed since the last major event. The results provide important insights into the physics of faulting near the brittle-ductile transition.
authors.affiliation: Seismological Laboratory, California Institute of Technology, Pasadena, CA 91125, USA.
authors.forename: Zachary E
authors.identifier: 0000-0002-6343-8400
authors.initials: ZE
authors.lastname: Ross
authors.affiliation: Seismological Laboratory, California Institute of Technology, Pasadena, CA 91125, USA.
authors.forename: Egill
authors.identifier: 0000-0002-6834-5051
authors.initials: E
authors.lastname: Hauksson
authors.affiliation: Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089, USA.
authors.forename: Yehuda
authors.initials: Y
authors.lastname: Ben-Zion
keywords: Fault zone structures; San Jacinto fault; Seismicity; damage zones; earthquake detection; fault geometry; template matching