Abundant off-fault seismicity and orthogonal structures in the San Jacinto fault zone
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 2016 M_w (moment magnitude) 5.2 Borrego Springs earthquake. In this area, the fault splits into three subparallel strands and is associated with broad V_P/V_S anomalies. 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. The M_w 5.2 Borrego Springs earthquake and past events with M > 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.
Additional Information© 2017 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. Distributed under a Creative Commons Attribution Noncommercial License 4.0 (CC BY-NC). Submitted 16 August 2016; Accepted 9 February 2017; Published 15 March 2017. We thank H. Kanamori for helpful discussions. We also thank D. Shelly and an anonymous reviewer for their constructive comments. Figures were produced with the Generic Mapping Tools. Funding: The study was supported by the NSF (grants EAR-1550704 and EAR-1551411), by the U.S. Geological Survey (USGS)/National Earthquake Hazards Reduction Program (NEHRP) (grants G15AP00095 and G16AP00147), and by the Southern California Earthquake Center, which was funded by NSF Cooperative Agreement EAR-1033462 and USGS Cooperative Agreement G12AC20038. Author contributions: Z.E.R. performed template matching and derived the aftershock locations. E.H. relocated the templates and calculated the focal mechanisms. Z.E.R., E.H., and Y.B.-Z. contributed to the analysis and wrote the article. The authors declare that they have no competing interests. Data and materials availability: All of the raw waveform data are publicly available from the Incorporated Research Institutions for Seismology (IRIS) and the Southern California Earthquake Data Center. SCSN data were provided by the California Institute of Technology/USGS SCSN (doi: 10.7914/SN/CI) and the Southern California Earthquake Center (doi: 10.7909/C3WD3xH1). All other data needed to evaluate the conclusions in the paper are present in the paper and/or the Supplementary Materials. Additional data related to this paper may be requested from the authors.
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Supplemental Material - 1601946_DatasetS1.xlsx
Supplemental Material - 1601946_DatasetS2.xlsx
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Supplemental Material - 1601946_SM.pdf