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Potential for large complex multi-fault earthquakes offshore southern California

Mark R. Legg, Christopher C. Sorlien, Craig Nicholson, Marc J. Kamerling, & Gerald G. Kuhn

Accepted June 15, 2018, SCEC Contribution #8123

The recent 2010 M7.0 El Mayor-Cucapah and 2016 M7.8 Kaikoura events emphasized that some major (M>7) earthquakes can be quite complex. Similar interacting high- and low-angle fault geometry exists on the major fault systems in the offshore Borderland region of southern California owing to structures developed during the middle Tertiary initiation and evolution of the Pacific-North America plate boundary. Oblique-rifting of the Western Transverse Ranges and Outer Borderland crustal blocks from the North America continental margin created the Inner Borderland Rift with major low-angle oblique normal faults (detachments) that may have reactivated former subduction thrust faults. New high-angle strike-slip faults were created in the rifted margin to accommodate the northwest translation of captured microplates and overriding continental crust. Offshore southern California, the Coastal fault zone includes complex 3-D fault geometry with widespread low-angle faults in the middle to lower crust that may link more steeply-dipping, segmented shallow crustal faults to produce large complex rupture sequences. This coastal system of segmented shallow crustal fault zones follows the initial breakaway for the rifted Western Transverse Ranges and Outer Borderland crustal blocks. Low-angle faults at seismogenic depths may be locked, allowing elastic strain energy to accumulate between large earthquakes. Moderate earthquakes, e.g., 1933 Long Beach (M6.3), may rupture individual segments, but paleoseismic data indicate the potential for infrequent large rupture events, which may include secondary faults along the coast. Although low slip rates reduce the probability of such events, fault proximity to major cities, harbors and associated infrastructure may produce significant risk for severe losses from potential large, complex multi-fault earthquakes.

Key Words
earthquake, coastal, California

Legg, M. R., Sorlien, C. C., Nicholson, C., Kamerling, M. J., & Kuhn, G. G. (2018, 06). Potential for large complex multi-fault earthquakes offshore southern California. Poster Presentation at 11th National Conference on Earthquake Engineering.

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