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White Paper: The California continental borderland

March 2020

By Jillian Maloney, Mark Legg, Craig Nicholson, and Thomas Rockwell

The California Continental Borderland (CCB) offshore southern California accommodates ~20% of Pacific-North American plate motion. It has experienced several M5+ events and hosts long, continuous fault zones capable of generating M7+ earthquakes and potentially tsunamis. Expansion of SCEC’s natural laboratory to include the CCB would address questions about long-term and recent plate boundary evolution and how current plate motion is distributed across the broad zone of faulting of the plate boundary in southern California. 

The highly complex structure of the CCB, and its location offshore, provide a unique opportunity to study step-over geometry, multi-segment ruptures, and the role these boundaries play in rupture initiation and termination. Resolving the hypotheses for multi-segment ruptures, particularly in the coastal Newport Inglewood-Rose Canyon fault zone, will be crucial for seismic hazard assessment for the densely populated coastal zones of Los Angeles, San Diego, and Tijuana, Mexico. The CCB is a region of deposition, so a more complete record of plate boundary deformation and fault interaction is preserved, as well as a dated syntectonic stratigraphy that can document dates and rates of active crustal deformation. Additionally, marine seismic methods are more efficient at mapping fault zone geometry over large distances compared to onshore methods. Nevertheless, the scarcity of slip rates and dearth of paleoseismic data for faults in the CCB mean that the recurrence intervals, dates of most recent events, event size, and pre-historic rupture patterns are mostly unknown. Evaluating the timing of events would contribute to improved understanding of earthquake cycles and transfer of slip between fault segments with variable geometry. Furthermore, differences between onshore and offshore regions of southern California are important for models of earthquake processes across the Pacific-North American plate boundary. 

Advancing research on faulting and earthquakes in the CCB will require [a] higher density and improved quality seismic reflection and bathymetric data (to map fault structures and related syntectonic stratigraphy, identify active faults, quantify slip rates, and determine paleoseismic history); [b] additional seismometers on offshore islands and ocean-bottom seismometers (to provide data for determining accurate 3D seismic velocity structure offshore and more precise location of seismicity and depth of faulting); [c] increased emphasis of onshore-offshore seismic imaging (to properly resolve the nature and geometry of coastal fault zones); and [d] more investigations on spatial and temporal seismicity patterns using new methods or improved offshore infrastructure (to identify repeating events or possible precursory activity to larger events).

→ Download the full white paper and response to NSF20-036 DCL.

Jillian Maloney is an assistant professor at San Diego State University. Her research seeks to understand tectonics and sediment processes on and beneath the seafloor. In particular, she investigates submarine earthquakes and landslides, which can present hazards to human life and infrastructure.

Mark Legg is the president of Mark Legg Geophysical, who specializes in strike-slip faulting and tectonics, offshore earthquakes and tsunamis, and risk assessment.

Craig Nicholson is a research geophysicist with the Marine Science Institute and adjunct professor of the Department of Earth Science at the University of California, Santa Barbara. His primary areas of research are subsurface imaging, earthquakes, active faulting, crustal structure, and tectonics.

Thomas Rockwell is a professor in the Department of Geological Sciences at San Diego State University, specializing in paleoseismology and neotectonics. He serves as an expert for local and global consulting companies on earthquake issues related to specific construction or modernization sites. He has been a member of the SCEC Board of Directors since 2015 and served on the SCEC Science Planning Committee before.


Return to the 2020 SCEC Community White Papers.