Collaborative Research: Slip-per-event Rupture History of the San Andreas Fault in the Carrizo Plain: Was the 1857 Earthquake Characteristic?

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On January 9, 1857 the magnitude 7.8 Fort Tejon earthquake ruptured a 400 km long section of the southern San Andreas Fault including the Carrizo section. The Carrizo section of the San Andreas Fault has been proposed to rupture only during the largest earthquakes, and thereby to control the occurrence of great characteristic earthquakes on the southern San Andreas Fault with centuries-long recurrence times in the Carrizo Plain. Recent findings suggest a more complex rupture pattern with significantly shorter recurrence times and possibly clustered earthquakes. Reported measurements of slip-per-event for the last 5 or 6 earthquakes, when combined with new dates of earthquakes, yield slip rates up to 50 mm/yr, well above widely accepted values of approximately 35 mm/yr. The mismatch suggests that either the strain released is not directly proportional to the time between ruptures, thereby casting doubt on time-predictable and slip-predictable elastic strain models, or some of the reported slip-per-event measurements represent cumulative slip for multiple events. Resolution of this conundrum should lead to fundamental insights into fault mechanics, as well as more effective methods for forecasting future large earthquakes. The goal of this project is to measure the slip from several recent well-dated ruptures of the San Andreas Fault in the Carrizo Plain. The challenge is in linking slip measurements with dates of earthquakes. Several methods will be applied, including three-dimensional paleoseismic excavation of the Bidart fan site, high precision radiocarbon dating, analysis of geomorphic offsets using high resolution airborne laser swath mapping data, and compilation of archival field data from unpublished studies. Results of this research could have significant impact on seismic hazard mitigation and planning for major earthquakes. The San Andreas Fault is the major source of seismic hazard and risk in the densely populated and economically important state of California, and its proven potential to generate a large earthquake makes it a significant threat to the nation. Data from this research will help to assess the probability, and forecast the location and magnitude of the next damaging San Andreas earthquake in southern California.