Collaborative Research: Multi-cycle Rupture History of the San Andreas Fault in the Carrizo Plain

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One of the most basic questions in earthquake physics, earthquake hazards, and neotectonics is "What controls the space-time pattern of seismicity?" A more fundamental question that must be answered first is "What is the space-time pattern of seismicity?" This question can only be answered by observing fault behavior at multiple temporal and spatial scales. Paleoseismic investigations provide data on the multi-cycle rupture history of a portion of a fault. The San Andreas fault is the largest source of seismic hazard in California. Data on the rupture history of the San Andreas fault form the basis of numerous models of fault behavior and seismic hazard. The high slip rate, short recurrence interval, great length and accessibility of the San Andreas fault make it the best target in the U.S. (and perhaps the world) for observational testing of fault behavior models. This project involves collecting a multi-cycle rupture history from a proven paleoseismic site along one of the most scientifically important and hazardous faults in the world: the San Andreas fault. There are only 4 sites along the southern San Andreas fault that have a rupture chronology of at least 5 events. One of these sites is the Bidart Fan in the Carrizo Plain. The Carrizo Plain is one of the best places to study the rupture history of the SAF because it has a proven paleoseismic record with excellent slip rate and slip per event measurements, and the potential to significantly enlarge the data set by increasing the length of the earthquake record. Previous work at the Bidart Fan shows that it contains a lengthy, datable record of surface ruptures. A total of four trenches have been excavated in the SAF zone on the Bidart Fan. These trenches yield a composite chronology consisting of 10 surface ruptures of the Bidart Fan over the last 3,000 years. The most recent 5 earthquakes are reasonably well dated, but uncertainty in the dates of the five earlier ruptures is much greater. Therefore, these five early earthquakes are the primary target of this project. Additional trenches placed across the Bidart Fan are revealing a better spatial and temporal record of deposition across the fan, leading to improved dates of previously documented earthquakes. Radiocarbon dates of samples will be obtained from University of California, Irvine's new Keck Accelerator Mass Spectrometry facility devoted to carbon isotopes using improvements in the application of radiocarbon dating to paleoseismology made at the Lawrence Livermore National Laboratory. This project includes a similar effort to improve the event record in the Carrizo Plain. It is doubling the temporal record of surface ruptures in the Carrizo Plain and providing a major paleoseismic site on the San Andreas fault for developing and testing models of fault behavior.