Clustering of earthquakes in the southern San Andreas Fault System based on 1100 years of paleo-earthquakes: Is the 200 year open interval for large earthquakes unusual?

Thomas K Rockwell

Tom Rockwell
San Diego State University

Wednesday, February 15, 2017
CSL  422 – 1 pm

Compilation of paleoseismic data from several dozen trench sites in the southern San Andreas fault system, along with geomorphic observations of displacement in recent and past earthquakes, allows for sequencing of the past 1100 years of large earthquakes for the southern 160 km of the main plate boundary faults. At least four generalizations are clear: 1) M7 and larger earthquakes account for most of the moment release in the southern San Andreas fault system over the past 1100 years; 2) large earthquakes on individual faults are quasi-periodic but display a relatively high coefficient of variation in recurrence time, similar to most long California paleoseismic records, and which may be a reflection of Coulomb stress interactions; 3) moment release has temporally varied during the past 1100 years but within potentially predictable bounds, and earthquakes appear to have occurred in clusters; and 4) the southern San Andreas fault system is currently moment deficit when compared to the previous millennium of moment release. Together, the record suggests that the southern San Andreas fault is late in the cycle but not necessarily “overdue”, and that a systems level approach may be more accurate in long-term earthquake forecasting than estimates made from individual faults. If the past is the key to the future, then the next century is likely to see far more large earthquake activity on the southern San Andreas fault system than was observed in the past two centuries.