Is the Southern San Andreas Fault Really Overdue For a Large Earthquake or Just Late in the Cycle?

Thomas K. Rockwell

Published May 31, 2016, SCEC Contribution #6291

Compilation of paleoseismic data from several dozen trench sites in the southern San Andreas fault system (Figure 1), along with more precise dating of Lake Cahuilla sediments that cross many of these sites, allows for sequencing of the past 1100 years of large (M6.5 and larger) earthquakes for the southern 150 km of the main plate boundary system. Major faults capable of larger earthquakes include the San Andreas, San Jacinto, Elsinore, Imperial, Cerro Prieto, Laguna Salada, and possibly the Earthquake Valley faults. Displacement data have been generated for most of these faults for the past one to several events. Using these observations on timing and displacement in past large earthquakes, and assuming reasonable seismogenic thicknesses, estimates of moment release through time can be made. Based on these estimates, at least three 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 records; and 3) moment release has temporally varied during the past 1100 years but within potentially predictable bounds. A forth observation is that inundation of Lake Cahuilla may have triggered some large earthquakes, as previously suggested, and that the lack of a lake in the past 300 years may partially explain the relatively long quiecence of the southern San Andreas fault system. 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 data generated from a single element of the fault system.

Rockwell, T. K. (2016, 05). Is the Southern San Andreas Fault Really Overdue For a Large Earthquake or Just Late in the Cycle?. Presentation at The 7th International INQUA Workshop on Paleoseismology, Active Tectonics and Archaeoseismology.