The Current Unlikely Earthquake Hiatus at California’s Transform Boundary Paleoseismic Sites

Glenn P. Biasi, & Katherine M. Scharer

Submitted August 14, 2018, SCEC Contribution #8601, 2018 SCEC Annual Meeting Poster #264

Paleoseismic and historic earthquake records provide data used to quantify earthquake recurrence rates for seismic hazard estimation. Earthquake recurrence rates are equally useful to test the likelihood of seismically quiescent periods. At principal paleoseismic sites in California on the San Andreas, San Jacinto and Hayward Faults, no ground rupturing earthquake has occurred in the last 100 years, or about three times the average inter-earthquake time for the ensemble of sites. We examine long paleoseismic records from those faults, as they carry most of the transform fault slip on the plate boundary, to see if the hiatus has any precedent in the last 1000 years. The selection of sites is designed to sample fault sections unlikely to have ruptured together, so their conditional probabilities of a hiatus can be combined as independent events. The current 100-year hiatus is not predicted by common time-dependent or time-independent recurrence models. Paleoearthquake dating uncertainties can allow long open intervals at individual sites or subsets of sites, but do not explain the observed gap in the ensemble. After approximately removing redundancies in the full paleoearthquake record, the time-independent probability of the current 100-year gap is of order 0.3%. This raises several questions. Do we live in a statistically exceptional time? Or does some wide-scale effect modulate earthquake occurrence among sites over longer timescales? Finally, how should we understand seismic hazard estimates in California if the recurrence models on which they rely seem, at least, incomplete? Whether or not some longer-term modulation of earthquakes on the transform fault ensemble we investigate is operating in California, the results emphasize that the last century has been exceptional.

Key Words
paleoseismology; earthquake recurrence; UCERF3

Citation
Biasi, G. P., & Scharer, K. M. (2018, 08). The Current Unlikely Earthquake Hiatus at California’s Transform Boundary Paleoseismic Sites. Poster Presentation at 2018 SCEC Annual Meeting.


Related Projects & Working Groups
San Andreas Fault System (SAFS)