SCEC Award Number 16114 View PDF
Proposal Category Collaborative Proposal (Integration and Theory)
Proposal Title Magnitude-Frequency Estimates From Paleoseismic Data Using the Stringing Pearls Methodology, San Andreas-San Jacinto Fault System
Investigator(s)
Name Organization
Glenn Biasi University of Nevada, Reno Thomas Rockwell San Diego State University
Other Participants
SCEC Priorities 2e, 4e, 6b SCEC Groups WGCEP, SoSAFE, Geology
Report Due Date 03/15/2017 Date Report Submitted 05/08/2017
Project Abstract
 We apply the “Stringing Pearls” methodology of Biasi and Weldon (2009) to new paleoseismic data of the San Andreas fault and a new application to the joint San Andreas-San Jacinto system. The low fault slip rates of the SAF along the reach from southern San Bernardino to the northern Imperial Valley, plus the relatively short earthquake recurrence intervals, make it unlikely that large, through-going ruptures occur commonly, and the data do not require them at all. Similarly, the relatively low fault slip rate of the northern 100 km of the San Jacinto fault make it unlikely that ruptures frequently include both the SAF and SJF, and the data do not require any. On the SJF, rupture sets fitting the slip data best focus on ruptures of 25 to 70 km, and only rarely up to ~120 km. These correspond to earthquakes in the M6.5 to 7.4 range. By contrast, UCERF3 would have most ruptures be much larger, M7.5 and up. The stringing pearls methodology thus brings independent data of event correlations and frequency that comprise an independent test of earthquake magnitude and frequency on the two highest slip rate faults of southern California.
Intellectual Merit Paleoseismic site records provide point estimates of the frequency of ground rupture, but do not provide the temporal resolution necessary to definitely link evidence from site to site, and thus make inferences about earthquake magnitude. The stringing pearls methodology provides an alternative approach, whereby ensembles of possible ruptures that could explain the field data are analyzed for useful and diagnostic features. A reanalysis of the San Andreas fault using new and revised paleoseismic data shows that if fault slip rates are accepted, 1857-style earthquakes are not normative for the SAF, and may have occurred only 2-3 times in the most recent 1200 years. The reanalysis also confirms that strike-slip ruptures through the southern San Bernardino to northern Imperial Valley part of the fault should be rare or non-existent. Such ruptures would be accompanied by large rupture displacements, which combined with earthquake recurrence rates at the paleoseismic sites, would imply strike-slip rates well above what is observed. Our method is based on linear strike-slip ruptures, and cannot be used to rule out other types of accommodations, such as thrusting. Through-going ruptures including the SAF and northern 100 km of the SJF likewise appear to be rare. Fault slip rates here are low, and when combined with paleoseismic slip rates, indicate average displacements are more consistent with M 6.5-7 than the M7.5+ events preferred by the UCERF3 model. Thus, the alternate view provided by stringing pearls shows that the Frazier Mountain data and the SJF Mystic Lake and Hog Lake records argue for more frequent, moderate to low M7 events over the last 1200 years, compare to the less frequent, much larger events that dominate UCERF3.
Broader Impacts This project developed a new collaboration between SDSU and UNR, which brings together the paleoseismic strength of SDSU (Rockwell's group) with the analytical strength of UNR (Biasi). This project focused on a major weakness in UCERF3 - the over-prediction of large earthquakes on the San Jacinto and San Andreas faults.
Exemplary Figure Figure 3. Stringing Pearls scenario for the San Jacinto fault; this is the best fit for displacement among 500 trials, with most of the misfit due to the northern 100 km, suggesting that ruptures involving both the San Jacinto and San Andreas faults are rare.