SCEC Award Number 14158 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Prospective test of earthquake rupture rates implied by paleoå_-seismic data for California
Investigator(s)
Name Organization
David Jackson University of California, Los Angeles
Other Participants Anne Strader
SCEC Priorities 2a, 2e, 4e SCEC Groups EFP, WGCEP, CSEP
Report Due Date 03/15/2015 Date Report Submitted N/A
Project Abstract
 Evidence for quasi-periodic recurrence comes largely from California paleo-seismic data, but these data are problematical. A team of experts compiled the most reliable data for use in the third Uniform California Earthquake Rupture Forecast (UCERF3). They reported dates of observed displacements at 32 sites on 13 named faults in California. The problem: recorded paleo-seismic events ceased at about the beginning of the instrumental seismic era, a result inconsistent with the inferred and modeled rates before that time.

The reported event rates for the ensemble of 31 sites sums to about 0.1 per year. Allowing generously for occurrences of earthquakes that rupture multiple sites simultaneously, the event rate is on the order of 0.04 per year. Yet the most recent paleo-event date is 1916. Such a long open interval would be extremely unlikely for a Poisson process and even less probable for an ensemble of quasi-period processes.

Possible explanations for the discrepancy include (1) extreme luck, (2) unexplained regional fault interaction, or (3) mistaken identification of near-surface displacements as evidence of large earthquakes. The first can be rejected with 99% confidence. There is no evidence for the second in the pre-1916 paleo-seismic history nor in any theoretical models yet published. The third could explain the observed quiescence because mistaken identity would be prevented by instrumental seismic data. In any case the inferred rates cannot be taken as long-term averages, and the implied recurrence can’t be trusted for the next century because they fail the last one.
Intellectual Merit SCEC develops earthquake probability models for use in hazard estimates and planning for future earthquakes. UCERF3 is a good example. In these projects, geological and seismological data are combined to give long-term earthquake rates. Internal consistency of the geologic (paleo-seismic) data has not before been examined comprehensively. Using techniques developed for prospective testing by CSEP, we examined tested the whole set of paleoseismic dates used in the UCERF3 report. The hiatus of the last century points to remarkable statewide clustering not previously recognized and not yet modeled, or to inconsistencies that could require corrections to UCERF3 earthquake rates.
Broader Impacts The project has involved graduate students Anne Stader and Deborah Weiser, both of whom developed important testing and statistical skills for their future careers.
Presntation of results at SCEC meetings and AGU and SSA conferences has stimulated dialog among seismologists, geologists, and computational modelers of earthquake occurrence and interaction.
Exemplary Figure Figure 1: Cumulative events vs. time since year 1060 for paleoseismic sites Santa Cruz, Wrightwood, Hayward S., Hog Lake, and Elsinore-Temecula. Adding results for other sites would increase the rate of events before 1916, but no other site had events after 1916. The hiatus after 1916 is unexplained.