Speaker/Affiliation: Mike Oskin, UC Davis
Title: Earthquake-Cycle Modality Revealed by Paleoseismic Inter-Event Time Distributions
When: Friday, January 16 12:00pm PST
Location: EMS B214
Abstract: Time-dependent seismic hazard models predict earthquake likelihood as a function of time elapsed since a prior event. The most widely applied model at present is the Brownian Passage Time (BPT), which is based on the periodic characteristic earthquake model with added noise that leads to aperiodic behavior. Using long paleoseismic records (n≥10 events) we show that time-dependent hazard for plate-boundary strike-slip faults in California and New Zealand is better fit by the Weibull distribution, defined by hazard increasing as a power-law of time. Unlike the BPT model, the Weibull model implies no fixed upper limit to the amount of loading a fault can sustain. Rather the likelihood of a large, surface-rupturing event continually increases with elapsed time and accrued loading since a prior large event. The Weibull model clearly distinguishes between two modes of fault behavior that can be explained by whether a fault tends to produce fewer, large, system-spanning ruptures or more frequent, smaller, partial ruptures. Quasi-characteristic systems such as the Alpine fault exhibit a system-spanning rupture mode, with strongly convex hazard functions (exponent k≥2). We hypothesize that this behavior is sustained by cyclic renewal: As the fault recovers from the previous system-spanning event, greater fault area contributes to the likelihood of rupture through a paleoseismic site. Systems with irregular recurrence such as the San Andreas and San Jacinto faults exhibit a partial-rupture mode, with hazard functions that are linear (k≈1) or decelerating (k<1) but still monotonically increasing (k>0). Events are more frequent on these systems, but most rupture only part of the fault; system-spanning events are rare. One mechanism to explain decelerating hazard on these systems is the survivor effect: Partial-rupture events on adjacent fault sections temper the likelihood of an earthquake initiating and propagating through a site. A second mechanism is super-cycles, when infrequent, system-spanning events are followed by unusually long periods of quiescence before returning to a partial rupture mode. A survey of other long paleoseismic records worldwide shows that the partial-rupture mode and hazard functions with k≤1 are the norm, and that quasi-characteristic behavior is unusual.
