A predictive model for earthquake rupture extents given an early warning epicenter

Steven G. Wesnousky, & Glenn P. Biasi

Published August 13, 2017, SCEC Contribution #7547, 2017 SCEC Annual Meeting Poster #130

Steps and bends in faults affect the probability that rupture will continue to propagate along a fault. Empirical relationships for rupture through individual step and bend features have been developed through examination of past surface ruptures (Biasi and Wesnousky, 2016; 2017). Given an event epicenter from an earthquake early warning (EEW) system or from a foreshock, bend and step relationships can be used to modify expected values of rupture length. For an EEW system, these values provide a probabilistic answer to the question of how far rupture could propagate given a first-alert event location.

The 2013 Uniform California Earthquake Rupture Forecast version 3 (UCERF3) provides rates of all ground-rupturing earthquakes considered possible on the known faults. Given a starting location, the list of possible ruptures reduces to the smaller set of ruptures that could start at that point. This subset of UCERF3 rupture rates provides a baseline distribution for probability of length given a hypocenter.

Because UCERF3 did not use the relative physical resistance to rupture propagation posed by bends and steps in the inversion for rupture rates, modifying rates to include bend and step effects adds information. This approach assumes a degree of linearity (local stability) in the UCERF3 solution; comprehensive application of bend and step effects would require a complete reinversion for rupture rates.

As an example, empirical data suggest a step passing ratio PR summarized as PR = 1.89 – 0.31x, where x is the step width at the surface. From a hypocenter, ruptures crossing steps decrease in probability of occurrence compared to UCERF3 where the step is not considered. Bends in fault traces have a similar effect. For example, in UCERF3 743 ruptures have one end at Bombay Beach on the SE end of the San Andreas fault (SAF). Long ruptures starting at Bombay Beach are strongly reduced in relative probability by fault bend and step effects. Ruptures originating on the San Jacinto fault have similar qualities, but the SJF is less continuous, and long ruptures decrease most in probability due to fault steps. For lengthy ruptures, probability decreases can reach several orders of magnitude relative to shorter ruptures. Bend and step effects push UCERF3 subsection magnitude-frequency distributions toward more traditional geologically estimated ranges.

Key Words
seismic hazard

Citation
Wesnousky, S. G., & Biasi, G. P. (2017, 08). A predictive model for earthquake rupture extents given an early warning epicenter. Poster Presentation at 2017 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology