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An Argument for Time-dependent National Seismic Hazard Maps

The U.S. Geological Survey National Seismic Hazard Maps depict time-independent seismic hazard. Hence current seismic design practice as reflected in the resulting International Building Code is based on time-independent probabilistic seismic hazard analyses. We argue that the National Seismic Hazard Maps should consider time-dependent hazard when paleoseismic records are sufficient to account for the elapsed times on faults. This is particularly critical if the elapsed times exceed the mean recurrence intervals suggesting that a large earthquake may be eminent. However, even when such fault recurrence information is not available, we believe some form of time-dependent hazard should be considered in the National Seismic Hazard Maps considering non-Poissonian recurrence (e.g., Biasi and Thompson, 2018). The key to an accurate time-dependent seismic hazard assessment for a fault is information on its mean recurrence interval, elapsed time since the most recent earthquake, and their uncertainties. Although such information is generally unavailable for most faults in the U.S., it is available for some of the most hazardous regions in the U.S. including along the San Andreas fault system, the Wasatch fault zone, the Cascadia subduction zone, and the New Madrid and Charleston seismic zones. We illustrate the time-dependent hazard using the forecast model of the Working Group on Utah Earthquake Probabilities for the Wasatch Front. The seismic hazard in this region is generally dominated by the five central segments of the 350-km- long Wasatch fault zone, which has ruptured repeatedly in M 6.8 and larger earthquakes. We illustrate the differences in time-independent and time-dependent hazard for three cities along the Wasatch Front where the mean recurrence intervals equal, exceed, or are a fraction of the elapsed time since the last large earthquake and compare these results with the National Seismic Hazard Maps.

Session: 50-State Update of the USGS National Seismic Hazard Models

Type: Oral

Room: Grand E-K

Date: 4/20/2022

Presentation Time: 09:00 AM Pacific

Presenting Author: Ivan G. Wong

Student Presenter: No

Additional Authors