USGS National Seismic Hazard Maps

Abstract

The U.S. Geological Survey (USGS) recently completed new probabilistic seismic hazard maps for the United States, including Alaska and Hawaii. These hazard maps form the basis of the probabilistic component of the design maps used in the 1997 edition of the NEHRP Recommended Provisions for Seismic Regulations for New Buildings and Other Structures, prepared by the Building Seismic Safety Council and published by FEMA. The hazard maps depict peak horizontal ground acceleration and spectral response at 0.2, 0.3, and 1.0 sec periods, with 10%, 5%, and 2% probabilities of exceedance in 50 years, corresponding to return times of about 500, 1000, and 2500 years, respectively. In this paper we outline the methodology used to construct the hazard maps. There are three basic components to the maps. First, we use spatially smoothed historic seismicity as one portion of the hazard calculation. In this model, we apply the general observation that moderate and large earthquakes tend to occur near areas of previous small or moderate events, with some notable exceptions. Second, we consider large background source zones based on broad geologic criteria to quantify hazard in areas with little or no historic seismicity, but with the potential for generating large events. Third, we include the hazard from specific fault sources. We use about 450 faults in the western United States (WUS) and derive recurrence times from either geologic slip rates or the dating of pre-historic earthquakes from trenching of faults or other paleoseismic methods. Recurrence estimates for large earthquakes in New Madrid and Charleston, South Carolina, were taken from recent paleoliquefaction studies. We used logic trees to incorporate different seismicity models, fault recurrence models, Cascadia great earthquake scenarios, and ground-motion attenuation relations. We present disaggregation plots showing the contribution to hazard at four cities from potential earthquakes with various magnitudes and distances.

Earthquake Likelihood Model Testing

“…We performed all tests (L-Test, N-Test, and R-Test) with two models. The first model H 0 is derived from the USGS 1996 model [Frankel et al, 1996] and the second model H 1 is a stationary model designed by Helmstetter et al [submitted]. We tested these models against the RELM catalog provided by Yan Kagan (http://moho.ess.ucla.edu/~kagan/relm_ index.html) as the observation.…”

“…The main model H 0 for the tests is a stationary grid-based model derived from the USGS 1996 model [Frankel et al, 1996]. We have derived the daily background rate from the long-term rate of this model, interpolated the characteristic fault information onto our grid, and extrapolated all rates Λ 0 into ∆M = 0.1 magnitude bins down to magnitude M = 5 using the Gutenberg-Richter relationship.…”

“…The characteristic fault information was interpolated to the given grid by: 1) projecting the fault to the surface, 2) distributing a large number of points over the surface projection of the fault, 3) counting the number of points that fell within a grid node, and 4) assigning the appropriate percentage of rates to each grid node, based on the percentage of overall points that fell within the node. The nodewise sums of expectations λ [Frankel et al, 1996]). …”

Initiation propagation and termination of elastodynamic ruptures associated with segmentation of faults and shaking hazard

“…[20] Figure 5b shows the peak velocity for the same probability of occurrence as in Figure 5a (the analogue of the 2% probability of exceedance in 50 year national hazard maps [Frankel et al, 2000] (see also http://eqhazmaps. usgs.gov)).…”

Forecasting the Locations of Future Large Earthquakes: An Analysis and Verification

“…Many such forecasts have been carried out, one example is the National Seismic Hazard Map for the United States (FRANKEL et al, 1996). This is a probabilistic estimate of the ground-shaking risk.…”