Documentation

Introduction

New seismic hazard maps have been prepared by the USGS for the conterminous United States. In June, 1996, these maps were placed on the Internet World Wide Web (http://gldage.cr.usgs.gov/eq). The color maps can be viewed on the Web or downloaded to the user's computer for printing.

Interim maps were placed on the Web site in December, 1995 for review and comment. The June 1996 maps are the result of revision in light of these reviews and comments. The June 1996 maps represent the products released for this round of map-making. We expect to revise these maps on a 1 1/2 to 2 year cycle as new fault parameters become available and as our methodology is further refined.

This document describes the procedures used to make the maps, the geologic/seismologic inputs to these maps, and the ground-motion relations used for the maps. Most of this documentation was used previously to describe the interim maps. We have added text which explains the changes made for the June 1996 maps. The maps and hazard curves included in this documentation have been revised from the interim documentation to reflect those used in the June 1996 maps.

The hazard maps depict probabilistic ground motions and spectral response with 10% 5% and 2% probabilities of exceedance (PE) in 50 years. These maps correspond to return times of approximately 500, 1000, and 2500 years, respectively. The maps are based on the assumption that earthquake occurrence is Poissonian, so that the probability of occurrence is time-independent. The methodologies used for the maps were presented, discussed, and substantially modified during 6 regional workshops convened by the USGS from June 1994-June 1995.

One of the primary features of the new maps is the use of smoothed historical seismicity as one component of the hazard calculation. This idea is similar in philosophy to the methodology used by EPRI (1986) and O'Hara (unpublished report, 1994), where locally-determined a and b-values were used as part of the hazard calculations. Our procedure is more similar to that of Jacob et al.(1994), although aspects of our hazard calculation differ from theirs. The mathematics of the procedure used in the USGS maps is described in Frankel (1995) and the reader is referred there for more details.

Another important aspect of this study is the use of alternative models of seismic hazard in a logic tree formalism. For the central and eastern U.S. (CEUS), different models based on different reference magnitudes are combined to form the hazard maps. In addition, we use large background zones based on broad geologic criteria as alternative source models for the CEUS and the western U.S. (WUS). These background zones are meant to quantify hazard in areas with little historic seismicity, but with the potential to produce major earthquakes. The background zones were developed from extensive discussions at the regional workshops.

For the WUS, a big advance in the new maps is the use of geologic slip rates to determine fault recurrence times. We have used slip rates from about 500 faults or fault segments in the June maps.

The hazard maps do not consider the uncertainty in seismicity or fault parameters. We used preferred values of maximum magnitudes and slip rates (see below). The next stage of this effort is the quantification of uncertainties in hazard curves for selected sites.

The national hazard maps are not meant to be used for Mexico, areas north of 49 degrees North latitude, and offshore the Atlantic and Gulf of Mexico coasts of the U.S.