We have produced updated seismic hazard maps for the conterminous United States, based on new seismological, geophysical, and geological information. The 2002 maps contain important changes from the previous version of the national seismic hazard maps made in 1996. However, most of these changes are incremental. For the 1996 maps, we developed a new methodology quite different from that used in prior USGS national seismic hazard maps (see Frankel et al., 1996; Frankel et al., 2000). In addition, we instituted an open, consensus-building process where there was feedback from geoscientists and engineers on the methodology and inputs for the maps during several regional workshops. The development of the 2002 maps followed the same open process.
Many of the changes in the 2002 maps were suggested by participants in four regional workshops that we held: Pacific Northwest (Seattle, March, 2000), Central and Eastern U.S. (St. Louis, June 2000), California (Pasadena, Sept. 2000), and Intermountain West (Salt Lake City, March, 2001). Modifications to the maps were also discussed in a user workshop convened by the Applied Technology Council (ATC) and the USGS in May 2001.
Draft maps were placed on our website (geohazards.usgs.gov/eq/) in January and August 2002 for comment. We requested review of the fault parameters from the western state geological surveys. Most of them provided comments. An external panel of non-USGS experts has also reviewed the maps and provided feedback during two meetings with project staff. The finalized maps presented here reflect many changes based on the comments we received on the draft maps.
The 2002 maps constructed to date are for peak ground acceleration (PGA), and 0.2 sec and 1.0 sec spectral acceleration (for 5% damping) at 10% and 2% probabilities of exceedance (PE) in 50 years (see Appendix B). We will also calculate spectral accelerations at periods of 0.1, 0.3, 0.5, 1.0 and 2.0 sec and will release uniform hazard spectra and hazard curves, as we did for the 1996 maps. The hazard maps are for a firm-rock site condition, where the shear-wave velocity averaged over the top 30m (Vs30) is 760 m/s (boundary of NEHRP site classes B and C).
The description below focuses on changes from the 1996 maps. The general methodology and organizing princ iples for the 2002 update are the same as the 1996 maps (see Frankel et al., 1996; available on our website). We typically use a combination of hazard curves calculated from gridded spatially-smoothed seismicity (see Frankel, 1995), large background zones, and specific fault sources. In some cases, areal zones with seismicity rates constrained by GPS deformation data are applied.
The California portion of the maps was produced jointly by the USGS and the California Geological Survey. The methodology for the 1996 maps for California is described in more detail in Petersen et al. (1996).
Throughout the revision, we have included more explicit treatment of uncertainties. In most cases we use the mean recurrence times, since we are first concerned with producing mean hazard maps. Therefore, it is not necessary to do a logic tree of recurrence rate when constructing a mean hazard map. We intend to produce maps of uncertainties after the maps derived from the mean hazard curves.
Our earthquake catalog was updated to include earthquakes through December 2001 and new seismicity grids constructed after dependent events were removed. In all regions, we made a more extensive effort for the 2002 maps to remove quarry blasts and other explosions from the earthquake catalog before calculating the seismicity parameters. For example, we removed blasts for the Kentucky catalog using the report of Street et al. (2002). In other cases we used additional information from the National Earthquake Information Center to identify suspected blasts in the catalog.
- Arthur D. Frankel
- Mark D. Petersen
- Charles S. Mueller
- Kathleen M. Haller
- E.V. Leyendecker
- Robert L. Wesson
- Stephen C. Harmsen
- Chris H. Cramer
- David M. Perkins
- Kenneth S. Ruckstales
Full Text Publication
OFR-02-420 - Documentation for the 2002 Update of the National Seismic Hazard Maps - full text of this documentation available for download.
This report is preliminary and has not been reviewed for conformity with the U.S. Geological Survey editorial standards or with the North American Stratigraphic Code. Any use of trade, firm or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.