FAULT CHARACTERIZATION FOR THE USGS NATIONAL SEISMIC HAZARD MAPS

The USGS, along with State geological surveys, academia, and the private sector, has compiled geologic information on thousands of faults throughout the United States that are known or suspected to have been sources of large magnitude (surface-deforming) Quaternary earthquakes. The data set and accompanying documentation provides the basis for fault-parameter characterization used in the 1996 and 2002 USGS National Seismic Hazard Maps. The maps are incorporated into building codes mandated by tens of thousands of governmental agencies to establish minimum construction requirements; they are also used for establishing earthquake insurance rates, land use plans, allocation planning for assistance funds, and other earthquake-related matters.

We use published information to characterize a fault's characteristic magnitude, slip rate (mm/yr) or recurrence interval (yr), fault location and geometry, and variability of these parameters for incorporation into calculations of the maps and other hazard assessments. However, in the lower 48 States, only about one fault in four has been sufficiently investigated to provide these parameters and most of the data are poorly constrained. Most faults are characterized by estimates shared by fault classes instead of fault-specific measurements. Future needs include better statistical estimates of uncertainties, priority on characterization of faults in or near cities, better constrained numerical input values on more faults to support time-dependent models instead of the Poissonian model used today. As we acquire more and better information on individual faults, the more accurate and precise hazard maps that result can reduce loss and casualties through cost-effective building practices.