Frequently Asked Questions (FAQ), Faults
Where do I need to look to find a fault map of the US?
Is one available in GIS format?
Where one would look depends on what one means. Faults may be found on the large format paper copies of the tectonic maps of the US and North America. However, many of these are not seismically active, but just represent features important in tectonics of very long ago. It's conceivable these could be available in GIS, but I don't know where.
To address the issue of active faults, around 1976, the USGS published a map of "Youthful and Possibly Active Faults." The problem with this map is that there was not much mapping of young faults done in most of the US, unless the faults were significant for mineral resources or for the design of critical facilities. Occasionally, young faults might have been mapped as a matter of interest to the researcher, even if not needed in application. As a result, some states had faults that were mapped as a secondary result of resource (e.g., coal) mapping. An adjacent state may not have had continuations of those same faults because they were not considered significant for coal resource mapping by geologists in the neighboring state.
Also, until recently most geologic investigations focused on bedrock instead of the overlying soil and loose sand, clay, and gravel. For earthquake fault mapping, much of the evidence for activity depends upon evidence found in this unlithified sediment that overlies the bedrock. This kind of mapping will have priority in high seismicity areas as a result of trying to understand earthquake hazard more precisely.
As a result, a national viewpoint for active faults is incomplete. It may take 50 years to understand the slip rates for all the suspected active faults just of the western US. Meanwhile, a few active faults east of the Rockies are being discovered through low-angle aerial photography and special local studies. Many eastern faults are well-buried and not available for study unless their movement has broken the surface, and the evidence of the breaking has not been covered or removed by weathering. Given the low rate of activity, the likelihood of both is small.
Our web site has tried to provide information about faults for which the slip rates are known and for which these rates are high enough to affect probabilistic ground motion hazard maps. For the scale of our maps, the traces of these faults do not have to be as accurate as one would need to provide local zoning restrictions. The faults for California are probably best documented. Nevada no doubt has more known active faults, but the activity rates are generally lower than those of California, and are largely determined by reconnaissance studies and aerial photography rather than detailed ground studies. Few other western states have active fault data bases as large or as documented as California or Nevada, but Oregon is in the middle of a very systematic study. For all the western states it is better to go to the states themselves for the most current information.
Meanwhile, some of our staff in Golden is collecting new fault information as it becomes available, for use in input to future USGS national hazard maps and also to be part of a world-wide active fault database. This will result eventually in a more complete and uniform database. But one does not exist now.
We frequently get inquiries about information on faults in places where faults are not known to be seismically active, e.g., western Florida, eastern Pennsylvania and New York. Information on faults in such areas are best found by asking at local universities or state geological surveys. In some few cases, extensive studies on particular faults have been undertaken during the licensing of nuclear power plants. Documentation is usually presented in "PSAR"'s and "FSAR"'s. These documents are supposed to be available locally and at the Nuclear Regulatory Commission.
