Open-File Report 2009-1140
The U.S. Geological Survey’s (USGS) Earthquake Hazards Program (EHP) has the responsibility to provide nationwide information and knowledge about earthquakes and earthquake hazards as a step to mitigating earthquake-related losses. As part of this mission, USGS geologists and geophysicists continue to study faults and structures that have the potential to generate large and damaging earthquakes. In addition, the EHP, through its External Grants Program (hereinafter called Program), supports similar studies by scientists employed by state agencies, academic institutions, and independent employers. For the purposes of earthquake hazard investigations, the Nation is geographically subdivided into tectonic regions. One such region is the Intermountain West (IMW), which here is broadly defined as starting at the eastern margin of the Rocky Mountains in New Mexico, Colorado, Wyoming, and Montana and extending westward to the east side of the Sierra Nevada mountains in eastern California and into the Basin and Range-High Plateaus of eastern Oregon and Washington. The IMW contains thousands of faults that have moved in Cenozoic time, hundreds of which have evidence of Quaternary movement, and thus are considered to be potential seismic sources.
Ideally, each Quaternary fault should be studied in detail to evaluate its rate of activity in order to model the hazard it poses. The study of a single fault requires a major commitment of time and resources, and given the large number of IMW faults that ideally should be studied, it is impractical to expect that all IMW Quaternary faults can be fully evaluated in detail. A more realistic approach is to prioritize a list of IMW structures that potentially pose a significant hazard and to focus future studies on those structures. Accordingly, in June 2008, a two-day workshop was convened at the USGS offices in Golden, Colorado, to seek information from representatives of selected State Geological Surveys in the IMW and with knowledgeable regional experts to identify the important structures for future studies. Such a priority list allows Program managers to guide the limited resources toward studies of features that are deemed to potentially pose the most serious hazards in the IMW. It also provides the scientific community with a list of structures to investigate because they are deemed to pose a substantial hazard to population centers or critical structures. The IMW encompasses all or large parts of 12 states, including Arizona, New Mexico, extreme west Texas, Colorado, Utah, Nevada, eastern California, eastern Oregon, eastern Washington, Idaho, western Wyoming, and western Montana. In Utah, and more recently in Nevada, geoscientists have taken steps to evaluate geologic data related to well-studied faults and to develop a statewide priority list of hazardous structures. In contrast to Utah and Nevada, the other IMW states contain substantially fewer Quaternary faults, so there have not been any previous efforts to develop similar priority lists. This workshop was organized to address this matter and create a more balanced perspective of priorities throughout the entire IMW region. Because working groups and workshops had already been convened to specifically deal with Quaternary fault priorities in Utah and Nevada, this workshop specifically emphasized structures outside of these two states.
First posted July 16, 2009
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Crone, A.J., Haller, K.M., and Maharrey, J.Z., 2009, Evaluation of hazardous faults in the Intermountain West region—Summary and recommendations of a workshop: U.S. Geological Survey Open-File Report 2009-1140, 72 p.
List of Appendixes
Figure 1. Diagram of the Western U.S. showing general area of the Intermountain West (IMW) in the gray shaded area
The Workshop Organization
First Day Activities
Overview: Need for the Workshop and Goals (Anthony Crone, USGS)
USGS External Grants in the IMW and the USGS National Seismic Hazard Maps (Mark Petersen, USGS, IMW Regional Coordinator and Chief, National Seismic Hazard Map project)
History and Utility of the Utah Quaternary Fault Parameters Working Group (Appendix 3a) (William Lund, Utah Geological Survey, Cedar City, Utah)
Hazardous Quaternary Faults in Arizona (Appendix 3b) (Phillip Pearthree, Arizona Geological Survey, Tucson, Arizona)
Hazardous Quaternary Faults in Colorado (Appendix 3c) (Vincent Matthews, State Geologist, Colorado Geological Survey, Denver, Colorado)
Hazardous Quaternary Faults in New Mexico (David Love, New Mexico Bureau of Geology and Mineral Resources, Socorro, New Mexico)
Hazardous Quaternary Faults in West Texas and Adjacent Areas (Michael Machette, USGS, Denver, Colorado)
Discussion of Hazardous Quaternary Faults in the Southern IMW (Anthony Crone, moderator, USGS, Denver, Colorado)
Hazardous Quaternary Faults in Idaho (Appendix 3d) (William Phillips, Idaho Geological Survey, Moscow, Idaho)
Hazardous Quaternary Faults in Montana (Appendix 3e) (Michael Stickney, Montana Bureau of Mines and Geology, Butte, Montana)
Hazardous Quaternary Faults in the Yellowstone Area, Wyoming (Appendix 3f) (Kenneth Pierce, USGS, Bozeman, Montana)
Hazardous Quaternary Faults in Southwest Wyoming (James McCalpin, GeoHaz Consulting, Crestone, Colorado)
Hazardous Quaternary Faults in Eastern Oregon (Appendix 3g) (Ian Madin, Department of Geology and Mineral Industries (DOGAMI), Portland, Oregon)
Discussion of Hazardous Quaternary Faults in the Northern IMW (Anthony Crone, moderator, USGS, Denver, Colorado)
Second Day Activities
IMW Quaternary Faults in the National Seismic Hazard Maps (Appendix 3h) (Kathleen Haller, USGS)
Table 1. IMW faults that are currently included in the National Seismic Hazard maps
Developing Criteria for Prioritizing Hazards Faults (Anthony Crone, USGS)
Table 2. Subjective criteria used to prioritize IMW faults
Prioritizing Faults for Future Studies (Anthony Crone, USGS)
Table 3. A proposed numerical rating system for IMW Quaternary faults
Table 4. Prioritized list of IMW faults
Concluding Remarks and Recommendations