The Evaluation of Hazardous Faults in the Intermountain West Region

A Regional List of Priority Faults for Future Investigations

The U.S. Geological Survey’s Earthquake Hazards Program (EHP) has the responsibility to provide information and knowledge about earthquakes and earthquake hazards nationwide 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, 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 regions where the tectonic setting is broadly similar.  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 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.  The IMW contains thousands of faults that have moved in Cenozoic time, hundreds of which have evidence of Quaternary movement (see Quaternary Fault & Fold Database for information about specific faults).

Ideally, each Quaternary fault should be studied in detail to evaluate its rate of activity and thus gauge the hazard it poses. However, the study of a single fault requires a major commitment of time and resources, and given the large number of IMW faults that should be studied, it is impractical to expect that all IMW Quaternary faults can be fully evaluated. A more realistic and efficient approach is to develop a priority list of IMW structures that potentially pose a significant hazard and to focus future efforts on those structures.

In Utah and Nevada, geoscientists have been taking steps to evaluate geologic data related to hazardous faults and to develop a statewide priority list of hazardous structures. In contrast, the other IMW states contain substantially fewer Quaternary faults, so there have not been systematic efforts to develop similar priority lists either on a state-by-state basis or for the entire region.

To facilitate the development of a region-wide list of priority structures, the USGS held a workshop in Golden, Colorado, in June 2008.  The objective of the workshop was to obtain input and advice from geologists who have knowledge and experience in the IMW and to identify those structures that potentially pose a substantial hazard and thus warrant additional study.  Because working groups and workshops had already been convened to specifically deal with Quaternary fault priorities in Utah and Nevada, this USGS workshop specifically emphasized structures outside of those two states.

A group of 21 geologists from State Geological Surveys in the IMW and knowledgeable regional experts met to present state-level perspectives on hazardous structures, to discuss criteria that could be applied to rank structures throughout the region, and to develop a priority list of IMW structures for future study. After considerable discussion, the attendees initially identified 43 structures and developed a ranked list from those candidates. This priority list allows Program Managers to guide the limited resources toward studies of features that are deemed to pose the most serious potential hazards in the IMW, and it provides the scientific community with a list of target structures to investigate.

A detailed description of the workshop proceedings, discussions, and presentations is now available as U.S. Geological Survey Open-File Report 2009-1140.  This report also contains the list of priority structures developed at the workshop (table 4).  It may be downloaded at: http: http://pubs.usgs.gov/of/2009/1140/.