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College of Arts & Sciences
Hazards & Vulnerability Research Institute

Seismic Hazard Susceptibility in Southwestern Montana: Comparison at Dillon and Bozeman

Arleen A. Hill

Advisor:  Dr. Jerry Bartholomew



A GIS was employed to integrate spatial distribution of active fault, earthquake, and other geologic data in developing a method for assessing seismic hazard susceptibility for southwestern Montana. Characteristic earthquakes were selected to represent the three overlapping stress fields affecting southwestern Montana. Fault-plane solutions for these three events and attitude data of identified active faults were compared to ascertain in which stress field each fault could be reactivated. Faults were then grouped based on favorable, unfavorable, or severe misorientation for reactivation within each stress field. Expected Modified Mercalli Intensity (MMI) values were delineated around each fault based on characteristic event isoseismal lines. Results of this study illustrate that Dillon and Bozeman are susceptible to MMI VIII and VII respectively.

This study documents the effectiveness of a method employing detailed geologic information to assess the spatial variability in hazard susceptibility. Comparison of results to previous studies suggests that the risk at Dillon and Bozeman differs due to proximity of active faults to the community. This method was limited by reliance on felt reports and characteristic events. Improved resolution of MMI > VII requires incorporation of fault segmentation data and regional attenuation parameters. Segment dimensions could be used to constrain maximum credible events for individual and linked segments. Attenuation curves would permit a more quantitative analysis of energy transmission and predicted acceleration values for southwestern Montana communities. This method can evolve, as additional information becomes available.

KEYWORDS: Seismic hazard, Southwestern Montana