RECENT SEISMICITY AND ITS RELATIONSHIP TO QUATERNARY FAULTS NORTH OF THE SNAKE RIVER PLAIN

Data from the Montana Regional Seismograph Network are used to locate seismicity in Montana and adjacent parts of Idaho. Recent seismicity clearly delineates the northern Intermountain Seismic Belt and Centennial Tectonic Belt (CTB). Both of these seismically active zones also encompass numerous Quaternary (Q) faults. However, only rarely can earthquake hypocenters be ascribed to slip at depth on mapped Q faults. The 1959 Hebgen Lake (M7.3) and 1983 Borah Peak (M6.9) earthquakes are the only two examples of historic surface rupture along Q faults north of the Snake River Plain (SRP). The ~2500 earthquakes that occur annually in this region as diffuse zones punctuated by dense clusters and aseismic patches, are rarely associated with mapped Q faults. Notable examples of earthquakes that are reasonably associated with slip at depth on Q faults include a 2006 M4.6 in the Centennial Valley, a 2007 M4.7 in the Ruby Valley, and a 2022 M4.5 in the Mission Valley. To support this association, these earthquakes have well constrained focal depths, focal mechanisms consistent with adjacent valley-bounding Q faults, and reasonable geometry with respect to the surface trace of Q faults. Seismicity clusters frequently occur near Q faults but typically within the footwall or off either end. Examples include the Tobacco Root Mountains south of Whitehall and the Tendoy Mountains south of Lima. The down-dip region of most Q valley-bounding, normal faults form conspicuous patches of low, or no seismicity. Focal mechanisms indicate that the region north of the SRP is extending. The average extension direction in central Idaho is N30E but rotates clockwise to N60E in SW Montana, excluding the eastern CTB. The extension direction in the eastern CTB-Yellowstone region is more complex and shows subgroups of events with extension directions similar to central Idaho and SW Montana plus a N10E direction, similar to the 1959 earthquake extension direction. Sparse focal mechanism data in NW Montana suggests E-W extension north of the Lewis and Clark zone, compatible with the orientation of the Swan and Mission faults. Despite the current aseismic nature of most recognized Q faults, abundant proximal seismicity and the prevailing stress field inferred from focal mechanisms are compatible with future slip on Q faults in western Montana and central Idaho.