RECENT SEISMICITY IN THE WESTERN PART OF THE 1959 EARTHQUAKE AFTERSHOCK ZONE

The 1959 Hebgen Lake earthquake occurred well before the advent of regional seismic monitoring networks, so pre-1959 seismicity patterns are unknown. Regional seismic monitoring by the Montana Bureau of Mines and Geology (MBMG) of the western part of the 1959 aftershock zone began in the early 1980s and dramatically improved in 2000 with the implementation of continuous digital seismic data recording, sharing data amongst networks and improvements to telemetered network stations in SW Montana. Since 2000, the MBMG has determined 1095 good-quality hypocenter locations in the area extending 20 km west of Hebgen Dam, covering the southern Madison Range and the Missouri Flats area of the southern Madison Valley, where most earthquakes occur. Eighty-nine percent of the earthquakes having well-determined hypocenter depths (vertical uncertainty < 2.0 km) occur between 5 and 13 km below the surface with the most frequent depth interval being 11-13 km. A recurrence analysis for earthquakes since mid-2000 indicates that the MBMG earthquake catalog for this area is complete down to magnitude 0.9 and that an average of 58 earthquakes above this threshold occur annually. Despite this high rate of seismicity, no earthquakes over magnitude 4.0 have occurred here since 1982 and only 16 earthquakes have magnitudes > 3.0. The vast majority of well-determined epicenters (horizontal uncertainty < 1.0 km) fall within the zone of 1959 coseismic subsidence (Plate 2, USGS Prof. Paper 435). The Missouri Flats seismicity cluster lies west of the north-trending, west-dipping Madison fault but 45 fault plane solutions and cross sections showing well-determined hypocenters provide no compelling evidence to indicate that recent seismicity is occurring at depth along the southern Madison fault. Instead, the hanging-wall block of the southern Madison fault is being brittlely deformed by distributed slip along strike-slip, oblique-normal, normal faults and also a few reverse faults. Most strike-slip and normal fault plane solutions exhibit NNE-SSW-oriented axes with minimum compressive stress similar to 1959 main shock focal mechanism (Doser JGR 1985 v 90 4537-4555) and consistent with horizontal strain observed on a trilateration network that covered the Hebgen Lake basin and southern Madison Valley (Savage et al. JGR 1985 v 90 10310-10320).