THE UNUSUAL 1959 MW 7.3 HEBGEN LAKE, MT, EARTHQUAKE: LITHOSPHERIC AND VOLCANO SOURCE CONTRIBUTIONS AND EARTHQUAKE HAZARD

The deadly 1959 Mw7.3 (Ms7.2-7.5, ML 7.6, mb 7.0) Hebgen Lake, MT normal-faulting earthquake occurred within the extensional stress regime of the eastern Basin-Range province interacting with processes associated with the Yellowstone hotspot and volcano system. The Hebgen Lake event, while commonly thought of as a single shock, was composed of 3 to 6 separate events tens of seconds apart that together produced a ~35 km long dual surface rupture with up to 5.7 m of offset. The source was double down-dip slip event dipping ~45° to 55° nucleation at ~16 km. The resulting co-seismic ground deformation was characterized by asymmetric hanging-wall subsidence greater than footwall uplift, a characteristic now seen in other normal faulting earthquakes. Moreover the relatively short fault length and large displacement of the Hebgen event produces an unusually high stress drop of 9.7 MPa (~100 bar). Post-seismic relaxation rates of the event, determined by leveling, trilateration, and GPS data, reveal extensional rates of up to 6.6 mm/yr across the fault and up to 6 mm/yr of hanging wall subsidence, that indicate continued surface extension of the Hebgen Lake fault that may last many decades. The location and source properties of the Hebgen Lake earthquake reflect effects of multiple processes: 1) broad scale lithospheric uplift from Yellowstone plume buoyancy, 2) Basin-Range extension, 3) YSRP flexural-tilt and contraction, and 4) stress interaction with the Yellowstone crustal magma system. A probabilistic seismic hazard assessment (PSHA) for the Yellowstone-Teton region shows that a repeat of a M7 earthquake on normal faults of this area could produce ground shaking affecting a large area. In addition, ground motions of a Y-T event accompanying co-seismic rupture due to dynamic fault stresses, during the time of the actual rupture, can produce ground velocities many times larger than are now assumed for the Intermountain region. Scenario ground shaking maps (Shakemaps) for the Hebgen Lake and other earthquakes of the Intermountain Seismic Belt emphasize the high earthquake hazard of the Y-T area. In conclusion, the Hebgen Lake event has been taken as the maximum credible earthquake (MCE) for the Intermountain region but we caution that it should be considered as an unusually large event for the observed seismic parameters, i.e. the Hebgen Lake earthquake was indeed big for its size.