Evidence for postseismic viscoelastic relaxation following the 1959 M = 7.5 Hebgen Lake, Montana, Earthquake

Abstract

A 1983 releveling survey conducted by the National Geodetic Survey, combined with previous surveys in 1923, 1960, 1967, and 1975, provides evidence for ongoing vertical deformation of a broad region (diameter of ∼150 km) surrounding the site of the 1959 Hebgen Lake earthquake. Deformation consists of relative uplift centered roughly on the coseismic fault and a smaller amplitude zone of relative subsidence south of the fault. Maximum observed elevation change during the postseismic period (1960–1983) exceeds 30 cm. Assuming that the entire uplift (i.e., 1923–1983) occurred following the 1959 earthquake (i.e., uplift measured between 1923 and 1960 was entirely postseismic), the rate of vertical deformation appears to have decreased exponentially with a characteristic decay time of about 10 years. The spatial pattern and time behavior of the observed movements are consistent with simple models of postseismic deformation following normal faulting in an elastic layer (thickness of ∼30–40 km) overlying a viscoelastic half‐space (viscosity of ∼1019 Pa s). This model may also account for the anomalously large horizontal strain around the Hebgen Lake region measured by repeated trilateration surveys during the period 1973–1984. The releveling measurements in the Hebgen Lake region appear to provide the first observations of viscoelastic relaxation in the asthenosphere following an intraplate earthquake in the United States.