Effects of lithospheric viscoelastic relaxation on the contemporary deformation following the 1959 M_w 7.3 Hebgen Lake, Montana, earthquake and other areas of the intermountain seismic belt

The 1959 M_w 7.3 Hebgen Lake, MT, normal-faulting earthquake occurred in an extensional stress regime near the Yellowstone volcanic field. Time-dependent crustal deformation data following this major earthquake were acquired by precise trilateration and GPS surveys from 1973 to 2000 around the Hebgen Lake fault zone. Modeling the changes of baseline lengths across and near the fault reveals a lateral variation of transient rheology, in which the lithosphere is stronger near the Hebgen Lake fault zone than in the vicinity of the Yellowstone volcano system. The models also imply that the lower crust is stronger than the upper mantle, in agreement with results from studies of postseismic and post-lake-filling relaxations (<∼100 years). In addition, evaluations of the postseismic motion produced by the Hebgen Lake and the 1983 M_w 6.9 Borah Peak, ID, earthquakes indicate that horizontal transient motion of up to ∼1 mm/yr contribute significantly to the contemporary regional crustal deformation near the epicentral areas. For the eastern Basin and Range, ∼500 km south of the Hebgen Lake fault, similar rheologic models were derived from the observed uplift associated with the Lake Bonneville rebound and were used to evaluate the postseismic deformation associated with six most recent paleoearthquakes of the Wasatch fault zone and three M ≥ 5.6 historic earthquakes of northern Utah. The results show ≤0.1 mm/yr of horizontal postseismic motion at present time that are within the horizontal uncertainties of continuous GPS velocity from the Basin and Range and significantly smaller than the contemporary extension of 1-3 mm/yr in the Wasatch Front.