LITHOSPHERIC RHEOLOGY FROM POSTSEISMIC DEFORMATION OF A M=7.5 NORMAL-FAULTING EARTHQUAKE WITH IMPLICATIONS FOR CONTINENTAL KINEMATICS

Time-dependent crustal motions were determined by GPS and trilateration measurements from 1973 to 2000 following the 18 August 1959 Ms=7.5 Hebgen Lake, MT, earthquake. The data were used to assess intraplate post-seismic deformation and to model lithospheric rheology. This largest historic normal-faulting earthquake of the western U.S. Basin-Range Province occurred at the northwest edge of the Yellowstone volcanic field in the extensional tectono-volcanic regime. Time dependent changes of baseline-length across the Hebgen Lake fault and its aftershock zone exhibited 4 to 6 mm/yr of extension up to 14 years following the earthquake. Rheological models derived by these data show that the lithosphere is stronger near the fault zone, and weaker in the vicinity of the Yellowstone caldera with much higher heat flow and a thinner brittle crust. Our models also imply a more viscous lower crust than the upper mantle, in agreement with a corollary that the continental mantle has relatively small long-term stress. We then employed the derived Hebgen-Lake rheological model to evaluate the postseismic deformation of a much larger area of the Intermountain region produced by the Hebgen Lake and the 1983 Ms=7.3 Borah Peak, ID earthquakes. The results suggest that the postseismic relaxation of these earthquakes produced horizontal ground motions up to ~2 mm/yr that must be considered for intraplate kinematic models. Similarly, a viscosity model based on observations of the Late-Quaternary loading and vertical rebound of Lake Bonneville of the eastern Basin-Range Province was also used to evaluate postseismic ground motions associated with six most recent, <1.5 ka, Holocene paleoearthquakes and three large, M≥5.6, historic earthquakes of the Wasatch fault zone, UT. These results demonstrate ≤0.1 mm/yr of contemporary horizontal motions induced by these earthquakes that are negligible compared to the ~1-3 mm/yr of E-W extension determined by GPS measurements. This study provides new insights into the widespread effects on regional deformation from postseismic relaxation of large earthquakes that need to be considered in kinematic models and earthquake hazards of intraplate tectonic regions.