GPS (global positioning system) studies of the Wasatch fault zone, Utah, with implications for elastic and viscoelastic fault behavior and earthquake hazard

Contemporary crustal deformation along the 370 km-long Wasatch fault, Utah, has been measured by the Global Positioning System (GPS) and modeled for elastic and viscoelastic mechanisms. The Wasatch Front GPS network, including 107 campaign sites surveyed in 1992--1995, 1999, and 2001, and 11 permanent stations operating continuously from as early as mid-1996, spans a 100-km wide area across the fault. Combining these GPS measurement data revealed surface velocities with horizontal components of 1.8 +/- 0.5 mm/yr and 2.2 +/- 1.0 mm/yr across the northern and southern part of the Wasatch fault, respectively, with directions nearly perpendicular to the fault (E-W). Analysis of the spatial variation of the strain rate field, moreover, revealed a notable strain concentration across the Salt Lake City segment of the Wasatch fault that may be produced by the interseismic fault loading.; Mechanisms other than fault loading that could contribute surface deformation signals to the Wasatch Front GPS observations were first examined, which include postseismic viscoelastic relaxation of the Earth's lithosphere and fluctuations of water table and the level of Great Salt Lake. Results showed that deformation signals induced by these effects are within the error ranges of GPS horizontal velocities, which imply that the Wasatch fault may be the main tectonic feature responsible for the contemporary deformation of the Wasatch Front area.; A nonlinear optimization algorithm was then implemented to the GPS observations to investigate the geometry and loading rate of the Wasatch fault zone. An optimal model that best fits the observed horizontal velocity field shows a fault plane dipping 27° and creeping at 7 mm/yr from depths of 9--20 km, which may correspond to the interseismic loading-zone of the Wasatch fault. Examining the rheological properties of crustal and fault-zone rocks, on the other hand, suggests the brittle thickness of 7 to 9 km for the Wasatch fault zone and the depth of 15 km for deep background earthquakes, similar to that predicted by my fault models and observed in the Wasatch Front area, respectively.; I also evaluated the integration of various types of potential seismic sources on the probabilistic earthquake ground-shaking hazard of a site in the Salt Lake Valley. Including paleoearthquake recurrence data obtained by trenching and geodetic moment rates from GPS, for example, increases the annual frequency of HPGA (horizontal peak ground acceleration) ≥0.25g by a factor of 1.7 compared with the result of using only the historic seismicity. These results address the importance of including GPS observations into the Wasatch Front earthquake hazard analysis that have relied primarily upon the historic seismicity and late-Quaternary fault slip rates.