CRUSTAL DEFORMATION OF THE WASATCH FRONT, UTAH, FROM GPS MEASUREMENTS, PALEOSEISMICITY, AND ELASTIC-VISCOELASTIC MODELING
CHANG, W.L., SMITH, R.B., and MEERTENS, C.M., Dept. of Geology and Geophysics, University of Utah, Salt Lake City, UT 84112, email@example.com; and HARRIS, R.A., Dept. of Geology, Brigham Young University, Provo, UT 94305
Contemporary crustal deformation of the 370 km-long Wasatch fault, Utah, has been measured by GPS and modeled for elastic and viscoelastic mechanisms. GPS campaign surveys of 90+ sites begun in 1992 and re-observed in 1993, 1994, 1995 and 1999 reveal a principal E-W horizontal strain rates of 20 to 45 nstrain/yr. This corresponds to a general E-W extension of 1.1 to 2.5 mm/yr across a 55 km-wide survey area spanning the central and southern Wasatch fault. Beginning in 1997, the University of Utah has installed five continuous GPS (CGPS) stations with baselines spanning the fault. Data from these sites and five additional stations of the BARGEN network in Utah are routinely processed. The CGPS results show that the strain rate is negligible east of the fault, but sharply increases to west across the fault to 50 nstrain/yr, essentially the same as that measured by the campaign GPS results. Strain rates vary laterally however, decreasing from north to south along the fault, with values of 50 to 30 nstrain/yr, respectively, corresponding to 2.5 to 1.1 mm/yr. Also we analyzed Wasatch fault paleoseismic fault slip data by analytically fitting ellipsoid functions to the measured displacement fields that provide a new estimate of fault loading rates that are compared with the GPS deduced loading rates. Using the strain and deformation results from the CGPS and the campaign GPS surveys, together with that from USGS GPS surveys to the south (Thatcher, et al., 1999) and EDM surveys to the north (Savage, et al., 1992), we have run nonlinear inversions on fault geometry and loading rates. Preliminary results for the central Wasatch fault suggest a best fit to the GPS data by a single infinite-edge dislocation with a N-S strike, a 40$\deg$W dip, a locking depth of 15 km, and a fault loading rate of $\sim$ 5-6 mm/yr that is notably higher than the rate derived from the paleoseismic data ($\sim$1-2 mm/yr). In addition, viscoelastic relaxation of prehistoric (M>7) and historic (M>4) earthquakes are being evaluated to examine the time-dependent effects of past earthquakes on the contemporary strain field. These results provide important new insights on temporal variations of the seismic cycle and constraints on related earthquake hazards.