TEMPORAL AND SPATIAL CLUSTERING OF PALEOSEISMICITY ALONG THE LOW-ANGLE SOUTHERN WASATCH FAULT ZONE, UTAH

The combination of GPS, paleoseismicity and structural data reveal strong evidence for both temporal and spatial clustering of paleo-seismicity along the southern Wasatch Fault.

At decadal time scales GPS velocity fields derived from both continuous and campaign data indicate mostly westward extension across the Wasatch Fault at rates of 1-3 ± 1 mm/yr. Nonlinear inversion of the velocity field (Chang et al., 2000) predicts that the southern Wasatch Fault dips west at around 40°, has a dip-slip loading rate of 4.2 mm/yr., and is locked at a depth of around 17 km. The meaning of the GPS velocity field, in terms of deformational mechanisms, remains controversial.

At a time scale of thousands of years 15-40 m offsets of Lake Bonneville terraces indicate dip-slip rates of 1.0-2.8 mm/yr. over the past 17 ka. However, pre-Bonneville alluvial fans may show maximum displacements of 200 m at time scales of 600 ka (<0.3 mm/yr).

Differences between short- and long-term rates at temporal scales of 105 years are manifest in the pattern of faceted spurs along the southern Wasatch Fault. Most facets show 250-350 m of nearly continuous slip interrupted by intervals of pediment development with little to no slip. The youngest facets mostly dip around 39° regardless of rock type, and are separated from the next oldest facet by pediments up to 100 m wide. The facets may indicate more or less continuous fault active for periods of around 100-200 ka that is interrupted by intervals of quiescence of unknown duration. Differences between loading rates indicated by GPS and slip rates inferred from paleo-seismic studies are reduced when calculated using a low fault dip angle (40°).

Measurements of footwall fault scarps, fault drag and the lowest faceted spurs along the Wasatch Fault indicate an average dip of the fault plane of around 40° west. Some fault planes show flat-ramp geometries, adjacent basins with shallow graben fill (<4 km), and wide zones of hanging-wall deformation. Gravity models of the fault (UGS) also predict fault dips of <40° with possible abrupt step structures. Low-angle fault dip may be explained by tectonic exhumation of deeper parts of a listric fault, and by reactivation of pre-existing structure as hypothesized by Smith and Bruhn (1984).