Contemporary Deformation of the Wasatch Front, UT, and its Implication on the Interseismic Loading of the Wasatch Fault Zone

The contemporary deformation field in the 370 km-long Wasatch fault zone, Utah, has been measured by continuous and campaign GPS observations since 1992. Six campaign surveys were conducted in 1992, 1993, 1994, 1995, 1999 and 2000, and 54 continuous stations have been operated since 1996 with their baselines straddling the fault. The GPS measurements reveal an average E-W horizontal extensional rate of 2.2-2.8 mm/yr across the Wasatch Front area, of which the deformation is assumed to have two components: block motion of the Basin-Range relative to the stable North America and loading on a locked portion of the Wasatch fault. We construct block models whose rotation poles and rates are based on geodetic strain, fault geometry, and earthquake distribution, and select a block configuration that best fits the GPS velocities. A nonlinear optimization algorithm for dislocation fault-modeling is then implemented to investigate the geometry and loading rate of the Wasatch fault zone that best explains the observed horizontal velocity field. Our preliminary results without considering block rotation show a dislocation dipping 27° and creeping 7 mm/yr at depths of 9-20 km. This dislocation may correspond to the interseismic loading-zone of the Wasatch fault, in which rocks are too weak to experience large brittle ruptures so aseismic creep accommodates most of the deformation. Focal mechanisms of large Basin-Range normal-faulting earthquakes, however, suggest that future ruptures of the Wasatch fault tend to propagate upward along steep planes (e.g., > 45°). Examining the rheological properties of crustal and fault-zone rocks suggests that the brittle/ductile transition zone of the crust is in the depth range of 6-14 km for the Wasatch fault area, in which crustal rocks are partly brittle and experience earthquakes, while fault zone rocks can be weaker than the adjacent rocks so that they undergo only quasi-plastic deformation. These results explain the locking-depths of the Wasatch fault zone of 7 to 9 km predicted by our dislocation models together with the observations of deeper background earthquakes, to the depth of 15 km, in the Wasatch Front area.