Recent activity of the Wasatch fault, Northwestern Utah U.S.A.

Abstract

The Wasatch fault trends north—south for a total length of 370 km, between Gunnison, Utah and Malad City, Idaho. The fault is in the eastern Basin-and-Range Province, adjacent to the Middle Rocky Mountains and the Colorado Plateau. The most recent fault displacements indicate normal slip, with the western block faulted down relative to the Wasatch Mountains. The Wasatch fault is within the Intermountain Seismic Belt of the western U.S.A.; however, it has been associated with only minor seismicity within historic time (140 yrs). There is no conclusive evidence of significant historic surface faulting along the Wasatch, although there is a concentrated 50–100-m-wide zone of damaged streets, curbs, houses, and buildings where the fault trends through Salt Lake City. This damage may be interpreted as evidence of tectonic creep; however, other possible interpretations include fault-controlled damage related to slope instability along the steep scarp or fault-controlled damage related to groundwater. Precise geodetic leveling across the Salt Lake City segments suggests that tectonic creep may be occurring, although the time span (7 yrs) has not been long enough to establish a consistent trend. Geomorphic evidence and tree-ring dating suggest that the most recent events of appreciable surface faulting are no older than a few hundred years. Fresh scarps in geologically young glacial and post-glacial deposits are 13–20 m high. Surface manifestations of active faulting along the Wasatch include multiple-fault scarplets in geologically young materials, graben on the downthrown block, backward tilting of the downthrown block near the fault, triangular facets along the fault scarp, lines of springs, and slickensided fault surfaces. In addition to the geologic evidence of active faulting, there are numerous landslides and large debris flows that have been faulted subsequent to their deposition, as linear scarps and other geomorphic features indicative of surface faulting cross them. Our current geologic research on the activity of the Wasatch fault is directed toward determining the recent history of surface displacements and the relationship of the fault with large-magnitude earthquakes, which is based upon the faulted sequence of Pleistocene Lake Bonneville beds and glacial deposits along the Wasatch Front.