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Active faults of the segmented Wasatch fault zone are next to the largest and growing population centers of central Utah. Structural fault segment boundaries play an unknown role in limiting large earthquakes.
Little Cottonwood Canyon sketch with scarps marked
G.K. Gilbert documented fault scarps (marked with red arrows) near the mouth of Little Cottonwood Canyon (modified from Gilbert, 1890). Similar scarps can be observed along the entire Wasatch Front and are evidence of recent, large-magnitude earthquakes on the Wasatch fault zone.

What do we already know?

For over a century, geologists have documented tectonic fault scarps along the Wasatch fault zone. In the 1880s, renowned geologist G.K. Gilbert first reported fault scarps at the mouth of Little Cottonwood Canyon, approximately 20 km (12 miles) south of downtown Salt Lake City, and suggested the scarps were evidence of prehistoric ground-rupturing earthquakes. Today there is a park at this location commemorating Gilbert's work and showcasing the geological features.

Since 1979, paleoseismologists, scientists who study past earthquakes, have excavated numerous paleoseismic trenches along the WFZ (white stars on location map, at right), with the goal of documenting the timing and frequency of past earthquakes (see Paleoseismology of Utah Series, Utah Geological Survey). Paleoseismologists used this information about the recent past to provide insight into the frequency of future earthquakes and ground shaking, for example, as with the 2014 USGS National Seismic Hazard Maps. In connection with the trenching work, geologists produced detailed maps of the Wasatch fault trace and the rock units and young alluvial deposits that it cuts.

Together, these contributions led to the subdivision of the WFZ into 10 structural fault segments, each approximately 30-60 km (19-37 miles) long. The central five fault segments have been active during Holocene time (the last 10,000 years) and produce large (magnitude 7.0) earthquakes about every 900-1300 years. The most recent earthquake on fault segments near Weber, Provo, and Nephi occurred 200-700 years ago. The most recent earthquake on the Salt Lake City segment occurred 1200-1600 years ago. It has been 2200-2800 years since the most recent earthquake on the Brigham City fault segment.

Earthquake timeline
Central Wasatch fault zone. Geologic studies show that at least 22 surface-faulting earthquakes (shown as starbursts on this generalized diagram) have occurred on the Wasatch fault during the past 6,000 years. Image from "Putting Down Roots in the Central US", Utah Geological Survey.

What is still unknown?

Paleoseismologic studies have provided a detailed record of Holocene earthquakes on the WFZ. This record generally suggests that individual magnitude 7.0 earthquake ruptures have been limited to one of the approximately 30-60 km-long fault segments. However, in a few cases the data could also be interpreted as multi-segment ruptures on the WFZ, where rupture spanned a segment boundary.

Multi-segment earthquake ruptures on the WFZ may have produced longer surface ruptures and generated larger (magnitude 7.4) earthquakes; however, they may also result in less frequent and fewer total surface-rupturing earthquakes. The extent and frequency of ruptures that span segment boundaries remains poorly known, adding uncertainty to seismic hazard models for this populated region of Utah.