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The AAPG/Datapages Combined Publications Database

Utah Geological Association

Abstract


Environmental and Engineering Geology of the Wasatch Front Region, 1995
Pages 1-9

Seismotectonic Setting and Recurrence of Large Earthquakes in the Wasatch Front Region, Utah

Suzanne Hecker

Abstract

Geologic and seismologic observations show that the Wasatch Front Region (WFR) is part of the tectonically active eastern margin of the Basin and Range Province and is susceptible to large-magnitude (~6.5-7.5) earthquakes. The region’s earthquake potential is not fully apparent from the short record of historical seismicity, with only one event large enough to cause fault rupture of the ground surface. However, geologic evidence of surface-faulting earthquakes is widespread and testifies to the presence of significant earthquake hazards in the region.

A survey of paleoseismic information for the WFR indicates that a minimum of 50 to 120 (with a preferred minimum estimate of 85) surface-faulting earthquakes have occurred on as many as 37 normal faults in the WFR during the past ~15,000 years. This estimate gives a minimum recurrence rate of 3.9 to 9.4 events per 10,000 years per 10,000 km2 (3,863 mi2) for the 85,000-km2 (82,833-mi2) region and a maximum regional recurrence interval of 125 to 300 years. The tally of earthquakes is considered a minimum estimate because some events near the threshold magnitude (~6.5) for surface faulting have probably not been recognized.

The low end of the range in the earthquake tally incorporates the possibility that surface-rupturing movement may occur on some faults without independently generating large earthquakes. In particular, faults that are associated with low-angle structures in the shallow crust (perhaps relicts of Sevier thrust faulting) may not have the deeply penetrating, higher angle geometries known to characterize seismogenic faults and thus may rupture, with mechanics that are poorly understood, without producing large earthquakes.

Surface faulting activity has been concentrated along the 340-km- (211-mi-) long Wasatch fault zone, a structure that bisects the WFR from north to south and underlies Utah’s largest cities. The five central segments of the Wasatch fault zone account for only ~15 percent of faults active during the past 15,000 years, but were the source of approximately half of the estimated 50 to 120 surface-faulting earthquakes in the region.


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