abstract

No historical earthquake associated with surface faulting have occurred along the Wasatch fault zone during at least the past 133 yr. Exploratory trenching, analysis of fault scarp morphology and scarp-derived colluvial deposits, and detailed mapping at two locations along the fault zone indicated there have been repeated moderate to large magnitude earthquakes (M612 to 712) associated with surface faulting during the late Pleistocene and Holocene. These detailed geological studies provide new data on the style of deformation associated with normal-slip faulting, amount of cumulative displacement, amount of displacement per event, and the recurrence intervals of surface faulting earthquakes. At the Kaysville site, 30 km north of Salt Lake City, at least three surface faulting events have produced 10 to 11 m of cumulative net vertical tectonic displacement since the middle Holocene; estimates of the net tectonic displacement for individual faulting events range from 1.7 to 3.7 m. The two most recent events occurred within the past 1,580 ± 150 yr and interval between these events was from 500 to 1,000 yr. Along this segment of the fault, the average recurrence interval is probably close to 1,000 yr. The middle to late Holocene slip rate is 1.8 (+1.0; −0.6) nm/yr. At the Hobble Creek site, 46 km south of Salt Lake City, six or seven surface faulting events have produced 11.5 to 13.5 m of cumulative net vertical tectonic displacement during the past 12,000 to 13,000 yr; the average net tectonic displacement per event is 0.8 to 2.8 m. The average Holocene recurrence interval at this location is 1,500 to 2,600 yr and the slip rate is 1.0 ± 0.1 mm/yr. If the recurrence intervals on these segments of the Wasatch fault zone are typical of the other segments of the zone, the recurrence interval of moderate to large magnitude earthquakes for the entire Wasatch fault zone may be 50 to 430 yr.

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