Paleoseismology of Utah Volume 3

The number and timing of paleoseismic events on the Nephi and Levan segments, Wasatch fault zone, Utah

Special Study 78, 1991, 23 pages, 3 plates, $6.50

ABSTRACT

The thermoluminescence (TL) and radiocarbon dating methods were used to constrain the paleoseismic history of the Nephi and Levan segments of the Wasatch fault zone (WFZ), Utah. Samples for dating were taken from the Deep Creek natural exposure and the Skinner Peaks trench, both on the Levan segment, and from the Red Canyon trench on the southern end of the Nephi segment.

At Deep Creek, a TL sample was taken from a soil buried by a large block of colluvium that spalled off the fault free face soon after rupture. The TL sample yielded an age estimate of 1000±100 yrs B.P. and provides a close maximum for the event. Offset stratigraphic layers, the amount of graben formation, and fault scarp morphology suggest this earthquake produced 1.8 in (5.9 ft) of net vertical tectonic displacement (NVTD).

A trench excavated near Skinner Peaks revealed evidence for 2 earthquakes that caused a minimum of 2.8 in (9.2 ft) of NVTD. The most recent event (MRE) is placed between l500 and 1000 yrs B.P. based on a TL age estimate of 2000 ± 300 years and a radiocarbon date of 1700 ± 200 yrs B.P., both from a burn horizon in the trench that is displaced by the MRE. The preferred time of faulting for the MRE is based on an estimate of the time required for an additional 0.5 in (1.7 ft) of sediment to accumulate on the burn horizon prior to the faulting event.

An older event is inferred due to a greater thickness of depositional units on the down versus the upthrown block of the fault. Age constraints on this event are provided by a TL age estimate of 3100 ± 300 yrs B.P. and a radiocarbon date of 3900 ± 300yrsB.P. These data and stratigraphic relations suggest that the older event occurred after 3900 yrs B.P. The dates for both events are in agreement with previous work on the segment.

The Red Canyon trench was cut across a scarp 5.5 in (I 8.0 ft) high near the southern terminus of the Nephi segment. Offset stratigraphy suggests three surface-rupturing earthquakes are responsible for approximately 5 in (16.4 ft) of NVTD. A maximum age for the MRE is provided by a TL age estimate of 1300 ± 500 and 1500 ± 400 yrs B.P. (most probable time of event 1200 yrs B.P.). The amount of displacement for this event is.4+-0.3m(4.6 ± 1.0 ft).

The penultimate event is constrained by maximum limiting radiocarbon ages of 3600 ± 400 and 3900 +500/-400 yrs B.P. (most probable time of event 3000-3500 yrs B.P.). The NVTD for this event is estimated at 1.5 ± 0.2 in (4.9 ± 0.66 ft). The oldest event has a maximum constraint of latest Pleistocene in age based on the morphology of the soil formed on the displaced alluvial-fan surface (most probable time of event 4000-4500 yrs B.P.). The amount of offset for this event is 1.7 ± 0.3 in (5.6 ± 1.0 ft).

Data from the Red Canyon trench are in general agreement with the three events in 5000 years determined at North Creek 20 km (12.5 mi) to the north.