Paleoseismology of Utah, Volume 1

Fault behavior and earthquake recurrence on the Provo segment of the Wasatch fault zone at Mapleton, Utah County, Utah

Special Study 75, 1991, 41 pages, $7.00

ABSTRACT

The results of a cooperative program between the Utah Geological and Mineral Survey and the United States Geological Survey to excavate trenches across the proposed Spanish Fork segment of the Wasatch fault zone (WFZ) provide new data on the size and timing of prehistoric earthquakes in the southern part of Utah Valley. The study was undertaken to determine if the subdivision of the original Provo segment of the WFZ into the proposed American Fork, Provo (restricted sense), and Spanish Fork segments could be substantiated on the basis of differences in timing of past earthquakes on the proposed segments.

Trenches were excavated across the WFZ on the proposed Spanish Fork segment at two closely spaced sites in Mapleton, Utah. At the north site, the WFZ is defined by a single scarp and graben in Holocene alluvial-fan deposits. Calendar-calibrated radiocarbon dates on charcoal from pre- and post-event deposits constrain the timing of the most recent surface faulting at 600 (t 80) yr B.P. Scarp profiles and the height of a buried scarp free face exposed by trenching indicate that an estimated 1.4 to 3.0 in (4.69.8 ft) of net vertical tectonic displacement occurred during the most recent surface-faulting earthquake (MRE) at the north trench site.

At the south trench site, the WFZ consists of two subparallel scarps that displace an upper Holocene alluvial fan. Trench exposures showed evidence for two prehistoric surface-faulting earthquakes. The MRE could not be dated at the south trench site. The penultimate surface-faulting earthquake occurred shortly before 2820 (+150, -130) yr B. P., based on radiocarbon and thermoluminescence age estimates obtained from a buried soil that was displaced prior to burial. The weak zonal development of the buried soil suggests that the penultimate earthquake occurred close in time to soil burial. The MRE at Mapleton is estimated to have had a maximum surface-wave magnitude of Ms 6.9 to 7.3 and an inferred moment magnitude of Ms 6.8 to 7.3.

The time range for the MRE at the Mapleton site (600 t 80 yr B.P.) overlaps that of the MRE at the American Fork Canyon site (550 t 100yr B.P.) on the proposed American Fork segment about 50 km (30 mi) to the north. Although less well constrained, timing of the penultimate event at Mapleton (shortly before 2820 +150, -130 yr B. P.) partially overlaps that of the penultimate event at the American Fork Canyon site (2650 t 150 yr B.P.).

The similarity in timing of the past two surface-faulting earthquakes at these two sites strongly suggests that both locations experienced the same surface-rupturing events and, therefore, define a single rupture segment (original Provo segment) of the Wasatch fault zone (WFZ) as suggested by Schwartz and Coppersmith (1984).