Paleoseismology of Utah, Volume 8

Paleoseismic investigation at Rock Canyon, Provo segment, Wasatch fault zone, Utah County, Utah

Special Study 93, 1998, 21 pages, 2 plates, $ 8.00

ABSTRACT

A trench and a natural stream cut at Rock Canyon near Provo, Utah expose fault-scarp-derived colluvium and faulted Holocene debris-flow and fluvial deposits that provide new information on the size and timing of the most recent surface-faulting earthquake on the Provo segment of the Wasatch fault zone. At the trench site, the Wasatch fault is expressed by a single, 5-meter-high scarp. The scarp becomes progressively lower to the north as displacement is partially transferred to a more westerly subparallel trace. Both fault traces intersect the stream cut about 50 meters north of the trench.

The trench exposed a single main fault, an antithetic fault, and an intervening 15-meter-wide graben. A single, wedge-shaped deposit of scarp-derived colluvium was present on the downthrown side of both the main and antithetic faults. The association of only one colluvial-wedge deposit with each of the faults is evidence for a single surface-faulting earthquake at the site in late Holocene time. Several geologic units, including three organic-rich paleosols, could be traced the entire length of the trench and are displaced across the main and antithetic faults. Net vertical tectonic displacement across the fault zone is 3.3 meters. Apparent-mean-residence-time (AMRT) radiocarbon ages on concentrated organics from the three paleosols provide broad constraints on earthquake timing. The event postdates burial of the middle paleosol at 1,600 cal yr B.P. and predates burial of the youngest (uppermost) paleosol by a post-event debris flow at 550 cal yr B.P. Precisely when the faulting occurred within the resulting 1,050-year window is unknown, but stratigraphic relations indicate it was probably closer to the end of the window (550 cal yr B.P.) than to the beginning.

Two fault strands are exposed in the Rock Creek stream cut. At the East fault, a single wedge of scarp-derived colluvium rests directly on an organic-rich paleosol. Concentrated organics from a bulk sample of the paleosol from beneath the colluvial wedge yielded an AMRT radiocarbon age of 1,330 ± 60 14C yr B.P., which represents an average age for all the carbon in the paleosol. In 1995, the uppermost 5 centimeters of the paleosol directly beneath the colluvial wedge were sampled to better constrain the age of the youngest carbon in the buried soil. Based on the resulting radiocarbon age of 830 ± 60 14C yr B.P., the paleosol was buried at about 650 cal yr B.P. Because the paleosol was buried by scarp-derived colluvium, the most recent surface-faulting earthquake at Rock Canyon occurred just prior to that time. Net vertical tectonic displacement across the East fault is 1.2 meters.

At the West fault, fluvial erosion has removed the fault scarp and all but the lower and distal pans of the scarp-derived colluvial wedge associated with the fault. An organic-rich paleosol is preserved on the downthrown side of the fault. Stratigraphic and structural relations show that two thin units (a debris flow and a fluvial deposit) overlying the paleosol were deposited after the earthquake. The two units are in turn overlain by the distal part of the colluvial wedge. Concentrated organics from a bulk sample of the paleosol collected from beneath the two units and the colluvial wedge gave an AMRT radiocarbon age of 1,520 ± 80 14C yr B.P., which represents an average age for all the carbon in the paleosol. Based on stratigraphic relations in the stream cut, the paleosols beneath the colluvial wedges at the East and West faults are believed to be the same soil unit. The approximate 200 14C year difference in their ages is within the expected resolution of the AMRT dating method and is attributed to natural variations in the soil-forming process at the two sample locations.

Extensive sloughing of the stream cut at the West fault precluded re-sampling the paleosol in 1995 to better constrain the age of the youngest carbon in the buried soil. Based on the available AMRT age (1,520 ± 80 14C yr B.P.), the paleosol was buried by the post-event debris-flow and fluvial units at about 1,100 cal yr B.P., and the earthquake would be somewhat older than that age. However, because of the large age span of the carbon in the paleosol, the calendric age provides a maximum limiting age for the event. The actual time of burial could be tens to hundreds of years younger than the calendric age estimate. Stratigraphic relations make radiocarbon ages on detrital charcoal from fluvial and debris-flow units adjacent to and overlying the West fault equivocal and of no use in constraining the time of most recent surface faulting. Because of extensive erosion of key deposits, net vertical tectonic displacement could not be determined across the West fault.

The new paleoseismic information obtained on the timing of the most recent surface-faulting earthquake at Rock Canyon indicates the event occurred shortly before 650 (+50/ -100) cal yr B.P. The timing of the most recent event at Rock Canyon is in general agreement with the timing of the most recent events at American Fork Canyon (500 ± 200 cal yr B.P.) and Mapleton (600 ± 80 cal yr B.P.) to the north and south, respectively, of the Rock Canyon site. The close similarity in earthquake timing at the three sites suggests that all three locations experienced the same surface-faulting earthquake and are situated on a single rupture segment.

Using a variety of fault parameters and paleomagnitude relations, magnitude estimates for the most recent surface-faulting earthquake at Rock Canyon range from Mw/Ms 7.2 to Ms 7.6. Because uncertainty exists regarding the maximum displacement and rupture length produced by the earthquake, we consider a magnitude range of 7.2 to 7.4 a conservative best estimate for the magnitude of the event.