Paleoseismology of Utah, Volume 7

Paleoseismic investigation on the Salt Lake City segment of the Wasatch fault zone at the South Fork Dry Creek and Dry Gulch sites, Salt Lake County, Utah

Special Study 92, 1996, 22 pages, 1 plate, $5.25

ABSTRACT

The Wasatch fault zone (WFZ) is one of the longest and most active normal-slip faults in the world. The fault trends through the densely populated Salt Lake City metropolitan area and is a potential source for large earthquakes that pose a significant seismic hazard.

Previous paleoseismic studies at two sites (Little Cottonwood Canyon and South Fork Dry Creek) on the Salt Lake City (SLC) segment of the WFZ showed that at least three large-magnitude surface-faulting earthquakes occurred in Holocene time (past 10,000 years), including two earthquakes in the past 6,000 years. Timing for these earthquakes suggested the average recurrence interval of surface faulting on the SLC segment was 4,000 + 1,000 years.

However, not all of the fault scarps at either site were trenched. A subsequent study at Dry Gulch discovered a previously unrecognized surface-faulting earthquake on an untrenched scarp at the nearby South Fork Dry Creek (SFDC) site, which reduced the average recurrence interval to 2,400 ± 500 years in the past 10,000 years and 2,150 + 400 years in the past 6,000 years.

To develop a comprehensive Holocene chronology of surface-faulting earthquakes on the SLC segment, the Utah Geological Survey reoccupied the SFDC site to complete the investigation started there in 1985. We excavated five trenches across fault scarps at this site. When combined with the 1985 study, all fault scarps at SFDC are now trenched.

Radiocarbon age analyses of buried soils and organic-rich sediment in two trenches on different scarps show that a previously unrecognized surface-faulting earthquake occurred about 3,950 years ago. These new data, combined with evidence from the previous studies, show that four surface-faulting earthquakes (rather than three) occurred on the SLC segment in the past 6,000 years: (1) a most recent event shortly after 1,300 (+250, -200) cal B.P., (2) a second event shortly after 2,450 + 350 cal B.P., (3) a third event shortly after 3,950 (+550, -450) cal B.P., and (4) a fourth event shortly after 5,300 (+450, -350) cal B.P.

The newly discovered earthquake reduces the average recurrence of surface faulting on the SLC segment in the past 6,000 years from 2,150 + 400 years to 1,350 + 200 years.