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

The South Fork Dry Creek and Dry Gulch sites lie within a few hundred meters of each other in the southeastern part of the Salt Lake Valley, and together provide the only location on the heavily urbanized Salt Lake City segment of the Wasatch fault zone where it is possible to develop a complete surface-faulting chronology for the segment since middle Holocene time (the past 6,000 years). Investigations at the two sites took place intermittently between 1985 and 1995 as permission was obtained to trench more and more of the scarps within the broad fault zone. The new information reported here on the size, timing, and especially recurrence of surface-faulting earthquakes on the Salt Lake City segment shows that the earthquake hazard presented by this segment of the Wasatch fault is greater than previously thought. Such information is vital to public officials, planners, and others making decisions regarding earthquake-hazard mitigation. 22 pages + 1 plate

This report presents the results of a paleoseismic investigation designed to date a long series of consecutive earthquakes on the Wasatch fault zone and to measure the variability of recurrence times between the events. Geologists have long recognized that the comparatively short average recurrence interval (compared to most other basin-and-range normal faults) between large surface-faulting earthquakes on the Salt Lake City segment of the Wasatch fault zone during mid- to late-Holocene time is potentially anomalous, and possibly affected by the rise and fall of Lake Bonneville. This study extends the paleoearthquake record back to Bonneville time, nearly doubling the previous record, and provides new information on the timing and periodicity of surface faulting on the Salt Lake City segment from the latest Pleistocene through the Holocene. The trench and accompanying auger hole for this study exposed 26 meters of vertical section, roughly four times that of a typical paleoseismic trench on the Wasatch fault zone, hence the name “Megatrench.”

This 43-page report presents new data from the Willow Creek site that provides well-defined and narrow bounds on the times of the three youngest earthquakes on the southern strand of the Nephi segment, Wasatch Fault zone, and refines the time of the youngest earthquake to about 200 years ago. This is the youngest surface rupture on the entire Wasatch fault zone, which occurred about a century or less before European settles arrived in Utah. Two trenches at the Willow Creek site exposed three scarp-derived colluvial wedges that are evidence of three paleoearthquakes. OxCal modeling of ages from Willow Creek indicate that paleoearthquake WC1 occurred at 0.2 ± 0.1 ka, WC2 occurred at 1.2 ± 0.1 ka, and WC3 occurred at 1.9 ± 0.6 ka. Stratigraphic constraints on the time of paleoearthquake WC4 are extremely poor, so OxCal modeling only yields a broadly constrained age of 4.7 ± 1.8 ka. Results from the Willow Creek site significantly refine the times of late Holocene earthquakes on the Southern strand of the Nephi segment, and this result, when combined with a reanalysis of the stratigraphic and chronologic information from previous investigations at North Creek and Red Canyon, yield a stronger basis of correlating individual earthquakes between all three sites.

Field work for this paleoseismic investigation was performed in 1997 at three sites (Winter Canyon, Roundy Farm, and Deep Canyon) on the Clarkston, Junction Hills, and Wellsville faults. These faults, along with several lesser associated faults nearby, comprise the West Cache fault zone on the west side of Cache Valley. No previous paleoseismic studies had been conducted on these faults. The information reported here on the size, timing, and recurrence of surface-faulting earthquakes on the West Cache fault zone is critical to public officials, planners, and others making decisions regarding earthquake-hazard mitigation in Cache Valley and the northern Wasatch front. 23 pages + 1 plate

Field work for this paleoseismic investigation at Rock Canyon was performed in 1988. It was one of three studies conducted in the late 1980s and early 1990s to determine if the Provo segment of the Wasatch fault zone should be subdivided into three smaller segments as tentatively proposed by Machette and others on the basis of their geologic mapping. This investigation was the last of the three studies performed. Those results, combined with the results of paleoseismic investigations at American Fork Canyon and Mapleton, showed that the Wasatch fault where it passes through Utah Valley probably consists of a single, almost 70-kilometer-Iong fault segment (Machette and others, 1992). Publication of the details of the Rock Canyon study has been delayed for several years, chiefly due to the press of new job duties on the part of the investigators. The information remains important and is presented here for the use of those individuals interested in earthquake hazards and seismic-source characteristics of the Wasatch fault in Utah Valley. 21 pages + 2 plates

The Salt Lake City metropolitan area is one of the most seismically hazardous urban areas in the interior of the western U.S. because of its location within the Intermountain Seismic Belt and its position adjacent to the active Wasatch fault. The elapsed time since the last large earthquake on the Salt Lake City segment of the Wasatch fault is approaching the mean recurrence interval based on the short-term paleoseismic record. In order to help raise the awareness of the general public and to help reduce earthquake risk in this area, we have developed nine microzonation maps showing surficial ground-shaking hazard. The maps are GIS-based and incorporate the site response effects of the unconsolidated sediments that underlie most of the metropolitan area within Salt Lake Valley. These nine maps, at a scale of 1:75,000, make up three sets, each consisting of three maps that display color-contoured ground motions in terms of (1) peak horizontal acceleration, (2) horizontal spectral acceleration at a period of 0.2 sec (5 Hz) and, (3) horizontal spectral acceleration at a period of 1.0 sec (1 Hz). One set of maps consists of deterministic or “scenario” maps for a moment magnitude (M) 7.0 earthquake on the Salt Lake City segment of the Wasatch fault. The two other sets are probabilistic maps for the two return periods of building code relevance, 500 and 2,500 years.

This report presents the results of the Utah Quaternary Fault Parameters Working Group (hereafter referred to as the Working Group) review and evaluation of Utah’s Quaternary fault paleoseismic-trenching data. The purpose of the review was to (1) critically evaluate the accuracy and completeness of the paleoseismictrenching data, particularly regarding earthquake timing and displacement, (2) where the data permit, assign consensus, preferred recurrence-interval (RI) and vertical slip-rate (VSR) estimates with appropriate confidence limits to the faults/fault sections under review, and (3) identify critical gaps in the paleoseismic data and recommend where and what kinds of additional paleoseismic studies should be performed to ensure that Utah’s earthquake hazard is adequately documented and understood. It is important to note that, with the exception of the Great Salt Lake fault zone, the Working Group’s review was limited to faults/fault sections having paleoseismic-trenching data. Most Quaternary faults/fault sections in Utah have not been trenched, but many have RI and VSR estimates based on tectonic geomorphology or other non-trench-derived studies. Black and others compiled the RI and VSR data for Utah’s Quaternary faults, both those with and without trenches.

The Farmington Siding landslide complex is in Davis County, Utah, about 25 kilometers north of Salt Lake City. The landslide complex covers approximately 19.5 square kilometers and is one of 13 late Pleistocene/Holocene features along the Wasatch Front mapped by previous investigators as possible liquefaction-induced lateral spreads. The Farmington Siding landslide complex is in a largely rural area, but state and interstate highways, railroads, petroleum and natural-gas pipelines, and other lifelines cross the complex. Continued population growth along the Wasatch Front increases the likelihood of urban development within and adjacent to the landslide complex. Development along the Wasatch Front has proceeded with little consideration of hazards associated with liquefaction-induced landslides. Slope-failure mechanisms, extent of internal deformation, and timing of landslide events are poorly understood, and these factors must be evaluated to enable local governments to effectively plan for development and implement hazard-reduction strategies as needed. The purpose of this study is to assess the hazard associated with future liquefaction-induced landsliding within and adjacent to the Farmington Siding landslide complex by evaluating slope-failure modes and extent of internal deformation within the complex, inferring the geologic and hydrologic conditions under which landsliding occurred, determining the timing of landsliding, and evaluating the relative likelihood of various earthquake source zones to trigger liquefaction-induced landsliding. We chose the Farmington Siding landslide complex for this study because of the distinctiveness of geomorphic features on the northern part of the complex and the presence of landslide deposits that are clearly of different ages. Furthermore, because much of the area is rural, appropriate land-use planning measures can still be implemented to protect future development.