Paleoseismic Investigation Of The Clarkston Junction Hills And Wellsville Faults West Cache Fault Zone Cache County Utah

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

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.

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.

Report summarizes a Utah Geological Survey project to characterize the relative level of activity of the Collinston and Clarkston mountain segments of the Wasatch Fault Zone.

The U.S. Environmental Protection Agency has recommended that states develop Pesticide Management Plans for four agricultural chemicals - alachlor, atrazine, metolachlor, and simazine - used in Utah as herbicides in the production of corn and sorghum, and to control weeds and undesired vegetation (such as along right-of-ways or utility substations). This report and accompanying maps are intended to be used as part of these Pesticide Management Plans to provide local, state, and federal government agencies and agricultural pesticide users with a base of information concerning sensitivity and vulnerability of ground water in the basin-fill aquifer (bedrock is not evaluated) to agricultural pesticides in Cache Valley, Cache County, Utah. We used existing data to produce pesticide sensitivity and vulnerability maps by applying an attribute ranking system specifically tailored to the western United States using Geographic Information System analysis methods. 28 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.