TEN DEBRIS FLOWS AND A HOLOCENE FAULT: TWO GEOLOGIC-HAZARD CASE STUDIES ON UINTA NATIONAL FOREST LAND, EAST OF SANTAQUIN, UTAH

Santaquin City, at the southern end of Utah Valley, has recently been an inadvertent host to two interesting geologic-hazard-related investigations. With ongoing development, Santaquin's eastern city limits abut the Uinta National Forest boundary along the western flank of Dry Mountain, part of the Wasatch Range and tectonically active Intermountain seismic belt.

On September 12, 2002, intense rainfall triggered fire-related debris flows in 10 drainages on Dry Mountain. Debris and floodwater from four tributaries flowed into developed areas of Santaquin causing property damage in two subdivisions. Prior to the event, Dry Mountain had a heightened debris-flow and flooding hazard due to the human-caused Mollie wildfire in the Uinta National Forest that burned the west side of the mountain during the summer of 2001. The debris flows initiated by rapid runoff of rainfall on steep slopes with sparse vegetation and water-repellent soils. Antecedent soil moisture was also a key factor in promoting surface-water runoff. Progressive down-drainage channel erosion and sediment bulking resulted in debris-flow volumes of up to 20,000 cubic yards. On July 26, 2004, rainfall again triggered smaller debris flows from two drainages, supporting earlier findings that the drainage basins still contained ample sediment for future debris flows following the 2002 flows.

In a paleoseismic study undertaken by the Utah Geological Survey during the spring of 2005, two trenches were excavated on Uinta National Forest land across a Holocene fault scarp that displaces older alluvial-fan deposits, a fan incidentally affected by the debris flows of both 2002 and 2004. In addition to trenching, our study included detailed surficial mapping and fault-scarp profiling. The paleoseismic study was undertaken in cooperation with the U.S. Geological Survey as part of an investigation of the northern part of the Nephi segment of the Wasatch fault zone to better constrain the timing, magnitude, and extent of earthquakes on the Nephi segment and contribute to our understanding of fault segment boundaries, the relationship between neighboring fault segments, and Wasatch Front earthquake hazards in general.