PALEOSEISMIC CHARACTERIZATION OF ACTIVE HORST AND GRABEN FAULTS ADJACENT TO WEST MOUNTAIN IN UTAH VALLEY ON THE HANGING WALL SIDE OF THE PROVO SEGMENT OF THE WASATCH FAULT

Normal faults within the hanging wall of the Wasatch fault zone (WFZ) pose significant hazard to the urban population of Utah. Lidar mapping has revealed that these types of faults are common within valleys of the Wasatch front, but they are still being characterized in terms of their earthquake recurrence and whether they move synchronously with large events on the Wasatch fault or rupture independently. A prominent hanging wall structure within Utah Valley is West Mountain, which reaches ~ 600 m above the valley and extends parallel to the Provo segment for about 14 km. Using lidar, we mapped discontinuous fault scarps extending along the entire length of both the west and east side of West Mountain. The fault scarps to the west of the range, named the Genola North fault (GNF), aligns with the Goshen and Long Ridge faults to the south. Fault scarps we mapped immediately to the east of West Mountain are not presently included within the Utah Quaternary fault and fold database but appear to be the Lincoln Point – Dry Hollow fault (LPDHF) mapped on the 1:24K Lincoln Point Quadrangle (2009) by Soloman and Biek of the Utah Geological Survey (M-232). Together, the west dipping GNF and east dipping LPDHF create a structural horst which forms West Mountain. To the east, the LPDHF scarps cut across mid to late Holocene surfaces within the Benjamin Slough. The vertical displacements on the youngest scarps range from 0.2 – 1 m. There is also topographic evidence of a prior event, demonstrating recurrence along this fault. On the west side, the GNF scarps range from 1 m on post-Bonneville fans up to 3 m on a Provo-age Lake platform at the West Mountain Paleoseismic Site (40.1146, -111.8406). Here, we excavated a single fault-perpendicular trench and documented evidence of two earthquakes since 6.4 ka and another older earthquake that occurred close-in-time to the Bonneville highstand. Taken together, our mapping and paleoseismic results show that the faults of West Mountain are responsible for at least 4 – 5 surface rupturing earthquakes since 20 ka. Importantly, the endpoints of these mapped faults are within 5 km of three neighboring faults to the south as well as faults mapped within Utah Lake to the north, suggesting that multi-fault ruptures are a strong possibility.