CONFIRMED: WEST CACHE FAULT ZONE WAS FAR MORE ACTIVE THAN EAST CACHE FAULT ZONE SINCE BONNEVILLE PLUVIAL

The Cache Valley consists of N-trending graben and horsts in N-central Utah and SE Idaho. The East Cache fault zone (ECFZ) was long considered the active master fault due to the high mountain in its footwall, its inversion of the Providence thrust fault, and the deep sedimentary basin in its hanging wall. However, trenching in 1990s identified four Holocene paleo-earthquakes along three faults of the West Cache fault zone (WCFZ), but only one Holocene paleo-earthquake along the central segment of the ECFZ (Black et al., 2000; McCalpin, 1994). We use two LIDAR datasets to document significant new evidence of post-Bonneville earthquakes within the WCFZ across dozens of new faults. The only unambiguous new fault scarp identified in the ECFZ is a multi-event fault scarp at the mouth of Hyde Park Canyon in the northern segment of the ECFZ. In contrast, we identify many new fault scarps and new fault traces with steps, branches and splays along ~105 km of the revised WCFZ. Our fault maps add >80 km of new fault traces and revise the recency of faulting along >20 km of the Dayton Oxford fault (DOF). We discovered post-Bonneville fault scarps in or near the towns of Mendon, Weston, Clarkston and Newton. Active deformation is spread across the floor of Cache Valley, where single-event fault scarps on > 6 newly identified fault zones present a surface-faulting hazard. Work is ongoing to fully characterize the Airport-Dump, Newton, Twin Lakes, Oxford, Sandhill and Cutler Reservoir fault arrays. Distributed normal faults extend the mountains as well, and the southern Wellsville Range and the central Bear River Range are in the process of turning into separate half horsts. Left-stepping faults in Weston, Idaho along the DOF separate post-Bonneville fault scarps in the south from degraded fault scarps in the north. Weston is a segment boundary. Cross faults with possible Quaternary slip connect the Mantua, Hyrum and East Cache faults in southernmost Cache Valley. Rather than being a collection of four en echelon faults, the WCFZ is a broad interconnected mesh of normal faults with evidence for numerous post-Bonneville earthquakes. If adjacent faults or segments rupture together, the maximum credible earthquake on the WCFZ could be larger than currently predicted.