EARTHQUAKES AND DEEP-SEATED LANDSLIDES ASSOCIATED WITH THE BONNEVILLE FLOOD NEAR ITS OUTLET IN NORTHERN CACHE VALLEY, IDAHO: FIRST RESULTS

New LIDAR data from north Cache Valley, Idaho and Utah reveal fault scarps, segment boundaries, deep-seated landslides, shorelines, sand dunes and scour channels that formed during the outburst flood. Deep-seated landsliding (sackung) is a significant process in northern Cache Valley and the Portneuf Range and we identify at least three areas of ridge collapse. The largest one near the outlet of Lake Bonneville has 5 slip surfaces, spans at least 5 square km and collapsed Oxford Ridge in the Bannock Range. We will explore the minimum earthquake magnitude required to produce these features. We also distinguish between fault scarps of different ages using their cross-cutting relationships. There are two distinctly different structural domains in northern Cache Valley: In the southern domain, 50-25 km from the outlet, most fresh scarps are localized along the basin-bounding Dayton-Oxford normal fault (DOF) and the most recent earthquake postdates the regressive Provo deposits of Lake Bonneville. Degraded pre-Bonneville (?) fault scarps dominate along the DOF from a newly identified rupture segment boundary in Weston, Idaho northward. In a northern domain, from Clifton to the outlet of Lake Bonneville, there are fresh fault scarps along small displacement N- to NNE-striking normal faults in the hanging wall of the DOF. These are preserved north of Oxford, Idaho and in the Twin Lakes horst block. None of the fresh fault scarps in the hanging wall of the DOF persist along strike through deposits of the Provo-level delta or the outflow channel of Lake Bonneville in Round Valley. Therefore surface-faulting earthquakes ruptured several hanging wall faults between ~19 to 16 ka, during the high stand of Lake Bonneville, its flood, or slightly afterwards. Identical cross-cutting relationships were previously identified along the Riverdale fault zone, a few km farther east (Jänecke and Oaks., 2011), and the high-resolution LIDAR confirms that interpretation. Reservoir-induced seismicity due to loading or rapid unloading of Lake Bonneville may have activated the hanging wall faults and the Riverdale fault zone. We continue to explore possible associations of the Lake Bonneville and earthquakes in northern Cache Valley and will report updates at the GSA meeting.