Marine and Lacustrine Turbidite Records: Testing Linkages and Estimating Ground Motions, Central Cascadia Margin, USA

Abstract:

We are investigating a potential paleoseismic record at Bull Run Lake, 165 km inland and 280 km landward of the tip of the plate boundary thrust, at the latitude of Portland, Oregon, central Cascadia margin. Bull Run is a landslide dammed lake in a cirque basin on the western flanks of Mt. Hood. Bull Run is potentially a good paleoseismic site, with no major stream inputs and a small catchment basin. The watershed and lake are faulted, and may contain ashes and evidence of crustal faulting. The lake was investigated by Raymond (1983), who cored the lake and found an orderly stratigraphic sequence with a number of minerogenic disturbance events (turbidites) and the Mazama Ash.  The bulk ages of several of the disturbance events dated in Raymond’s cores overlap well-known Cascadia earthquakes, including the AD 1700 event and several prior earthquakes, suggesting potential for this site. We collected full coverage high-resolution multibeam and backscatter data, along with a high resolution grid of CHIRP sub-bottom profiles, and seven new sediment gravity cores. We find that the turbidite record in the lake is well imaged in the high-resolution chirp data, and is found throughout the lake, including at least one basin isolated from the main basin. The continuity of the turbidite record shows little or no relationship to the minor stream inlets, suggesting the disturbance beds are not likely to be storm related. Many faint laminae may contain a storm record. Subtle channels from north and south sides of the lake feed an axial channel that terminates at the eastern shore. Lake sidewall failures are evident on the north and south walls, and occur with and without imageable tabular blocky slide debris where sedimented slopes exceed ~ 22-25 deg. Smaller failures visible in backscatter data are found on slopes as low as 12 degrees. We conducted diver investigations of several of the landslide areas, collecting hand push core samples and in-situ vane shear torquemeter measurements. Initial slope stability models suggest that slopes less than ~ 25 degrees are statically stable. We are investigating the levels of ground motion required to destabilize surface sediments around the lake, and radiocarbon dating the disturbance events for comparison to other paleoseismic records, including new offshore cores at a similar latitude.