THE SEQUIM FAULT AND IMPLICATIONS FOR SEISMIC HAZARD ON THE NORTHERN OLYMPIC PENINSULA

Recent evaluation of the Sequim fault, located about 2 km south of Sequim, WA on the northern Olympic Peninsula, confirms Holocene activity and has implications for regional seismic hazard. Lidar-based mapping indicates the roughly east-west-striking Sequim fault consists of several subparallel north- and south- facing fault and fold scarps, extending discontinuously for about 13 km across Dungeness River alluvial fan surfaces and Vashon glacial upland surfaces on either side of the river. Fault scarps are up to 15-feet high across the older glacial surface compared with 5-feet high across the younger alluvial surface, suggesting the older surface has experienced more earthquakes. New geomorphic mapping combined with an existing radiocarbon date indicate that the most recent deformation postdates the 7.6 ka age of the oldest alluvial surface. Inset alluvial terraces, assumed to be on the order of a few thousand years old are not apparently deformed. The scarps align with a south-dipping fault to the east that was imaged on an existing marine seismic reflection profile and demonstrates late Quaternary and possibly Holocene deformation. The scarps are also approximately colinear with previously recognized down-to-the-north geophysical contrasts that follow the northeast margin of the Olympic Mountains and cross the northwest Puget Sound. Sequim fault and fold scarp morphologies suggest reverse faulting, which is consistent with the apparent geophysical fault geometry. Taken together, this information suggests the Sequim fault may represent an eastern continuation of the Holocene active North Olympic fault zone (NOFZ) and/or rupture in tandem with the NOFZ. Geophysical data suggests the Sequim fault may extend east to the Southern Whidbey Island fault zone. If this extension is considered, the fault system represents a considerably longer seismic source (132 km) than is currently in the U.S. National Seismic Hazard Maps model (52 km) for the NOFZ. This larger source is likely to have a significant impact on regional seismic hazard and will be included in a site-specific probabilistic seismic hazard analysis for a local reservoir project.