ACTIVE FAULTING ALONG A NEWLY-FOUND SEGMENT OF THE SADDLE MOUNTAIN FAULT ZONE, SOUTHEASTERN OLYMPIC MOUNTAINS, WA: A PALEOSEISMIC TRENCHING STUDY

A 20 meter-long paleo-seismic trench was dug across a LiDAR-detected, Holocene-age fault scarp near Price Lake during the fall of 2008 on the eastern-facing flank of Dow Mountain. The scarp is located approximately 0.5 miles southeast of and trends sub-parallel to the mapped traces of the demonstrably active Saddle Mountain East and West faults. Scarp height at the trench site was measured at 3.2 m; greater scarp heights were noted west of the trench location on the western portion of Dow Mountain.

The trench exposed a southwest-striking, steeply-dipping fault which offsets bedrock (Eocene Crescent basalt), Vashon-age glacial deposits, and surficial soil. In addition, multiple buried colluvial wedges interpreted to have originated from scarp collapse and their associated soil horizons were encountered in the footwall. The vertical separation of bedrock is the same as the total scarp height, yet the Vashon deposits in the hangingwall of the fault are significantly thicker than those in the footwall. From slickenlines measured on both the active fault surface and within the fractured bedrock surfaces in the hangingwall, the direction of fault movement is uncertain, although data are highly suggestive that older movement on the fault (measured on fracture surfaces in bedrock adjacent to the active fault surface) was southeast- to east-directed oblique reverse. Holocene-age movement (measured on slickenlines within the clay-lined active fault surface) was sub-horizontal to slope-parallel, which implies left-lateral strike-slip offset when coupled with the vertical sense of bedrock offset. However, the scarp facing direction (southeast) suggests right-lateral offset, making the determination of slip direction problematic.

The relationships between faults, colluvium and soil development within the trench give strong evidence for multiple (possibly four) Holocene-age faulting events on this newly-found segment of the Saddle Mountain fault system. A lack of bedrock clasts in any of the footwall colluvial wedge material indicates scarp height was never sufficient to expose bedrock, suggesting a short recurrence interval of relatively small magnitude earthquakes. Radiocarbon age estimates of charcoal sampled from four buried soil horizons in the footwall of the fault are forthcoming.