Active Faulting Along the Saddle Mountain fault zone, Southeast Olympic Mountains,WA

Excavations across two strands of the Saddle Mountain fault zone show that a minimum of two Holocene earthquakes deformed glacial and post-glacial deposits and underlying bedrock along the southeast flank of the Olympic Mountains. Strands of the fault zone are clearly visible on LiDAR topography as multiple, prominent, northeast-trending scarps that extend at least 6-7 km between Hood Canal and Lake Cushman. We excavated two trenches to determine style and ages of faulting, one along Saddle Mountain East fault (SMEF) on Saddle Mountain, and the other along a newly-identified, sub-parallel fault strand located about 1km to the SE of SMEF on Dow Mountain. A trench excavated across the NW-facing SMEF scarp exposed Eocene Crescent basalt unconformably overlain by late Quaternary glacial deposits, a Holocene buried soil, and a Holocene colluvial wedge. A steeply dipping, NE-trending fault cuts and vertically separates by 3m the basalt and overlying glacial deposits and soil. The colluvium deposited over the soil also appears to be cut and deformed by the fault, suggesting a younger event. Although the main component of faulting that generated the scarp appears to be of NW-verging reverse sense, structural evidence from striations on the fault surface point to lateral motion in the final event. Rake measurements of striations etched on the clay-lined fault surface consistently plunge gently SW, implying left-lateral motion. Owing to a lack of evidence for strike-slip in the geomorphology, and given the height of the scarp and the shallow plunge of the striations, we infer that two earthquakes produced the observations: earlier reverse or oblique faulting generated the bedrock vertical separation and scarp relief, and subsequent oblique motion in a smaller event cut the colluvium and left the striations on the fault surface. Stratigraphy of the Dow Mountain fault strand also records evidence for at least two earthquakes, possibly as many as four. The trench across the SE-facing scarp exposes a steeply NW-dipping fault with 3m vertical offset of Crescent basalt and the overlying late Quaternary glacial deposits, similar to the SMEF trench. A thick colluvium package rests on the glacial deposits and contains as many as three colluvial wedges, separated by thin soils. The entire colluvium-soil package is cut by the fault, suggesting 4 events. However, internal stratigraphy of the package is somewhat ambiguous. Two of the colluvial wedges instead might be a single deposit due to uncertainty in interpreting the intervening soil, so the actual number of earthquakes might be fewer. Like the SMEF trench, striations on the fault plane suggest left-lateral strike-slip in the last event. Yet, also like the SMEF trench, reverse motion on the fault likely was the dominant contributor to vertical offset and scarp formation.