A Post-Glacial Record of Large, Strike-Slip Earthquakes on the Sadie Creek Fault, Northern Olympic Peninsula, WA

Date: 4/24/2019

Time: 11:15 AM

Room: Puget Sound

Active faulting in the northern Olympic Peninsula of Washington reflects ongoing shortening and dextral transpression of the Cascadia subduction zone forearc. The Sadie Creek fault (SCF), a recently discovered splay of the Lake Creek – Boundary Creek fault (LCBCF), extends for ~15 km WNW from the northwest shore of Lake Crescent, immediately south of the Strait of Juan de Fuca. The surface trace of the SCF, identified and mapped using airborne lidar, reveals abundant geomorphic offsets suggesting a history of repeated post-glacial earthquake surface ruptures dominated by dextral displacement along a steeply dipping fault zone. We excavated two trenches across the SCF to investigate the timing and extent of surface ruptures and to assess their relationship to earthquakes identified in trenches on the adjacent LCBCF. The SCF trenches, along the eastern and central portions of the fault, were sited in fault-bounded depressions where uphill-facing scarps locally pond N-NW-flowing drainages. Stratigraphy in these trenches reveals till and post-glacial outwash overlain in the down-dropped block by progressively buried, organic-rich, forest and wetland soils developed on scarp-derived colluvial wedges. Complex faulting, fracturing, and tilting of these deposits strongly suggests an overall dextral sense of displacement, combined with dip-slip separation similar in magnitude to the scarp height (~2-5 m), consistent with the fault geomorphology. From these observations we infer 3-5 earthquake surface ruptures along the SCF since retreat of the Juan de Fuca lobe of the Cordilleran. Preliminary radiocarbon dating of charcoal fragments in gouge cores extending beneath the trenches restrict these events to younger than ~13.5 ka. Additional dating of abundant charcoal and woody plant fragments will further constrain the timing of these earthquakes for comparison with Holocene records in nearby Lake Crescent and the LCBCF to the east.

Presenting Author: Colin Amos

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