RELATIONSHIP BETWEEN SLIP ON THE LAKE CREEK-BOUNDARY CREEK FAULT SYSTEM, NORTHERN OLYMPIC PENINSULA, AND THE UNDERLYING CASCADIA SUBDUCTION ZONE

Seismic hazard in the Pacific Northwest is posed by the Cascadia subduction zone (CSZ) and active faults in the overriding North American forearc. Upper-plate faulting in western Washington is driven by interactions among crustal blocks of the forearc, as well as interseismic and coseismic stresses from the CSZ. The stresses from these two sources promote different kinematics, slip rates, and timing of earthquakes on forearc faults. We investigate long and short-term fault kinematics on the northern Olympic Peninsula by integrating field observations with crustal deformation modeling. We focus on the Lake Creek-Boundary Creek fault (LCBC) and related strands, with known Holocene dextral-reverse motion, but which may have an earlier history of sinistral slip. We use new lidar data to map the surficial geology along the fault zone and to assess Holocene fault geometry and kinematics. Dextral offset of channels incised into glacial deposits and post-glacial surfaces ranges from 3 to 20 m with a horizontal:vertical offset ratio ~10:1. We will compare Holocene slip rate estimates derived from geomorphic and paleoseismic work with crustal deformation models that simulate the contemporary interactions between CSZ earthquake cycle processes and LCBC slip. Specifically, we use an elastic boundary element method model, with coupling on the CSZ constrained by GPS velocities, to estimate the slip rate distribution on the LCBC system required to relieve stresses imposed during the interseismic period. Preliminary modeling suggests that interseismic stresses from the CSZ drive left-lateral motion up to 1 mm/yr on the LCBC, with a smaller component of reverse slip up to 0.5 mm/yr. Coseismic stress produces the opposite pattern, dominated by dextral motion and subordinate normal fault slip. The Sadie Creek strand, marking the northwest limit of the LCBC system, features the fastest estimated slip rates. Synthesis of crustal deformation modeling, paleoseismic trenching, and geomorphic mapping will enable direct testing of whether CSZ earthquake cycle processes are capable of driving seismic slip on forearc crustal faults producing long-term permanent deformation. Our work will also have implications for the relative stability of CSZ coupling and slip over multiple earthquake cycles.