SLIP AND STRAIN ACCUMULATION ALONG THE SADIE CREEK FAULT, OLYMPIC PENINSULA, WA

Upper-plate faulting in the Olympic Peninsula of Washington State reflects the interaction of crustal blocks within the Cascadia forearc as well as contributions from various earthquake cycle processes along the Cascadia subduction zone (CSZ). In this study we utilize high resolution airborne lidar, field mapping of deformed surficial deposits and landforms, optically stimulated luminescence (OSL) dating and ¹⁴C dating to reconstruct fault slip rates since Late Pleistocene deglaciation on the Sadie Creek fault (SCF), located north of the Olympic Mountains. This mapping reveals the SCF as a ~14 km-long NW-striking, subvertical, dextral strike-slip fault with a subordinate dip-slip component. Field and lidar measurements of 48 scarp profiles and 11 laterally offset stream channels indicate that faulting of late Pleistocene and younger surfaces varies along strike with dextral slip ranging from 4.0–26.0 m (average of 14.3 ± 7.5 m) and dip-slip displacement ranging from 0.7–6.5 m (average of 3.4 ± 1.6 m). Reevaluation of fault slip on the adjacent Lake Creek Boundary Creek fault (LCBCF), which connects with the SCF beneath Lake Crescent, shows a slightly higher range of dextral slip (4.5–29.7 m, average of 15.9 ± 8.9 m) and lower range of dip-slip displacement (0.8–4.6 m, average of 2.3 ± 0.9 m). Preliminary OSL and ¹⁴C results constrain the retreat of the Juan de Fuca lobe of the Cordilleran ice sheet at ~14 ka and produce a preferred dextral slip rate of 1.3–2.3 mm/yr. Comparing this slip rate to geodetically constrained models of forearc deformation, we determine how shorter-term (decadal) stresses contribute to fault slip and strain accumulation within the upper plate. This approach uses an elastic block model and a boundary element method model to estimate the stress on the SCF and LCBCF as a result of different earthquake cycle processes in the forearc and on the CSZ. Models of coseismic stress transfer from a full-length rupture on the CSZ and elastic block models – which together consider the interactions of forearc blocks and CSZ coupling – both produce comparable estimates to the post-glacial slip rate and kinematics of the SCF and LCBCF. As such, the SCF/LCBCF plays an important role in the permanent accumulation of strain observed in the GPS velocity field but may be modulated by stress transferred from CSZ earthquakes.