Slip and Strain Accumulation Along the Sadie Creek Fault, Northern Olympic Mountains, WA

Date: 4/24/2019

Time: 11:00 AM

Room: Puget Sound

Upper-plate faulting in the Olympic Peninsula of Washington State reflects the interaction of crustal blocks within the Cascadia forearc, as well as poorly constrained contributions from various earthquake cycle processes along the Cascadia subduction zone (CSZ), including interseismic coupling, megathrust earthquakes, and aseismic slow slip events. In this study we utilize high resolution airborne lidar, optically stimulated luminescence (OSL) dating, ¹⁴C dating, and field mapping of deformed surficial deposits and landforms to reconstruct fault slip rates since Late Pleistocene deglaciation on the recently discovered active Sadie Creek fault (SCF), located north of the Olympic Mountains. Our mapping shows the SCF is a ~14 km-long NW striking, subvertical, dextral fault with a subordinate dip slip component that connects with the longer Lake Creek Boundary Creek fault (LCBCF) beneath Lake Crescent. Field and lidar measurements of 48 scarp profiles and 11 laterally offset stream channels indicate cumulative slip on Late Pleistocene and younger surfaces varies along strike with vertical slip ranging from 1-6 m (average of 3.5 m) and cumulative dextral slip ranging from 6-26 m (average of 14.8 m). Recent work on the adjacent LCBCF presented a slightly higher range of cumulative dextral slip (11-28 m) and lower range of cumulative vertical slip (1-2m) suggesting that slip on the SCF may be more oblique than on the LCBCF. Preliminary ¹⁴C ages of deposits within offset stream channels suggest a slip rate of 1-2 mm/yr on the SCF since deglaciation (~13-15ka) and will be refined with pending OSL and ¹⁴C dates of other offset deposits. We will compare millennial slip rates to a geodetically-constrained boundary element method model which estimates the stress on the SCF and LCBCF as a result of different shorter-term (decadal) earthquake cycle processes on the CSZ. Taken together, our results will provide insight into how strain from the CSZ persists over multiple timescales on upper plate faults.

Presenting Author: William C. Duckworth

Authors