NEW INSIGHTS ON ACTIVE FAULTS AND CRUSTAL ARCHITECTURE OF THE KITTITAS VALLEY, WASHINGTON, USA: A MULTI-SCALE LOOK UTILIZING SEISMIC REFLECTION PROFILES AND PALEOSEISMIC TRENCHING

The Kittitas Valley, located in central Washington State, USA, is a low-relief basin within the northwest corner of the Yakima fold province, a region of active NNE-SSW shortening in the backarc of the Cascadia subduction zone. Our study focuses on the Dead Coyote and Craigs Hills faults, two faults that transect the valley and parallel the prominent and active fault-cored anticlinal ridges to the south. To understand the subsurface geometries, relative timing, and rate of deformation of these individual structures in the Kittitas Valley, we combine observations from multi-scale seismic reflection datasets, ranging from deeper (~4.5 km) imaging legacy industry profiles to ~7.5 km of high-resolution shallow imaging (<1.5 km) profiles, with tectonic geomorphic mapping and paleoseismic trenching. Analysis of the seismic data reveal evidence of shallow north-vergent anticlinal folding beneath an east-west trending set of fluvially incised knolls that define the queried Craigs Hill fault trace in the middle of the valley. Conversely, south-vergent folding is observed below the Dead Coyote fault scarp ~6 km to the north. Stratigraphic and structural relationships of late Pliocene and Quaternary deposits within two trench exposures across the Dead Coyote fault scarp suggest multiple episodes of late Quaternary surface faulting above a south-verging thrust fault. Pedogenic characteristics of buried soils developed on scarp-derived colluvial wedges suggest long recurrence intervals with the most recent earthquake during the Holocene and the penultimate earthquake likely sometime during the late Pleistocene. Our findings suggest that these seismogenic faults within the populated Kittitas Valley accommodate and transfer transpressional strain westward across the Cascade Mountains.