History of Recent Surface-Rupturing Earthquakes on the Burbank fault, Yakima Folds, Central Washington

Clockwise rotation and crustal shortening in the upper plate of the Cascadia subduction zone occurs >500 km from the plate boundary. In the Yakima folds region of central Washington, GPS data indicate active shortening is oriented NNE, slightly oblique to thrust faults and folds. We use 2-meter lidar to identify and map scarps along the NW-SE-striking Burbank fault, a back-thrust related to the Quaternary-active, 120 km-long Umtanum Ridge-Gable Mountain fault-cored fold. We excavated two paleoseismic trenches across a west-facing scarp that splays off the primary fault trace and cuts across Miocene lithologic contacts. The southern trench was excavated across a 4-m-high scarp, exposing a 5-m-wide fault zone with scarp-derived colluvium in steep fault contact with Miocene basalt. At least three surface-rupturing earthquakes are inferred from colluvial-wedge stratigraphy, fault terminations, and soil formation. Each colluvial wedge is 0.2-0.3 m-thick, suggesting 0.5 m of throw per event, if colluvial deposition filled half of the scarp height. Colluvial wedge thickness and oblique-slip kinematic data (most slickenlines plunge <25°) indicate that slip per event may exceed 1 m. Ages from 10 radiocarbon and 6 luminescence samples will constrain prehistoric earthquake timing. The northern trench was excavated across a 10-m-high portion of the same scarp, exposing a 10 m-wide fault zone with scarp-derived colluvium in steep fault contact with Miocene sandstone and basalt. Distinct carbonate horizons occur at depths of 0.6 m, 0.9 m, and 1.8 m. U-series ages from these horizons, along with luminescence and radiocarbon ages, will constrain the timing of punctuated co-seismic subsidence and colluvial wedge aggradation. Additional paleoseismic studies on this structure are required to estimate rupture length and magnitude of paleoearthquakes. Results will aid hazard analyses for sites that might be shaken by earthquakes on the Umtanum Ridge-Gable Mountain structure.