Constraining the Paleoseismic Record on the Wallula fault in Southeastern Washington from Widespread Liquefaction Features: Nice scarp, Where’s the Fault..?

Abstract:

The Wallula fault zone is composed of northwest striking faults and folds that extend for ~120 km across southeast Washington. Prior paleoseismic study indicate at least three late Pleistocene surface rupturing earthquakes west of the Columbia River and an episode of liquefaction during the Holocene. We identify from lidar a ~0.5 m-high south-facing Holocene scarp east of the Columbia River that trends along the base of the north-facing escarpment of the Horse Heaven Hills for ~20 km. In this study, we combine observations from geomorphic mapping, paleoseismic trenching, and an outcrop study to place constraints on the kinematics and paleoseismic record of the Wallula fault. Observations from the trench show the scarp to be the result of a thickness change within the capping loess, while the underlying glacial-outburst flood deposits (aka. Touchet Beds; 18–15 ka) do not display any detectable vertical offset below the scarp. Rather the beds dip gently (<5⁰) to the north under the scarp forming a gentle monoclinal fold where large-scale ‘flame structures’ composed of fragmented Touchet bed material within the loess overlie large sills and dikes of liquefaction north of the scarp, indicating a post-glacial liquefaction event. Although the process of scarp formation at this site remains unclear, radiocarbon dating results from charcoal collected at the base of a thinly laminated capping silt bed, interpreted to be slope-wash deposition, suggest a maximum age of scarp formation at 584±56 ca. yr BP. Additional observations and pending OSL dating results at a near-by outcrop of similar large-scale liquefaction dike features that cross-cut the interbedded Mt saint Helens set C tephra (15.5 cal k.y. B.P.) provide an opportunity to constrain earthquake timing. We suspect the scarp to be the surface rupture trace of the of the 1936 Milton-Freewater M6 earthquake and representative of a partial rupture along the Wallula fault zone.