Evidence for Strong Holocene Ground Shaking on the Wallula Fault: Nice Scarp, Where’s the Fault...?

Session: Cryptic Faults: Assessing Seismic Hazard on Slow Slipping, Blind or Distributed Fault Systems [Poster]
Type: Poster
Date: 4/28/2020
Time: 08:00 AM
Room: Ballroom
Description: 

The Wallula fault zone is composed of northwest striking faults and folds that extend for ~120 km across southeast Washington. Prior paleoseismic study west of the Columbia River indicate at least three late Pleistocene surface rupturing earthquakes and an episode of liquefaction during the Holocene. We use lidar images to identify a Holocene scarp east of the Columbia River that trends parallel to and along the northern flank of the Horse Heaven Hills for ~25 km within the lower loess-capped surfaces. The scarp is ~0.5 m-high, south- and uphill-facing, parallels an adjacent broad (1-5 m-wide) discontinuous trough to the south and displaces the ground surface 0.2 – 1.3 m down to the south. We will present structural and stratigraphic observations from a recent paleoseismic trench emplaced across the scarp within loess capped rhythmatic glacial-outburst flood deposits (aka. Touchet Beds; ca. 18 – 15 ka) located at the western end of the scarp. The trench exposure shows the scarp to be the result of a thickness change within the post-glacial loess and the underlying Touchet Beds (10-30 cm-thick) 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. Large-scale ‘flame structures’ composed of fragmented Touchet bed material within the loess overlie large bodies of liquefaction within the Touchet beds north of the scarp and indicate a post-glacial liquefaction event. We interpret a thinly laminated silt bed within the upper loess package to be slope-wash deposition sourcing from the steeper slopes to the south. If correct, deposition of this unit predates the scarp and pending results of ¹⁴C age dating of charcoal collected at the base of this unit will place a maximum age to scarp formation. The process of scarp formation at this site remains unclear; however, the liquefaction event provides evidence for strong Holocene ground shaking at this locality and supports prior observations. We suspect this Holocene scarp to be related to the 1936 Milton-Freewater M6 earthquake.

Presenting Author: Stephen Angster

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