Observations of widespread liquefaction and stratigraphic and structural relationships in a trench across an ambiguous scarp are used to constrain the timing of Holocene earthquakes on the northwest-striking Wallula fault zone in southeast Washington and Oregon. Additional observations and age constraints from OSL analysis of samples collected from large-scale liquefaction features that crosscut the Mount St Helens J tephra (13.8-13.7 ka) exposed at a nearby outcrop suggest up to 3 Holocene regional liquefaction events, any of which were likely triggered by seismic shaking sourced from either the Wallula fault and/or faults of the Yakima fold and thrust belt. Our observations provide plausible evidence supporting that the scarp formed during the M6 1936 Milton-Freewater earthquake. In addition, stratigraphic relationships observed in this study indicate that the end of the Missoula Floods in the southeast Washington region occurred sometime between 13.8–13.5 cal. k.y. B.P., approximately 1,000 years earlier than prior estimates.