32-2 Insights into the spatial, temporal, and structural evolution of the Wallula fault zone in the Walla Walla Basin of Oregon and Washington, USA.

Session: Latest Research Advances in Structural Geology and Tectonics (Posters)

Poster Booth No.: 220

Presenting Author:

Eduardo Guerrero

Authors:

Guerrero, Eduardo F.¹, Burgette, Reed J.², Swenton, Vanessa M.³, McClaughry, Jason D.⁴, Franczyk, Jon J⁵, Azzopardi, Carlie⁶

Abstract:

The Wallula fault zone is a structurally complex portion of the Olympic-Wallowa lineament in northeast Oregon and southeast Washington. Limited GNSS data shows low strain rates in eastern Cascadia compared to the subduction zone. However, the region has had significant earthquakes.  Previous work suggests that the 1936 M6.0 earthquake epicenter was located close to the Oregon-Washington border, south of Umapine in Umatilla County, and had subsurface rupture along the south-eastern margin of the Walla Walla Basin. This event had Mercalli intensities of VII+, making it Oregon's most damaging crustal earthquake until the 1991 Klamath Falls and 1993 Scotts Mills events.  The southern margin of the Walla Walla basin is defined by the Wallula fault (WF) system, where it is actively deforming the Columbia River Basalt Group (CRBG) and younger units along the main WF and to the south by sets of Riedel shears, including Pine Creek North fault and Little Dry Creek fault.  Correlation of the Frenchman Springs member of the Wanapum Basalt (CRBG) across the main WF trace in this area suggests ~325 m of vertical offset since 15.9 Ma. And a previous study suggested up to 7.5 km of NE-SW directed extension based on a gravity low, interpreted to have evolved as the Wallula fault encounters the NNE-striking Thorn Hollow segment of the Hite fault (HF). In October 2024, DOGAMI conducted a paleoseismic investigation on the Pine Creek North fault (PCNF), conjugate of the WF, with evidence of oblique left-lateral displacement.  Deformed sediment packages suggest a minimum of 2 earthquakes since the Missoula floods occupied the Walla Walla basin ~15 ka.  The CRBG units are overlain by Holocene to lower Pleistocene loess deposits in the south and upper Pleistocene Missoula Flood deposits in the north.  We will additionally provide an update on geologic mapping of the Waterman, Smeltz, and Helix 7.5’ quadrangles, including results from refraction microtremor seismic profiles deployed in support of geologic mapping, and local updates on Columbia River Basalt stratigraphy from XRF analyses of samples collected in 2024 and 2025.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-4404

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