Document Type

Thesis

Date of Degree Completion

Spring 2023

Degree Name

Master of Science (MS)

Department

Geological Sciences

Committee Chair

Dr. Walter Szeliga

Second Committee Member

Dr. Anne Egger

Third Committee Member

Dr. Breanyn MacInnes

Fourth Committee Member

Dr. Stephen Angster

Abstract

Recently discovered fault scarps along the Dead Coyote Fault (DCF) in the northern Kittitas Valley (KV) of central Washington suggests active faulting in the late Quaternary, but constraints on the timing and potential magnitude of earthquakes along the fault zone are limited. The KV lies at the northwestern edge of the Yakima Fold and Thrust Belt (YFTB), a low-strain region where individual structures are capable of producing M~7 earthquakes.

This investigation uses surficial geologic mapping and topographic analysis of the DCF scarps, ground penetrating radar transects, and paleoseismic trenching to determine the rupture history of the DCF. The trench was excavated across a 30-cm-high scarp in an alluvial fan surface and exposed two shallow, north-dipping, thrust fault zones within a silicic caliche. My interpretation suggests the DCF has had two major ruptures since the mid-Pleistocene based on mapped stratigraphic and structural relationships. Within the trench, measurements of correlated units across both fault zones indicate 72–76 cm of cumulative vertical separation, with the most recent rupture accounting for 36 – 45% of the total vertical separation.

Topographic profiles across the DCF scarps suggest that a greater amount of slip was accommodated along the western portion of the DCF and the estimated minimum surface rupture length of the most-recent rupture is 27.3 – 28.3 km suggesting a moment magnitude between Mw 6.5 - 6.7.

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