The Doty Fault Zone: A Cryptic Fault in Southwest Washington

Session: Cryptic Faults: Assessing Seismic Hazard on Slow Slipping, Blind or Distributed Fault Systems
Type: Oral
Date: 4/28/2020
Time: 02:30 PM
Room: 240
Description: 

The Doty fault zone is a set of west-striking, north-dipping faults in southwest Washington State (WA). It may accommodate shortening from oblique subduction and northward migration of the Oregon/WA forearc block. It appears to resemble the active and hazardous Seattle fault, but it is unclear if the Doty fault is active and poses similar threat. Lidar along the Doty fault zone exhibits no clear fault scarps. Therefore, to understand the fault zone’s place in WA forearc deformation, we employ a multi-disciplinary approach that includes geologic mapping, Quaternary geochronology, study of lidar data, aeromagnetic and ground-magnetic data, regional and high-resolution gravity data, shallow geophysical methods and regional seismic data. These data might answer many questions: 1) How long is the fault zone and does it connect to other faults? 2) Are there active blind strands? 3) What is the fault zone’s slip rate? And 4) do Quaternary surfaces preserve uplift related to the Doty fault zone or associated faults? Preliminary results from potential-field studies indicate that the Doty fault zone extends from the Doty Hills to the St. Helens seismic zone—77 km—thus representing a major discontinuity in the upper crust. The combined length of kinematically linked faults is ~96 km suggesting that the faults could be capable of Mw 7.6 earthquakes if they rupture together. In the Doty Hills, the fault dips steeply (50-60°); geologically constrained potential field data modeling indicates ~1 km slip since 16.4 Ma. Apparent kinematic linkages to adjacent faults suggest additional oblique slip. Our constraints result in a minimum slip rate of ~0.06 to 0.08 mm/yr, much lower than the Seattle fault, suggesting that large magnitude earthquakes could rupture the Doty fault, but less often than the Seattle fault. Along the eastern end of the Doty fault zone, geologic mapping shows NW-SE and conjugate NE-SW trending structures, dipping >65° to near vertical. Future work will assess interaction of these faults with the Doty fault zone.

Presenting Author: Megan L. Anderson

Authors