Paleoseismic Evidence for a Near Historic Rupture within the Seattle Fault Zone
Holocene fault scarps on Bainbridge Island, Washington State contain evidence for multiple late Holocene ruptures of secondary faults within the hanging wall of the Seattle fault zone (SFZ). These secondary faults are possible independent sources of local seismic hazard and provide insight into the history, geometry and kinematics of rupture on the broader SFZ. In this study, we investigate the paleoseismic history of a 1.5 m-high north-facing scarp on south Bainbridge Island. A hand-excavated trench exposed north-vergent monoclinally folded strata of the Tertiary Blakely Harbor Fm., Vashon-age glacial till, and post-glacial lacustrine deposits. Holocene colluvium associated with two paleosols infills a structural trough at the northern end of the trench and records at least two earthquakes. The most-recent earthquake raised a former intertidal sand flat, preserved on the western shore of south Bainbridge Island, which is inset into the marine terrace raised by a larger (M7+) earthquake on the main thrust of the SFZ ~1 ka yrs ago; indicating that the most-recent earthquake (MRE) occurred <1,020 cal. yrs B.P. Dendrochronology of submerged tree snags in a scarp-dammed pond east of the trench suggests that the trees died circa 1820, possibly as the result of the scarp damming the pond during the MRE, indicating a near historic earthquake within the SFZ. Preliminary Bayesian geochronologic modeling using radiocarbon ages of charcoal samples collected from horizons above and below the lower paleosol in the trench further indicate that the penultimate earthquake occurred between 16,172 – 10,486 cal. yrs B.P. Our observations provide supporting evidence to the hazard potential of these surface deforming secondary Holocene faults within the SFZ and extend the record of rupture within the SFZ to the early Holocene.
Session: Distributed Deformation from Surface Fault Rupture [Poster]
Type: Poster
Room: Evergreen Ballroom
Date: 4/22/2022
Presentation Time: 08:00 AM Pacific
Presenting Author: Stephen Angster
Student Presenter: No
Additional Authors
Stephen Angster Presenting Author Corresponding Author sangster@usgs.gov U.S. Geological Survey |
Brian Sherrod bsherrod@usgs.gov U.S. Geological Survey |
Jessie Pearl jessie.pearl@tnc.org The Nature Conservancy |
Wes Johns wjohns@nevada.unr.edu University of Nevada Reno |
|
|
|
|
|
Paleoseismic Evidence for a Near Historic Rupture within the Seattle Fault Zone
Category
Distributed Deformation from Surface Fault Rupture
Description