Paleoseismic Segmentation in Cascadia: Possible Link to Juan De Fuca/Gorda Rift Propagator Wakes

Session: Amphibious Seismic Studies of Plate Boundary Structure and Processes [Poster]
Type: Poster
Date: 4/29/2020
Time: 08:00 AM
Room: Ballroom
Description: 

A 2012 integration of onshore and offshore paleoseismic data proposed approximate segment boundaries based on the along-strike continuity of turbidite event beds offshore and compatible onshore evidence. Segmentation and shorter ruptures were largely based on the offshore data, with more sensitive onshore sites such as Bradley Lake likely recording some of these events. The threshold sensitivities of offshore sites can now be demonstrated to be at or below Mw 7.2, while onshore sites require higher magnitude thresholds for generation and preservation of earthquake evidence, thought to be ~ Mw 8.2-8.4. The segment boundaries were data limited in some cases; that is, the segments ended where the data ended, leaving uncertainties due to data gaps as well as the poorly known triggering distances from the rupture terminations. Inspection of potential linkages of the proposed segment boundaries to subducting features yielded no obvious matches, with the exception of the subducting Blanco Fracture Zone and one rift propagator in the Gorda plate. Additional 2017 core data relieved the data limitations and shifted several of the segment boundaries northward for segments terminating in Oregon. Re-examination of the JDF plate and its associated structure suggests that the revised segment boundaries may be associated with the rift propagators in the JDF plate as mapped by Wilson (1988, 2002). These features are mapped with magnetic anomalies primarily and form as a result of one ridge segment growing at the expense of another, leaving a “wake”. These wakes are commonly weak points in the lithosphere, may have an association with volcanic seamounts and commonly have topographic expression. When subducted, these features, also known as “pseudofaults” may either be activated as faults or represent roughness on the subducting plate or both. New structural mapping in Cascadia reveals deformation of the accretionary prism associated with subducting rift propagators and their associated seamounts that may also be compatible with revised paleoseismic segmentation.

Presenting Author: Chris Goldfinger

Authors