Deflection of the Juan de Fuca Plate Beneath the Cascadia Continental Margin Beneath an Upper Plate Load: Direct Evidence for a Compliant Subducting Plate

Session: Amphibious Seismic Studies of Plate Boundary Structure and Processes
Type: Oral
Date: 4/29/2020
Time: 04:30 PM
Room: 120 + 130
Description: 

Amphibious seismic data acquired in 2012 have been used to generate a 3D Vp model of the upper and lower plates in Central Cascadia from the trench to the Coast Range and from ~44-45°N. This segment of Cascadia is characterized by clusters of seismicity on or near the plate boundary in the region generally thought to be locked. In addition to Pg and Pn arrivals used to define the velocity structure, PmP wide-angle reflections have been modeled to define a surface interpreted to be the base of the crust of the subducted Juan de Fuca plate. Intra-crustal reflections are also observed, but their distribution is patchy and their origin ambiguous. The Pg data indicate the presence of a narrow, vertical, northwest-trending high velocity (and presumably high density) slab within the upper plate that is likely the feeder dike for the Yachats volcanics that crop out near the coast in this region. The base of the crust, defined by PmP arrivals that are clearly observed on many instruments, is deflected downward by ~4 km beneath this load. Converting the velocity anomaly to density and assuming a line load on an unbroken elastic plate results in an estimate for the elastic thickness of the subducting plate of only a few km. While on the low end of estimates for the elastic thickness of 10-million year old oceanic lithosphere derived from seamount and trench outer rise topography, this estimate is not unreasonable and suggests that the subducting plate, as well as the overriding wedge, is deforming in response to plate convergence. Lower plate compliance should, therefore, be considered when interpreting forearc geodetic data. Moreover, weak plates can deform aseismically and store a lot of energy before rupturing, perhaps explaining the anomalous lack of seismicity throughout the Cascadia subduction zone. Finally, we note that high-resolution 3D models of subduction structure are rare, and that the results reported here would not be detected with 2D transects.

Presenting Author: Anne Tréhu

Authors