Seismic Imaging of the Gorda Slab Subduction Interface Near the Mendocino Triple Junction Using Converted Phases

Date: 4/24/2019

Time: 06:00 PM

Room: Grand Ballroom

Geodetic observations show that the interseismic coupling of subduction interfaces can vary strongly in both the along strike and downdip directions. Identifying the structural and material properties responsible for these variations is often hindered by a lack of high-resolution imaging at seismogenic depths. We use seismic phases from local earthquakes in the downgoing plate (Ps or Sp) converted at the subduction interface of the Gorda plate to constrain its location and material properties. We use data from a dense onshore-offshore seismic array deployed during the Cascadia Initiative experiment near the Mendocino triple junction. The abundant seismicity generates coherent waveform sections at each station with clear Ps and Sp converted phases that are observed at both onshore and offshore stations.

In the shallow part of the subduction zone near the deformation front, Ps and Sp converted phases from the slab interface arrive at about 2.5 s after P or before S on horizontal or vertical components. Other velocity discontinuities are also observed within the overlaying sediments. Using the converted phase arrival times and a previously studied active source seismic line, we estimate the average Vp/Vs ratio in the overlaying sediment ranges from 2.3 to 3.0. Given these constraints, the converted phases imply that the slab interface in the vicinity of the deformation front is ~2-4 km shallower than the Slab 1.0 model.

For several onshore stations, Sp converted phases are observed. Their arrival times are nearly consistent with the slab 1.0 model. The amplitude and waveforms of the Sp phases at nearby stations can vary significantly, indicating strong along-dip and along-strike variations in material properties near the interface. Our dataset includes phases that convert at depths from 2-25 km and span significant transitions in interseismic coupling that will be utilized to explore the relationship between material properties and coupling.

Presenting Author: Jianhua Gong

Authors