Décollement initiation at Cascadia Subduction Zone from Full-Waveform Inversion

Date: 4/24/2019

Time: 06:00 PM

Room: Grand Ballroom

The plate-boundary décollement at subduction zones is expected to follow a mechanically weak stratigraphic horizon where low frictional strength material resides or high pore fluid pressure develops. The stratigraphic level at which the décollement forms directly controls the amount of sediments being subducted, thus affecting the amount of fluid entering the subduction zone and the plate interface properties. At Cascadia subduction zone where the incoming Juan de Fuca plate is covered by 3-4 km thick sediment, the stratigraphic level of décollement varies significantly along strike, resulting in minimum sediment subduction offshore Washington, yet a 1.4-1.7 km thick layer of under-consolidated sediments is being subducted offshore Central Oregon. To investigate the sediment properties associated with décollement initiation and the conditions that give rise to different décollement depths off Central Oregon and Washington, we conducted 2-D elastic full waveform inversion (FWI) of multichannel seismic (MCS) data and resolved the fine-scale velocity structure of the incoming sediments within 40 km seaward of the deformation front. Sediment velocity from pre-stack depth migration and oceanic crustal velocity derived from travel-time tomography of ocean bottom seismometer data were used to build the starting model. We inverted for the refracted and then the reflected energy in downward extrapolated MCS data. Our preliminary results from offshore Central Oregon show that a low-velocity zone initiates ~10 km seaward of the deformation front at the stratigraphic boundary between Astoria fan sediment and abyssal plain turbidites. This low velocity zone is likely associated with anomalous high porosity that developed in response to the horizontal compression from the wedge. Further landward, this horizon becomes the décollement where the frontal thrusts shoal. We will present a comparative study of the velocity structures offshore Oregon and Washington at the meeting.

Presenting Author: Shuoshuo Han

Authors