金曜セミナー(Pascal Audet氏)

Toward resolving plate structure from the trench to the arc in the Cascadia subduction zone from ocean-bottom and land-based seismic data.

Prof. Pascal Audet(University of Ottawa)

 

Teleseismic receiver function studies in the forearc of the Cascadia subduction zone have resolved structures associated with deep fluid cycling, such as the basalt-to-eclogite reaction and overpressure of the subducting oceanic crust, as well as the serpentinization of the forearc mantle wedge. Unfortunately, the updip extent of the over-pressured zone, and therefore the possible control on the transition from episodic slow slip to seismic slip, occurs offshore and is not resolved in those studies. The Cascadia Initiative has provided an opportunity to extend this work to the locked zone using teleseismic receiver functions from the deployment of a dense line of ocean-bottom seismograph (OBS) stations offshore of Washington State, from the trench to the coastline. However, high quality receiver functions using OBS data are notoriously difficult to obtain and to interpret due to reverberations due to the effect of the water column and low-velocity sedimentary layers. Here we model receiver functions for a variety of oceanic lithospheric structures to investigate the possibilities and limitations of receiver functions using OBS data. These modeling results indicate that receiver functions from OBS data are difficult to interpret in the presence of marine sediments, but shallow-water sites in subduction zone forearcs are suitable for constraining various crustal elements around the locked megathrust fault. We also propose using a complementary approach based on transfer function modeling that bypasses receiver functions altogether and estimates crustal properties directly from the waveforms. Using data from a combination of OBS and land-based seismic stations, we show how calculated transfer and receiver functions can be used to constrain seismic properties of the subducting oceanic crust from the trench to the arc. Preliminary results indicate that the elevated P-to-S velocity ratio of the downgoing oceanic crust interpreted as high pore-fluid pressure extends updip into the locked zone, indicating that the megathrust fault is intrinsically weak at all depths.