Teleseismic receiver and transfer function modeling of OBS data: Resolving plate structure in the locked zone of Cascadia

Abstract:

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 overpressured zone and its possible control on the transition from episodic slow slip to seismic slip occur offshore and are not resolved in those studies. The Cascadia Initiative has provided an opportunity to extend our capabilities to study 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 the presence of a water column that produces P-wave reverberation above the recording stations. 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 real data examples from the Cascadia Initiative, we show how calculated receiver and transfer functions can be used to constrain seismic properties of the crust in both shallow (Cascadia forearc) and deep (Juan de Fuca Ridge) ocean settings. Interestingly, 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 may be intrinsically weak at all depths, consistent with other geophysical data.