Synthesis of Results From a Dense Nodal Geophone Array Deployed Along the Cascadia Subduction Zone

Date: 4/25/2019

Time: 04:45 PM

Room: Cascade II

In the summer of 2017, the University of Utah in collaboration with the University of Arizona, the University of Oregon, the University of New Mexico, and the PASSCAL Instrument Center deployed 174 3-component 5-Hz nodal geophones in central Oregon. The approximately trench perpendicular line started near the coast in Waldport, OR and traversed the Coastal Range, the Willamette Valley, and the Western Cascades with ~500-meter station spacing and recorded continuous data for ~40 days. The specific array design was chosen to be comparable to a previous broadband deployment with ~5-kilometer station spacing that recorded continuous data for ~1 year. The results from this ambitious deployment have yielded three publications and formed the basis of two larger subsequent nodal deployments in Alaska (~400 nodes, winter 2019) and Cascadia (~700 nodes; summer 2020).

Our receiver function results (Ward et al., 2018) show notable agreement with first-order features interpreted in previous studies. We also employ double beamforming of the interstation ambient noise cross-correlations to extract Rayleigh wave phase velocities and invert them for a shallow (<25 km) 2‑D crustal shear-wave velocity structure (Wang et al., in-revision). Local earthquake P-wave tomography (Dunham and Kiser, in-prep) agrees particularly well with our shear-wave model and existing 3-D magnetotellurics imaging. Between the three methods, we interpret a prominent low-velocity zone in the mid crust under the Willamette Valley in central Oregon as fluids from the mantle wedge percolating through the mafic Siletzia terrane. The top of the Siletzia terrane is well imaged by the receiver functions as a shallow positive arrival (1-5 km) and is consistent with offshore seismic imaging of the terrane. Both velocity models image a high velocity crust consistent with previous estimates of the eastward extent of the accreted Siletzia terrane. Cumulatively, these results illustrate how this deployment strategy could be scaled for detailed 3-D imaging of a subduction zone.

Presenting Author: Kevin M. Ward

Authors