1University of Wyoming; 2University of Nevada; 3University of Oklahoma; 4University of Washington; 5Oregon State University; 6University of Wisconsin; 7Western Washington University
The Cascadia margin, where the Juan de Fuca and Gorda plates subduct beneath North America, poses substantial (but poorly understood) earthquake and tsunami hazards to the Pacific Northwest. Several major scientific infrastructure and research initiatives are focusing effort on the Cascadia margin. These include GeoPRISMS, EarthScope, encompassing the Plate Boundary Observatory (PBO), the Cascadia Initiative of ocean-bottom seismometers (OBS) with extensive onshore seismometers and geodetic stations associated with the Amphibious Array Facility, the Ocean Observatories Initiative (OOI) and NEPTUNE/CANADA cable observatories, and the SeaJade OBS program off Vancouver Island. GeoPRISMS has selected Cascadia as a focus site, and the first deployment of the Cascadia Initiative OBSs included a concentration of instruments off Grays Harbor, Washington (see GeoPRISMS Newsletter Issue 27, 2011). Here we report on a recently completed, open-participation/open-access geophysical survey of the Cascadia margin off central Washington, which provides new opportunities to participate in Cascadia studies.
Figure 1. (A) Map of COAST track lines (labeled 1-11), plotted on multibeam bathymetric grid acquired during MGL1212. (B) Inset map showing location of COAST survey on Cascadia continental margin. Bathymetry contoured at 500 m intervals. (C) Example of post-stack time migration across deformation front on Line 4 (yellow line, Fig. 1A).
The COAST (Cascadia Open-Access Seismic Transects) survey comprised a successful, two-week cruise of the R/V Langseth in July 2012 that acquired diverse geophysical data, including multichannel seismic reflection, multibeam bathymetry, gravity, and magnetic data in a high-priority corridor of the Cascadia margin off Grays Harbor. The scientific goals of this project include (1) constraining the position of the plate boundary, which is poorly known in this region; (2) imaging downdip variations in the character of the subduction thrust across the transition from aseismic creep to seismogenic rupture; (3) quantifying pore fluid pressure, fluid budgets, and upstream inputs to the zone of episodic tremor and slip; and (4) determining the geological controls on methane distribution in the forearc. Substantial shipboard processing efforts produced seismic sections processed through post-stack migration, as well as bathymetric data that provide nearly complete coverage of the forearc region (Fig. 1). Shipboard processing of the data provides the following initial observations:
(1) The Pleistocene accretionary wedge is well imaged and shows landward-vergent thrust faulting throughout our survey area. An outboard series of ramp-and-thrust structures gives way to a region characterized by folds that separate “oases” of undeformed sediment. (2) The oceanic basement reflection is strong and clear outboard of the deformation front but becomes much weaker beneath the Pleistocene wedge. At this stage of processing it is not clear whether this reflects inaccurate processing, loss of energy by scattering off a complex surface, or (more intriguingly) a physical change in the plate boundary structure. (3) Where it is imaged beneath the margin, the top of oceanic crust appears gently dipping beneath the Pleistocene wedge, then bends into a steeper inclination beneath the Miocene wedge. (4) A widespread methane hydrate system, indicated by bottom-simulating reflections, exists in the outer wedge and upper slope of the study area. Increased amplitudes of the Bottom Simulating Reflection (BSR) in tilted sediments suggest that fluid flow along bedding planes controls methane flux.
The COAST program was the first Langseth cruise conducted as an open-participation/open-access cruise. Participants were selected by an open application process, through which seventeen members of the science party were selected from over 60 applicants. Of the twenty members of the visiting science party, eight had not previously been aboard a research vessel, and an additional five (13 total) had never participated in a marine seismic reflection survey. A robust daily shipboard education program included science lectures, scheduled tutoring on seismic processing, and informal data interpretation.
All cruise data are open-access and available immediately. Raw geophysical and seismic data can be downloaded from the LDEO website. Seismic sections processed shipboard through post-stack time migration can be downloaded from the UTIG seismic data base. The cruise report can be downloaded here. We encourage all interested parties to make use of the COAST data in any way desired, including writing proposals to process and analyze the data, integrating the data with other recent and ongoing Cascadia initiatives, and incorporating the data and images in the classroom.