The great Tohoku earthquake and tsunami of 2011 has reminded us of the hazards to life and property that attend such events, and of the need for a focused scientific effort to understand both their causes and effects. Cascadia, which extends from northern California to southern British Columbia, will be the site of a future great earthquake that will affect a number of large metropolitan centers, including Seattle and Portland. The Cascadia Initiative is an effort to gain a better understanding of seismicity in the region that includes the deployment of an array of instruments both onshore and offshore. Several ancillary studies are also planned or under way; one such study is a sophisticated 3D (three dimensional) seismic survey to be carried out off the coast of Washington. The suite of 2D seismic profiles that will be acquired by this project will provide essential data for planning and carrying out the 3D survey. The chief broader impact of this study is the very high societal relevance of gaining an improved understanding of the seismic risk in this region.
This award sponsored the acquisition of seismic reflection data across the plate boundary offshore Washington state, and produced seismic images of the plate boundary structure. The data and the images produced are expected to be used by scientists in a variety of disciplines, many in the earth sciences but also in other disciplines (e.g. marine biology), for wide-ranging studies including hazard assessment, studies of methane vents and gas hydrates, and studies of plate tectonic processes at subduction margins. All of the data collected by this award are openly-accessible and were made so immediately after the cruise. Raw geophysical and seismic data can be downloaded from the LDEO web site (www.marinegeo.org/tools/search/entry.php?id=MGL1212). Seismic sections processed shipboard through post-stack time migration can be downloaded from the UTIG seismic data base (www.ig.utexas.edu/sdc/cruise.php?cruiseIn=mgl1212). The research cruise sponsored by this award trained 17 scientists, most early career, from 14 different institutions. These scientists received extensive training on-ship, including in the acquisition and processing of seismic reflection data, and processing and interpretation of the acquired images on this cruise across the Washington margin. The cruise provided these scientists with experience on the research vessel Marcus G. Langseth, which a number of these scientists will likely use in their careers. Major research results or findings from the initial processing of the geophysical data include the identification of an active methane system, identification of a seafloor scarp along a listric normal fault in the accretionary prism, imaging of undeformed sediment oases within the outer accretionary prism, and hints of variable reflectivity along the plate boundary as the plate descends, possibly caused by variable pore fluid pressure along the megathrust. These initial findings are informative for hazard assessment, fluid processes at plate margins, and subduction processes, and will be followed-up with detailed imaging studies.
