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.
The Cascadia margin is the site of active subduction, with the Juan de Fuca plate subducting under the North American plate at a rate of ~35 mm/yr. This system is of great scientific and societal interest, as it is capable of very large (Mw~9) earthquakes, creates volcanic hazards in the Cascades, and hosts periodic episodic tremor and slip (ETS) episodes. Extensive paleoseismic work illuminates a 10,000 yr history of quakes with an average recurrence rate of ~500 yrs for magnitude ~9 events that rupture the entire subduction zone from northern California to the Nootka fault off Vancouver Island. The last event occurred in 1700 based on a tsunami recorded in Japan. Despite evidence that the system has generated large megathrust earthquakes, very few earthquakes occur on the plate boundary on human timescales, creating large uncertainties in the position, structure, and physical state of the plate boundary. The COAST (Cascadia Open-Access Seismic Transects) project funded by this NSF grant conducted an open-access, open-participation 2D seismic survey of the Cascadia subduction margin in July 2012. The cruise acquired a grid of 2D seismic reflection profiles and associated geophysical data in a high-priority corridor off Grays Harbor, Washington. The cruise provided benchmark seismic images to address key scientific issues regarding the location, physical state, fluid budget, and associated methane systems of the subducting plate boundary and overlying crust. These issues include (1) determining the location of the offshore plate boundary beneath a segment of the Cascadia margin that ruptures in very large, infrequent earthquakes, (2) constraining sediment subduction and plate boundary roughness, (3) estimating pore fluid pathways, (4) determining controls on methane distribution, and (5) imaging compressional and extensional structures that may pose geohazards on the Cascadia margin. An important secondary achievement of this project was to conduct a fully open-participation, open-access cruise (the first of its kind), with an organized shipboard education and training program, and immediate, full release to the community of all geophysical data. The open-participation aspect of the cruise was a great success. From a group of about 60 applicants, the Principal Investigators selected 17 participants from 14 institutions, comprising thirteen graduate students, two postdocs, and two faculty. The focus on introducing this diverse group of scientists to the science conducted aboard the U.S. National Marine Seismic Facility resulted in an empowered, highly motivated shipboard science party that was actively involved in all phases of shipboard operations, data processing, and analysis. Shipboard seismic data processing was conducted primarily by the "newbies," most of whom had little or no previous experience with marine seismic reflection processing. As a result, we substantially increased the number of students and early-career scientists with experience on this important U.S. research facility. The open-access aspect of this cruise was equally successful. The raw data, shipboard migrated stacks, and cruise reports were made available to the public for use without restriction immediately following the cruise. As a result, data from this project have been in high demand: 604 files from this cruise have been downloaded from the University of Texas in the two years since the cruise (more than 89 Gb of data downloads). Several researchers have submitted successful NSF proposals to fund follow-up work on this project, demonstrating the viability of the open-access model for funding marine seismic work in the U.S.
