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.
Washington State’s Pacific coast faces no hazard greater than its next great Cascadia earthquake. The expected effects of this mega-thrust earthquake include many minutes of strong shaking, a fast-arriving tsunami, tectonic down warping that effective raises sea level by a meter or more, and consequent disruption of the coastal economy. The earthquake source is a tectonic plate boundary, the Cascadia Subduction Zone, that dives beneath the Pacific coast of North America between Vancouver Island and Cape Mendocino, California. It forms the huge active Cascadia Subduction Zone fault, dwarfing the San Andreas, and conveys the Juan de Fuca Plate beneath the overriding North America Plate. This subduction zone fault relentlessly produces great mega-thrust earthquakes, at intervals averaging about 300 to 500 years. The most recent of these earthquakes, on 26 January 1700, spawned a trans-Pacific Ocean tsunami that not only overran Native American fishing camps but also caused documented damage in Japan. Today the fault is mostly or entirely locked offshore along its 1,100 km length, accumulating plate motion that it can spend during its next great earthquake where the most recent 1700 AD earthquake was in the size range M 8.7-9.2. This subduction zone fault 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 Earthscope, the Plate Boundary Observatory (PBO), the Ocean Observatories Initiative (OOI) and NEPTUNE/CANADA cable observatories, GeoPRISMS, and the Cascadia Initiative of ocean-bottom seismometers (OBSIP) with extensive onshore seismometers and geodetic stations. 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 provided 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. (3) Where basement 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 BSR amplitudes in tilted sediments suggest that fluid flow along bedding planes controls methane flux. The MCS data provided by the Langseth cruise is currently being used by a variety of scientific research programs, including my present GeoPRISM cruise (August, 2013) to conduct heat flow and fluid flux measurements on Line 4 of the COAST survey shown below.
