Subduction is a process in which oceanic crust and upper mantle are consumed beneath the margins of island arcs, such as Japan and Sumatra, or the margins of continents, as along the northwest coast of the U.S. Subducton gives rise to violent volcanic eruptions as well as great ?megathrust? earthquakes and tsunamis. Dramatic demonstrations of the destructive effects of subduction have occurred recently in Indonesia, Japan, and Chile. Water released from the subducted plate is a significant, but poorly understood, contributor to these events; this seismic survey of the Juan de Fuca Plate has the specific aim of characterizing the plate from its formation at the ridge, through alteration and hydration of the plate as it ages, to subduction of the plate along the Cascadia margin. Owing to the very high risk of large earthquakes in Cascadia, improved understanding of the role of water in the subduction process is a primary broader impact of the survey. The project is also an important adjunct to the Cascadia Initiative, a multi-year deployment of instruments both onshore and offshore to study the structure and seismicity of the Cascadia region.
