9527001 Trehu The coastal region of northern California from latitude 39oN to 42oN represents a region in which the active tectonic regime changes abruptly from subduction to transform motion at the Mendocino Triple Junction (MTJ). Knowledge of the crustal and upper-mantle structure in this region, when combined with information on the plate-tectonic history, geologic structure, seismic activity, and timing and extent of heat influx and consequent surface volcanism, places constraints on the processes that control continental growth and evolution. Because the triple junction has been migrating north along the coast of North America, an understanding of crustal structure and evolution in this region will provide insights into the geologic history of the entire U.S. west coast continental margin and will contribute to better assessments of the seismic hazards represented by the Cascadia subduction zone and northern San Andreas Fault system. In 1993 and 1994 the principal investigators collected a network of large-aperture seismic profiles to image the crustal and upper- mantle structure beneath northern California and the adjacent continental margin. The data include approximately 650 km of onshore seismic refraction/reflection data, 2000 km of offshore multichannel seismic (MCS) reflection data, and simultaneous onshore and offshore recording of the MCS airgun source to yield large-aperture data. Scientists from more than 12 institutions were involved in data acquisition, which was led by a team of principal investigators from Oregon State, Lehigh, Rice, and Stanford Universities and from the U.S. Geological Survey. This award provides a fourth year of support from the Continental Dynamics program to permit the principal investigators to complete data analysis and presentation. Support for a workshop is also provided during which they will discuss their seismological results with the broader community of earth scientists interested in geological, geod ynamic, and natural hazard issues associated with plate interactions in along the western margin of North America. ***
