Descr: Massive, magnitude 9, earthquakes occur along the Cascadia subduction zone every 200-600 years, while magnitude 7 events occur approximately every 30 years. The Cascadia Collaborative Seismic Experiment (CASSIS) is designed to determine the structural and geodynamical controls on these major earthquakes in southwest British Columbia. The proposed experiment includes a passive phase consisting of the deployment, by the Canadian Vessel Tully, of 15 short-period and 2 broad-band Japanese Ocean-Bottom Seismometers off the coast of Vancouver Island and Washington during May, June and July, 2003, and an active phase during August, 2003. The active phase includes the deployment of about 60 Japanese and 21 Canadian Ocean-Bottom Seismometers, and about 120 PASSCAL REFTEK short-period, three-component seismometers. These seismometers, as well as a reflection streamer, will record three weeks of airgun shooting off the coast of Vancouver Island from the 12000 cu in airgun arrays towed by the Japanese seismic vessel Kairei. An additional 10 large dynamite shots on land will be recorded during this time. The plan is to deploy 80 Refteks on Vancouver Island, 20 on mainland British Columbia and 20 on the Olympic Peninsula, Washington. A major focus is on a detailed comparison of the structure and mechanics of the Cascadia subduction zone with the similar Nankai subduction zone of southwest Japan. We will map the 3D geometry of the inter-plate boundary, including the "seismogenic zone" where the descending and overriding plates are well-coupled and elastic strain accumulates. We will identify small-scale variations in fault topography that can concentrate stress and cause frequent earthquakes. Simultaneous analysis of earthquakes and active-source data will reveal the 3D seismic velocity structure within the subducting and overriding plates, allowing the inference of lithologies at depth. We will determine whether changes in lithology, particularly those associated with metamorphic reactions that release fluid and affect pore pressure, are associated with earthquake occurrence. The state of stress in the downgoing oceanic plate will be determined from the focal mechanisms of recorded local earthquakes.
