U.S. Geological Survey Woods Hole Science Center

Cruise Report for RV WECOMA 9406A

Summary

This cruise represents part of a two-year onshore/offshore seismic acquisition program to image the crest and upper mantle of northern Califomia. The primary objective is to place constraints on how the northward migration of the Mendocino triple junction affects the crustal architecture of the Califomia continental margin, which is undergoing a transition from a compressive to a transpressive tectonic regime. A better understanding of the processes associated with triple junction migration in this region will contribute to our understanding of the geological history of western California and of the seismic and volcanic hazards associated with the San Andreas fault system and the Cascadia subduction zone. During the cruise described in this document, we deployed and retrieved ocean bottom seismographs (OBS) operated by the U. S. Geologicai Survey and ocean bottom hydrophones (OBH) operated by the Woods Hole Oceanographic Institution in order to record airgun shots from from a 8385 in3 (137.5 liter) tuned airgun array deployed by the RV Ewing, thus providing large-aperture seismic data for derivation of. crustal velocity and structure. This information complements the multichannel seismic reflection information obtained simultaneously by the RV Ewing. Additional seismographs were deployed on land to record the shots. This field effort builds on an onshore seismic refraction/reflection program conducted during August, 1993, using approximately 600 seismometers and large explosive sources. The need to coordinate the efforts of two ships and a large onshore field crew during the 1994 field experiment placed unusual constraints on the timing of the OBS/OBH operations that were conducted from the RV WECOMA. A secondary objective of the cruise was to obtain high-resolution subsurface images around sites that have been proposed to the Ocean Drilling Program (ODP) as part of a major effort to constrain the paleo-oceanographic effects of the California current through acquisition of a series of long cores recording the sedimentary history of the Califomia continental margin. Logistically, this program complemented the OBS/OBH effort because several proposed drilling sites fall within the boundaries of the OBS/OBH experiment, and time for surveying was available between OBH/OBS deployments. Digital 3.5 and 12 kHz echo-sounding data and digital single channel seismic reflection data using an 80 in3 water gun as a source were acquired on grids surrounding 4 of the proposed drilling sites. Finally, during the transit to and from Newport, we acquired several Acoustic Doppler Current Profiles (ADCP) to map currents offshore Cape Blanco.

Cruise Narrative for RV WECOMA 9406A

Cruise Participants

• Anne Trehu - Oregon State University - Chief Scientist
• Mitch Lyle - Boise State University - co-Chief Scientist
• Christof Lendl - Oregon State University
• John Chambers - Boise State University
• Vee-Ann Cross - US Geological Survey
• Jim Dolan -Woods Hole Oceanographic Institution
• David Dubois - Woods Hole Oceanographic Institution
• Tim Holt -Oregon State University
• Don Michaelson - Oregon State University
• Greg Miller - US Geological Survey
• Ken Peal - Woods Hole Oceanographic Institution
• Brian Wendler - Oregon State University
• F. Beecher Wooding - Woods Hole Oceanographic Institution

Cruise Data