R/V SONNE. Cruise report SO 108. San Francisco - Astoria 14.4. - 23.5.1996 ORWELL Oregon and Washington exploration of the lithosphere - a geophysical experiment

The main goal of the R/V SONNE cruise SO108 was to investigate the large-scale structure of the Cascadia subduction zone to better assess the earthquake hazard it presents. The cruise was carried out in April and May of 1996 cooperatively by GEOMAR and the USGS. Ocean Bottom Seismometers (OBS) and Hydrophones (OBH) were deployed at 116 stations and collected wide-angle reflection and refraction seismic data along 12 profiles with a total length of 1385 km across and along the continental margin off Washington and Central Oregon and on the adjacent ocean floor of the Juan de Fuca plate. Both wide-angle reflection and refraction data and the MCS data could detect the subducting oceanic crust under the continental margin to a distance of 20 km from shore. Three major differences between the continental margins of Washington and Oregon were discovered. The second major difference is the average velocity of the sedimentary and crustal rocks overlying the subducting plate. Offshore Washington, the rock velocity never exceeds 5.3 km/s even at depths as deep as 10-12 km. Offshore Oregon, the rock velocity is up to 6.5 km/s and is 5 km/s or higher below 5 km depth. The third difference is in the thickness of sediments underlying the continental shelves. Offshore Oregon sediment thickness doe not exceed 3 km, whereas offshore Washington a deep (up to 8 km) partly fault-controlled elongate basin underlies the shelf. Refraction data from the Juan de Fuca plate show that the crust is generally thinner than the global average thickness of ocean crust and is overlain by 1-3 km of low-velocity sediments. Clear S-wave arrivals from the top of the igneous crust are common. Further processing and analysis of the wide-angle and multichannel seismic data will enable us to map in detial the geology of the Cascadia subduction zone and to characterize the probable zone of earthquake nucleation. (orig./HK)Available from TIB Hannover: RR 1846(49) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman