Download or read book Seismic Velocity Structure Associated with Gas Hydrate at the Frontal Ridge of Northern Cascadia Margin written by Caroll López and published by . This book was released on 2008 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: At the frontal ridge near the base of the slope off Vancouver Island, wide-angle ocean bottom seismometer (OBS) data were acquired in summer 2005, in support of the Integrated Ocean Drilling Program (IODP) Expedition 311. Marine gas hydrate is present beneath the ridge based on the observation of the 'Bottom Simulating Reflector' (BSR) that is interpreted to coincide with the base of the methane hydrate stability zone. Hydrate was also observed in downhole logs and drilling by IODP. The BSR has been identified on single-channel seismic data at -250-260 m depth beneath the ridge crest and on its seaward slope. The OBS data have been analyzed with the objective of determining the velocity structure in the upper portion of the accretionary wedge especially the hydrate stability zone and underlying free gas. As identified by a clear refracted phase, the velocity structure above the BSR shows anomalous high velocities of about 1.95 (?0.5) km/s at shallow depths of 80 - 110 m. On vertical incidence data, high amplitude reflectors are observed near this depth. Below the BSR, the velocities increase to -2.4 km/s at sub-seafloor depths of about 600 m. A strong refracted phase with a velocity of 4.0 km/s is generated at a depth of about 1700 mbsf. Velocities from traveltime inversion of OBS data are in general agreement with the Integrated Ocean Drilling Program (IODP) X311 downhole sonic velocities. In particular, on the log data, a layer with low porosity and high velocities of 2.4 - 2.8 km/s was observed at depths of 50 - 75 m. This probably corresponds with the 1.95 km/s layer at depths of 80-110 m interpreted from the OBS data. The refraction data thus suggest that this high-velocity layer varies laterally through the frontal ridge region, out to distances of at least 4 km from the drillhole. BSR depths (250-280 m) estimated in the present work also agree with the IODP X311 depths. From the velocity structure, we can make estimates of hydrate concentration in a region close to the deformation front, where fluid flow velocities are expected to be large. The gas hydrates concentrations vary from -35% for the shallow phase to -22% for the layer above the BSR. The deep refracted phase with a velocity of 4.0 km/s at 1700 m depth indicates the presence of highly compacted accreted wedge sediments. On the SW side of the frontal ridge, a collapse structure is observed in newly acquired multi-beam bathymetry data from the University of Washington and in seismic reflection data. The BSR is present in the region surrounding the slump. There are only weak indications of its presence within the slide region. Since hydrates may prevent normal sediment compaction, their dissociation in sediment pores is thought to decrease seafloor strength, potentially facilitating submarine landslides on continental slopes. The head wall of the frontal ridge slide is -250 m high, extending close to the BSR depth, and the slump has eroded a -2.5 km long section into the ridge, along strike. Migrated seismic reflection data image a set of normal faults in the frontal ridge striking NE-SW, perpendicular to the strike of the ridge and the direction of plate convergence. These faults outcrop at the seafloor and can be traced from the surface through the sedimentary section to depths well below the BSR in some locations. Seafloors scarps show that fault seafloor displacements of -25 m to 75 m are generated. The two faults with the largest seafloor scarps bound the region of slope failure on the frontal ridge, suggesting that the lateral extent of slumping is fault-controlled. The triggering mechanism for the slope failure may have been a combination of various effects. The possible mechanisms explored include gas hydrate dissociation, high pore pressure fluid expulsion along the faults, and salinity elevation in faults which would inhibit the formation of gas hydrates along the faults. However, an earthquake may induce initial slope failure, which can not only start gas hydrate dissociation but also increase fluid expulsion and pore pressure.