Seismic Velocity Structure Associated with Gas Hydrate at the Frontal Ridge of the Northern Cascadia Margin

In order to estimate the velocity structure of the sediments and their association with gas hydrates, ray-trace traveltime modeling has been conducted on an array of eight Ocean Bottom Seismometers (OBS) at the frontal ridge just landward of the deformation front at the base of the slope off Vancouver Island. Marine gas hydrates are present beneath the ridge based on the observation of the Bottom Simulating Reflector (BSR) that is assumed to coincide with the base of the methane hydrate stability zone. The velocity structure above the BSR shows anomalous high velocities of about 2.0 km/s at shallow depths of 50 - 100 m. On vertical incidence data, high amplitude reflectors are observed near this depth, and a series of steep normal faults extend above the reflectors to intersect and offset the seafloor. Below the BSR, the velocities increase to ~2.4 km/s at subseafloor depths of about 400 m. A strong refracted phase with a velocity of 4.0 km/s is generated at a depth of about 1 km, and this indicates the presence of highly compacted accreted wedge sediments. Velocities from traveltime inversion of OBS data are in general agreement with the Integrated Ocean Drilling Program (IODP) X311 downhole sonic velocities. In particular a layer with low porosity and high velocities of 2.4 2.8 km/s was observed at depths of 50 75 m. This corresponds well with the 2.0 km/s refracted phase in the OBS data. BSR depths (250-260 m) estimated in the present work also agree with the IODP X311 depths. The origin of the shallow high velocity layer is currently unknown. Ongoing interpretation of wide angle and vertical incidence data will provide information on this layer by extending the velocity and structural model beyond the drillhole.