SEISMIC STRUCTURE AND GAS HYDRATE STUDY ON THE SOUTHERN CASCADIA MARGIN USING MULTICHANNEL SEISMIC DATA

Abstract:

Multichannel seismic data were acquired in 1994 by the U.S. Geological Survey (USGS) to understand tectonic activities (volcanic and earthquake hazards) as well as the crustal structure on the Pacific Northwest and Cascadia margin in the vicinity of Cape Blanco. Specifically, this survey was designed to image the north-south variation in crustal structure, and to investigate differences in the crustal structure across an east-west shear zone in the Cape Blanco area. Reprocessing and tomographic velocity analysis of one of the margin perpendicular lines from this survey reveals prominent structures.

The migrated seismic section clearly imaged the Cascadia basin sediments, the accretionary wedge and the top of the oceanic crust. About 500 m of sediment section appears to be intensely deformed with several landward-dipping thrust faults. In addition, fold-thrust geometries are observed within this section. The top of the subducting Gorda plate appears to be fractured showing horst and graben structures. Several landward-dipping faults are observed within the sediment section, some of which reach the top of the crust. A prominent bottom-simulating reflector (BSR) is observed in the accreted sediment wedge over a 10 km distance at a depth of 270-537 ms. Seismic velocities of the hydrated sediments range from 1.8-2.0 km/s. In general velocities increase landward indicating over-consolidation of accreted sediments. Seismic velocities of 3.5-4.0 km/s are observed on top of the crust. Velocities of 4.0-4.5 km/s are observed beneath the deformation front, which coincide with an opaque zone in the migrated seismic section. This zone is likely acting as a backstop for the incoming sediments, which are then uplifted and folded to form the frontal accretionary ridge. These preliminary results highlight the complex relationship of margin sediments and structure that will assist further studies of hydrate in this area.