A three-dimensional seismic reflection survey was carried out over a gas hydrate field offshore Vancouver Island in the area of Ocean Drilling Program Leg 146, Site 889/890. The area of investigation is located on the accretionary prism of the Cascadia margin in water depths of 1200--1500 m. Several seismic blank zones were observed in the seismic data over a frequency range from 30 Hz to 3.5 kHz, where the degree of blanking increases with seismic frequency. Time slices and selected horizon slices were used to define the spatial distribution of the blank zones. The zones range from 80 m to several hundred meters in width. One blank zone, almost circular with a diameter of ~400 m, has a distinct seafloor expression. It shows the characteristics of a mud/carbonate mound and is probably associated with free gas expulsion. Hydrate was found at four sites with a piston corer within this blank zone at depths of 3--8 m below the seafloor. We present a tectonic model that explains the blank zones to be a result of differential uplift along thrust faults. The blank zones represent conduits for fluids and gas migrating upward. Blanking of the seismic energy is believed to be mainly the result of increased hydrate formation in lenses within the faults. The blanking effect is enhanced locally because of scattering at carbonates formed at the seafloor.