The inner continental margin of northern California is underlain by a well-defined and extensive acoustic reflector that crosses other reflectors and mimics the surface of the sea floor. This bottom-simulating reflector (BSR) lies at a typical subsurface depth of about 250 m and has been mapped continuously beneath the Klamath Plateau and upper slope (water depths of 800 to 1200 m) for a distance of more than 130 km; it covers an area of at least 3000 km2. Limited data show that the BSR extends northward into Oregon and seaward at least to the base of the slope (3000 m).
The water depths, subsurface depths, and pervasive nature of the BSR all suggest that it represents the base of a natural-gas hydrate. Using standard phase boundary diagrams for hydrate stability, we estimate the local geothermal gradient within the gas hydrate to be about 55 °C/km. This value is higher than that of most subduction margins and may be a result of the youthfulness of the subducting oceanic crust. This acoustically inferred gas hydrate is the first to be mapped on the western conterminous United States continental margin.