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ABSTRACT: Submarine Landslides Associated with Shallow Seafloor Gas and Gas Hydrates off Northern California

Michael E. Field

The continental margin off California north of Cape Mendocino contains more landslides than any other region along the west coast of the U.S. Factors contributing to the abundance of landslides are high levels of seismicity (historically one event of M6 or greater per decade), tectonic uplift and deformation, and large quantities (20 to 30 × 106 tons) of fluvial sediment delivered to the margin each year.

More recently, interstitial gas derived from biogenic and possibly thermogenic sources, and from degraded gas hydrates, has been recognized as another potentially important factor in causing some of the slides.

One of the more prominent slides is the Humboldt slide zone, west of Eureka on a 4° slope at water depths of 250 to 500 m. The slide zone consists of large back-rotated blocks that failed in a retrogressive manner. In cross section, individual blocks are up to 500 m wide and extend 50 m below the sea floor. In plan view, the slide zone is visible on GLORIA long-range side-scan sonar images as a series of continuous arcuate ridges.

The evidence for shallow gas is abundant. Acoustic masking and enhancement of reflectors below the slide are evident on high resolution records. Hundreds of pock marks (seep craters) up to 25 m in diameter are scattered throughout the area. Shallow cores indicate elevated levels (>10,000 mL/L) of methane gas in the upper 2 m of sediment. Similarly, the presence of gas hydrates is well documented. Initially inferred on the basis of a bottom simulating reflector (BSR), samples of gas hydrates have recently been obtained from the upper 1 m of the sea floor.

Gas in bubble phase can markedly increase the pore fluid pressure and thereby decrease the effective stress of seafloor sediment and ultimately lead to failure. Gas hydrates contain enormous quantities of gas, and thus their presence, along with the abundant evidence of free gas, in the failure zone indicates a possible link between the gas hydrates and the slides.

AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990