Geologic Controls on Seafloor Fluid Seepage in Cascadia from New Multibeam Echosounder Data

Multibeam bathymetry, backscatter, and water-column data were examined for evidence of seafloor fluid seepage and integrated with high-resolution seismic reflection profiles to assess geologic controls on seepage and implications for earthquake and submarine landslide hazards in Cascadia. Data were collected in 5 areas covering more than 8,850 km² offshore Oregon and northern California on cooperative USGS-NOAA surveys conducted in 2018 and 2019. The survey areas were on the outer shelf and slope in water depths ranging from 95-1900 m. More than 714 seeps were identified by the presence of bubble plumes in the water column using water column backscatter. In addition, more than 250 inactive seep-related features, including pockmarks and authigenic carbonate hardgrounds (a common by-product of methane seepage) were identified from analysis of seafloor bathymetry and acoustic backscatter data.

Seeps were found to occur in a variety of seafloor environments including seafloor ridges, depressions, pockmarks, mud volcanoes, faults, and associated with submarine landslides. There is no clear relationship of seep occurrence with water depth. Most active seeps in the survey areas occur on or near the crests of seafloor highs or ridges. Seismic reflection profiles show that most of these ridges are underlain by anticlinal folds associated with reverse faults. In places, seeps occur along the traces of faults, including a high-angle fault that cuts the seafloor offshore northern Oregon, and offshore Coos Bay, where an alignment of pockmarks interpreted to mark sites of past seepage follows the trace of a buried fault imaged on seismic reflection profiles. More than 150 seeps, about 20 percent of the total identified, are spatially coincident with two seafloor landslide features. Although there is a clear association of increased seep activity near these slides, it is not clear whether this is due to locally higher fluid flow that preconditioned slope failure, or if areas of slope failure subsequently promote seafloor seepage, possibly due to local (near-surface) extension and development of cracks that serve as fluid pathways.