Anomalous Concentration of Methane Emissions at the Continental Shelf Edge of the Northern Cascadia Margin

Abstract

AbstractA recent compilation of methane plumes detected offshore Washington State includes 1,772 individual bubble streams issuing from 491 discrete vent sites. The majority of these plume sites form a narrow 10‐km‐wide band located shallower than 250‐m water depth, with most sites located near the 175‐m‐deep continental shelf break that tracks the head scarps of large submarine canyons. Archive multichannel seismic profiles over the Cascadia shelf and uppermost margin that were co‐located within a few hundred meters with active emission sites show that methane bubble streams arise from listric/normal faults and triangular‐shaped regions of disturbed seismic reflectors that intersect the seafloor and extend several kilometers into the subsurface. Geological processes were evaluated for producing the narrow emission site depths including nonuniform distribution of methane within the Cascadia accretionary sediment wedge and horizontal transfer of groundwater from onshore subaerial sources. A model of enhanced sediment permeability arising from a contrasting response between the inner and outer portions of the accretionary wedge deformation during a megathrust earthquake cycle appears the most likely mechanism. This faulting is generated during extension of the overriding plate during megathrust earthquake cycles, with semicontinuous permeability enhancement of the fluid pathways from excitation by contemporary incident seismic waves.