Sediment consolidation at the Cascadia margin deformation front and its impact on megathrust slip behavior

At the Cascadia subduction zone, the megathrust shows along-strike variation in slip behavior that is not well explained. From land-based geodetic observations, the plate interface is currently partially creeping between 43°N and 46°N but is strongly locked to the south and north (Schmalzle et al., 2014). Paleoseismic studies identify four rupture segments that vary in rupture frequency (Goldfinger et al., 2010). The consolidation state of sediments entering subduction zones is one key control on pore fluid pressure and slip behavior of the plate interface, but has not been examined in detail. Here we quantify sediment properties across the deformation front, and from this infer in situ stress and pore pressure along two cross-margin transects where different slip behaviors are inferred: a Washington transect at 47.4°N crossing a locked northern paleo-rupture segment, and an Oregon transect at 44.6°N crossing the partially creeping zone and is at the boundary between two middle segments. Vp models for the sediment are derived from long source-receiver offset (8 km) multichannel seismic data acquired from the Juan de Fuca Ridge-to-Trench study, and then converted to porosity, effective stress, and pore fluid pressure using empirical relationships calibrated by core and log measurements from ocean drilling that link Vp-porosity-effective stress. On the WA transect where the decollement is only 150 m above the basement, we observe over-consolidated sediments near the deformation front, and further consolidation in the outer accretionary wedge. In comparison, along the OR transect, the 1 km thick sediment section beneath a proto-decollement is under-consolidated with velocities of 3.5-3.6 km/s, significantly slower than the sediments at similar depth along the Washington transect (4.0-4.1 km/s). This difference corresponds to a significantly lower ( 40%) mean effective stress offshore OR. The thick under-consolidated sediments subducting beneath central OR and associated excess pore pressure, may contribute to partial creeping of the plate interface in this region, relieving stress on the plate interface and creating a local barrier for earthquake ruptures.