Submarine landslide distribution and potential sources of seismoturbidites along the Cascadia Subduction Zone Margin

Abstract:

The offshore record of large (>M7) earthquake recurrence in the Cascadia Subduction Zone is derived from the deep sea turbidite record, and relies on the assumption that submarine canyon systems are the primary conduits for synchronous turbidity flows generated during shaking events. However, the sources of these turbidity flows are poorly constrained, leading to uncertainties in the connections between ground shaking, slope failure, and deepwater turbidites. Using a combination of high-resolution multibeam bathymetry, Chirp subbottom profiles, and multichannel seismic surveys, we are developing a comprehensive spatial database of seafloor failure scarps in this region to investigate potential source areas for earthquake triggered turbidity currents along the margin. To date, more than 5000 seafloor scarp features have been identified, with many of these representing coalesced failures.

In northern Cascadia, well-developed submarine canyon systems exhibit propagating headscarps with extensive sidewall and channel levee failures. Open slope gullies drain canyon interfluves and incise a steep upper slope escarpment. Larger, more discrete failures occur on the lower slope where the flanks of steep-sided landward-vergent folds exhibit slab failures with rectangular headscarps. In contrast, in central and southern Cascadia, mass wasting primarily occurs outside of submarine canyon catchment systems where the steep outer wedge of the lower slope is characterized by a mix of large, coherent block failures, coalesced slab-style failures, slumps, gully erosion, and other small disintegrative failures. Upper slope failures are notably absent in large portions of central and southern Cascadia, where seafloor gradients are lower due to forearc basins that extend seaward onto the slope. Our results suggest disintegration of the steep outer wedge away from canyon catchment systems is a significant potential source of turbidity flows triggered by earthquake shaking, and may be the primary source of fine-grained seismoturbidites found in southern Cascadia. Given the extensive mass wasting also present on the steep lower slope of central Cascadia, these deposits may extend farther north than previously thought, and may have important implications for the record of earthquake recurrence across the region.