A High Wave Speed Basal Sedimentary Layer Identified From Seismic Imaging of the Plate Boundary in Central Cascadia

The Cascadia subduction zone is known to produce large tsunamigenic ruptures but has enigmatically little microseismicity in historic times. With the paucity of microseismicity and recent long-offset multichannel seismic imaging in the past 20 years, poor constraints are available for Cascadia on plate boundary structure, fluid pressure, and mechanical properties, the combination of which control seismogenic behavior. Here we present prestack time-migrated seismic images from two profiles across the central Cascadia megathrust offshore southern Washington, focused on the plate boundary. Our results indicate that (1) a similar to 1 km-thick sedimentary unit with high seismic wave speed (over 4.5 km/s) directly overlies oceanic crust and (2) the decollement is at the interface between oceanic crust and the high wave speed unit. The high wave speed begins seaward of the deformation front and is consistent with porosity near or below 5%, interpreted to result from a combination of depth compaction and diagenesis over young, hot oceanic crust. The location of the decollement is identified by the participation of the entire sedimentary package in upper-plate compressional deformation. We suggest that the low-porosity basal unit may focus fluid escape from oceanic crust along faults and may be mechanically strong similar to Sumatra, where a high-wave speed, diagenetically altered basal unit facilitated megathrust rupture to the trench. A uniformly strong basal unit above the plate interface at Cascadia may inhibit microseismicity while building stress released in great earthquakes.