SHEAR-WAVE VELOCITY STRUCTURE AND SEISMICITY OF THE CASCADIA SUBDUCTION ZONE FROM OBS STUDIES

The up-dip region of subduction zones is difficult to study using land-based seismic and geodetic networks, yet documenting its ability to store and release elastic strain is critical for understanding the mechanics of great subduction earthquakes and tsunami generation. Here we summarize results from recent OBS deployments that reveal the mechanical structure and seismicity of the Cascadia forearc region. As part of the SeaJade experiment we deployed 10 instruments on the continental slope offshore of Vancouver Island. Using acceleration measured by broadband seismometer and seafloor pressure measured by differential pressure gauge, we estimate the compliance spectrum and invert for the S-wave velocity, P-wave velocity, and density structure. By including constraints on the Vp distribution from active-source studies, these parameters are well constrained down to the plate interface (~4-6 km depth). There is a clear difference between stations close to the deformation front and those further up the continental slope indicating a region of unconsolidated, high-porosity sediment. These regions with low S-wave velocities and high Vp/Vs ratios vary rapidly in both the along-dip and along-strike directions. The slip distribution during future great earthquakes will likely be influenced by these variations, possibly as regions with extremely low rupture velocities and/or high dynamic stress concentrations both of which may indicate a connection between shear-velocity structure and tsunami generation.

The Cascadia Initiative has provided a high-resolution seismicity dataset from the Mendocino Triple junction region. At least an order of magnitude more seismicity can be detected and located by the OBS network than is archived in the onshore catalogs. In addition to considerable activity on the Mendocino transform, there is seismicity in the downgoing slab including along the rupture plane of the January 2010 Mw 6.5 earthquake. The OBS data is of sufficient quality to constrain earthquake corner frequencies and radiated energy for events in the magnitude 2 to 4 range. We are currently evaluating the variations in stress drop and apparent stress for the various fault systems near the triple junction and within the subducting Gorda Plate to understand the distribution of fault strength in this complicated region.