Collaborative Research: Inferences on Cascadia Deformation Front and Plate Interface Properties from Advanced Studies of Active Source Seismic Data

The Cascadia subduction zone extends offshore western North America from northern California to British Columbia and has been the site of past great earthquakes comparable to the Tohoku Mw~9 earthquake in 2011. The last great earthquake at Cascadia occurred in 1700 and produced a trans-oceanic tsunami recorded in Japan. With estimated recurrence intervals of 300-500 yrs it represents a significant seismic and tsunami hazard for the dense coastal population of the Pacific Northwest. Furthering our understanding of the characteristics of this subduction zone is of significant societal interest. At present, Cascadia is unusually quiet seismically, with few of the frequent small earthquakes detected at most subduction zones, and the properties of this subduction zone are poorly understood. Furthermore, over 3 kilometers of sediment buries the Cascadia subduction fault or megathrust zone making it inaccessible to direct sampling. Marine seismic methods provide x-ray like images beneath the seafloor that can be used to illuminate the fault zone as well as constrain the properties of the fault and bounding sediments, which contribute to seismogenesis. This project will use previously acquired marine seismic data to investigate the structure of the Cascadia megathrust zone within the shallow portion where great earthquakes initiate. It will help elucidate properties of the fault zone in a region offshore central Oregon that may currently be creeping and which may act as a barrier to rupture in future earthquakes. This area will be compared with another region offshore Washington that is believed to be fully locked at present with potential for rupture over a larger area in future earthquakes. The understanding of megathrust properties obtained from the project will benefit other major ongoing and planned scientific programs in the region including studies of the subduction zone at central Oregon using NSF's Ocean Observatory Initiative and planned future subduction zone monitoring studies making use of the International Ocean Drilling Program. This study will support two graduate students and two early career scientists, helping to build the next generation of marine seismologists.

The project makes use of 8 km long-streamer multi-channel seismic and wide-angle ocean bottom seismometer data acquired in 2012 along two lines that cross the margin offshore central Oregon and Washington extending from ~40 km seaward of the deformation front to ~20 km from the coast, and one ~400 km line parallel to the coast located ~10-15 km seaward of the deformation front and spanning the 2 dip lines. The seismic data will be used for detailed studies of the properties of the basal sediment section and top oceanic crust near the Cascadia deformation front, including velocity-derived effective stress and pore fluid pressures, and for improved imaging of the forearc accretionary wedge within the Washington and Oregon regions of contrasting subduction zone properties. Sediment properties will be evaluated from compressional-wave velocity (Vp) models derived from Pre-Stack Depth Migration (PSDM) and Full Waveform Inversion, shear wave velocity (Vs) and splitting studies, and Amplitude vs Offset studies. Forearc accretionary wedge structure will be examined using PSDM imaging and waveform modeling of the plate interface. Specific project goals are: 1) to determine the seismic characteristics (Vp, Vs, Vp/Vs, anisotropy) and derived physical properties (porosity, effective stress, pore fluid pressure, and crack density) of the basal sediment section prior to subduction, and how these vary along the margin, and 2) to examine what conditions control the stratigraphic position of the proto-décollement and how these conditions are related to the different structural styles of the forearc and 3 to determine how the physical properties of plate interface zone at Cascadia vary in the down-dip direction both in the strongly-coupled Washington region as well as in the partially creeping central Oregon section, and how they relate to upper plate forearc deformation.