High-Resolution Imaging of Material Properties in the Cascadia Subduction Zone
The Cascadia Subuction Zone offshore of Northern California, Oregon, and Washington is capable of generating devastating magnitude 9 earthquakes. Since we have not observed one of these great earthquakes with modern instrumentation, the seismological community has significant uncertainties about what such an earthquake will look like in detail. Hazard estimates for this type of earthquake are very dependent on how far onshore the rupture propagates. Currently this is best estimated by computer models of the rupture which rely on matching GPS data to validate the material property distributions that control the model behavior. This study will use a new, combined onshore-offshore dataset to investigate the detailed spatial variations in material properties along the main Cascadia thrust fault. The results will hopefully inform the computer models of future ruptures and will also provide new data to the entire scientific community. The overall goal of the study is to understand the distribution of strength on actively deforming faults within the Cascadia subduction zone. Are the faults that rupture within the downgoing slab really two orders of magnitude stronger than the immediately adjacent (~5km away) and locked thrust interface? While we cannot measure stress directly, the researchers can measure related physical properties that underlie the typically strength distributions assumed in models of these faults. The primary project goals are to determine if there is any evidence for a downdip change of fluid pressure within the immediate (~1km) fault-zone producing thrust interface seismicity. Leading fault mechanics models typically assume a discontinuity (>1 order of magnitude change) in effective normal stress at the base of the locked zone. However, this has not been observationally evaluated at the resolution need to see the fault-zone or across the locked-creeping boundary which requires offshore data. They will also examine whether there is any evidence for pervasive serpentinization within the fault-zones in the mantle of the subducting Gorda Plate. Both of these types of studies will be made possibly by the recently deployed, community seismic experiments onshore and offshore in the Southern Cascadia subduction zone which cover the one region of the Cascadia thrust interface with abundant microseismicity, and the use of a modern techniques for determining earthquake locations, seismometer clock corrections and ambient noise techniques.