CASCADIA SUBDUCTION ZONE LOCKING

Between 43°N and 46°N the central Cascadia subduction zone has reduced interseismic uplift observed in geodetic data. Using a mechanical block model, we estimate subduction thrust ‘‘decade-scale’’ locking and crustal block rotations from three-component continuous Global Positioning System (GPS) time series from 1997 to 2013, as well as 80 year tide gauge and leveling-derived uplift rates. Modeled locking distributions suggest a wide locking transition zone that extends inland under central Oregon. In the same region paleoseismic records of multiple great earthquakes along Cascadia indicate less subsidence. We suggest that the Cascade thrust under central Oregon may be partially creeping for at least 6500 years (the length of the paleoseismic record) reducing interseismic uplift and resulting in reduced coseismic subsidence. Thick accretions of the Siletzia terrane between 43°N and 46°N may increase creep if it is less permeable compared to surrounding terranes; thereby increasing pore fluid pressures along the fault interface, resulting in a wide zone of persistent fault creep.