DEFORMATION OF THE CENTRAL CASCADIA ACCRETIONARY PRISM FROM A 2-YEAR ONSHORE/OFFSHORE SEISMIC ARRAY

In September, 2007, we deployed an array of ocean bottom seismometers across the central Oregon continental margin from the shelf to the abyssal plain in the region where moderate-size, low-angle thrust earthquakes have recently been documented on the nominally locked part of the plate boundary (Trehu et al., 2008). In January 2008, six additional stations from EarthScope's USArray FlexArray instrument pool were added to COLZA (Central Oregon Locked Zone Array) to replace stations of the transportable array and increase station spacing. The instruments were redeployed for a second year in 2008. The array spans the plate boundary from the deformation front, across the outer accretionary prism and continental shelf, to the Coast Range. We will focus on results from the outer accretionary prism, where data from the first deployment suggest bursts of microtremor activity lasting several hours that may be related to fluid flow. Similar activity has been reported from offshore Coast Rica (Brown et al., 2005). We also discuss new constraints indicating that seismic tremor beneath the Coast Range originates on the plate boundary at a depth of 30 km and that some of the microearthquakes beneath the continental shelf also originate on the plate boundary.