Seismic activity in the outer Cascadia accretionary prism from a 2-year onshore/offshore seismic array

In September, 2007, we deployed an array of ocean bottom seismometers from the NSF OBSIP across the central Oregon continental margin from the continental shelf to the abyssal plain. The Central Oregon Locked Zone Array (COLZA) was designed to span the subduction zone from the deformation front to deeper zone of ductile slip across a segment where moderate-size, low-angle thrust earthquakes have recently been documented on the nominally locked part of the plate boundary (Trehu et al., Geology, 2008). In January 2008, six additional stations from EarthScope’s USArray FlexArray instrument pool were added to COLZA to replace stations of the transportable array and increase station spacing. The instruments were redeployed for a second year in 2008. In this poster, we focus on results from the outer accretionary prism, where data from the first deployment (2007-2008) suggest bursts of broadband microtremor activity lasting several hours that may be related to fluid flow events. Similar activity has been reported from offshore Coast Rica (Brown et al., EPSL, 2005). We also observe narrow-band vibrations near 6 Hz with a weak apparent tidal modulation that are similar to vibrations recently reported for several other sites in the global ocean as well as a variety of other vibrations that result from a wide range of poorly-understood sources of both natural and man-made origin. During the second OBS deployment (2008-2009), we were able to compare data for different deployment configurations, which should provide insights into the sources of these enigmatic signals. For related results from COLZA, see the companion poster by Williams and Trehu (this session).