Improved Epicentral Locations for Earthquakes Near Explorer Ridge

The tectonics and structure of the Explorer region, which is the northern boundary of the subducting Juan de Fuca plate, help to inform our assessments of the seismic hazard in the Pacific Northwest. Our understanding of this tectonically complex area is largely based on morphology of the seafloor from swath bathymetric data, potential field anomalies, and the calculated locations of contemporary earthquakes in the region. However, the Navy Sound Surveillance System hydrophone network, the Canadian National Seismic Network, the U.S. Advanced National Seismic System, and the Harvard Centroid Moment Tensor Catalog report significantly different epicentral locations for swarms of earthquakes near Explorer Ridge in August and October 2008. We relocated the larger (M>5) earthquakes in the August 2008 swarm using data from both U.S. and Canadian networks to improve azimuthal coverage. Absolute locations were determined for the largest events in the swarm, and the smaller events were relocated relative to the largest using a double difference method. To better understand why the locations from land-based seismic networks differ from those computed from the hydrophone arrays, we also examine T-phases from regional events recorded on Ocean Bottom Seismometers from the COLZA and Cascadia Initiative experiments and evaluate the potential for using T-phases to improve the epicentral locations of submarine earthquakes in the Pacific Northwest region.