Defining static correction for jointly relocated earthquakes along the Blanco Transform Fault Zone based on SOSUS hydrophone data

The Blanco Transform Fault Zone (BTFZ) is a complex /spl sim/350 km long segment of the boundary between the Pacific and Juan de Fuca plates, located /spl sim/200-500 km west of Point Blanco on the Oregon coastline. Individually located earthquake epicenters reported by the International Seismological Centre and the U.S. Geological Survey´s National Earthquake Information Center along the BTFZ are not well correlated spatially with the active transform boundary due to poor azimuthal distribution of recording stations, inaccurate crustal velocity models or random reporting errors. More accurate locations of 120 m/sub b/ /spl ges/ 5.0 earthquakes relative to each other have been obtained using the joint-relocation method of Jordan and Sverdrup (1981). The Jordan-Sverdrup method simultaneously relocates epicenters in clusters of up to 20 events, yielding improved locations for epicenters within the cluster relative to each other. Six of the earthquakes in this set have been independently located using data from the U.S. Navy´s SOSUS hydrophone array, enabling specification of a static correction to obtain improved absolute locations for the epicenters relative to the BTFZ. Of several methods evaluated, the preferred method to define the static correction in this case used the pole to the best-fit great circle through the six reference epicenters as one of the two necessary fiducial points, and the projection of the largest earthquake onto the best-fit great circle as the other fiducial point. The fiducial points were used to define the rotation parameters for the static correction. The 95% confidence ellipses of the relocated epicenters generally fall on or very near active structural features along the BTFZ after application of the static correction.