Tripartite arrays of sonobuoys were used to invesigate microseismicity at two locations on the central portion of the northwest trending Blanco fracture zone 450 km west of Oregon. The two arrays operated for 14 and 12 hours at locations separated by 50 km. Individual sonobuoys were tracked by radar as they drifted across the fracture zone. Epicenters determined for 32 of the 38 events detected show close association with topographic features of the fracture zone, in contrast to teleseismically determined epicenters, which are offset approximately 30 km from the fracture zone. The events at array 3 do not lie in the central trough but along the southern flank of a linear ridge that is parallel to the trend of the fracture zone. The events at array 4 lie within a topographic depression that offsets the center of the fracture zone and are probably related to steep scarps observed on the bathymetry. Magnitudes assigned to 23 events at array 4 range from -0.5 to +0.5.