Earthquake activity at the Kodiak continental shelf, Alaska, determined by land and ocean bottom seismograph networks

abstract The spatial pattern of earthquakes determined by a combined land and ocean bottom seismometer (OBS) network in the Kodiak Island shelf region differs systematically from the pattern determined by a land network and from locations determined by the International Seismological Centre (ISC). As a part of a larger study of seismic risk on the continental shelf near Kodiak Island, we augmented the University of Alaska land network by deploying 11 recoverable OBS units south of Kodiak Island for 2 months in the summer of 1979. Despite a relatively short operation time and various instrument malfunctions, the combined network detected 19 locatable earthquakes in the shelf region. Because of the structural heterogeneity of this area, the earthquakes were located with a scheme which allowed different velocity models to be used for travel-time calculations of phases traveling to different stations in the network. The locations of earthquakes determined using data from both land and OBS networks were displaced about 12 km from the hypocenters of the same earthquakes determined using only land network data. For these events on the continental shelf, azimuthal control of the joint land-OBS network is excellent, and thus the joint land-OBS network locations are considerably more reliable than locations determined with the land network data alone. When the locations of the combined land-OBS network are compared to 15 yr of teleseismic locations reported by the ISC, the center of teleseismic activity appears to be about 20 to 30 km north of the center of activity determined in our study. This difference between locally determined and teleseismically determined location is similar to that observed in other studies of earthquakes and nuclear explosions in the Aleutian arc.