Abstract:

Recently, there has been a surge of interest in monitoring small-scale, mid-ocean ridge seismicity as made possible by the increasing availability of data from military-run hydrophone arrays. In the past, long-term ridge seismicity has been studied using the global networks of seismic stations. Since the young lithosphere at the ridge is relatively weak, it is not able to support large earthquakes and much of the seismicity associated with active deformation is not detected by the seismic networks. In addition to seismic body waves, mid-ocean ridge earthquakes usually produce a T phase---a water-borne acoustic wave---which travels great distances with little attenuation and is well recorded by hydrophones. This paper examines earthquakes from a section of the Mid-Atlantic Ridge near Ascension Island, using data collected from five U.S. Air Force hydrophones that surround the island and the seismic station ASCN located on the island. By integrating the hydroacoustic T phase and the local body-wave phases, earthquakes can be located down to a magnitude of 2.8 on a segment of the ridge with an accuracy of about 5 km.