Seismoacoustic recordings of a spreading episode an the Mohns Ridge

A period of very active seismicity near 72.7 degrees N, 4 degrees E marks a n episode of seafloor spreading on the Mohns Ridge. The earthquakes were re corded from November 1995 to January 1996 by onshore seismic stations and b y U.S. Navy hydrophone arrays in the North Atlantic. Both the temporal and spatial histories of the activity suggest that volcanism accompanied the te ctonic events. The hydrophone arrays recorded 2-3 orders of magnitude more events than the onshore seismic arrays with up to 1000 events per day obser ved during the most intense phase of activity. A level of 50-200 events per day was sustained throughout the episode. Initial locations of the events were obtained from the seismic bulletin. Further refinement of the epicente rs was possible using P, S (converted to an acoustic phase at the seafloor) , and T waves in the hydrophone data, Analysis of arrival time differences between these phases indicates that one main area and two subsidiary areas along the rift were active during the swarm. A few events occurred at a mor e distant location. The activity tends to concentrate in one area or anothe r for short periods (a few days), but at times it is clear that events occu r simultaneously at more than one location. We have not found evidence of s teady migration of activity, such as might accompany propagation of a magma -filled dike. We thus infer that despite the 50-70 km length of ridge invol ved in the spreading episode, rupture and magmatic eruption at the seafloor probably only occurred in a few discrete areas.