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 an episode of seafloor spreading on the Mohns Ridge. The earthquakes were recorded from November 1995 to January 1996 by onshore seismic stations and by U.S. Navy hydrophone arrays in the North Atlantic. Both the temporal and spatial histories of the activity suggest that volcanism accompanied the tectonic events. The hydrophone arrays recorded 2-3 orders of magnitude more events than the onshore seismic arrays with up to 1000 events per day observed 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 epicenters 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 more distant location. The activity tends to concentrate in one area or another for short periods (a few days), but at times it is clear that events occur simultaneously at more than one location. We have not found evidence of steady 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 involved in the spreading episode, rupture and magmatic eruption at the seafloor probably only occurred in a few discrete areas.