Abstract:

A swarm of earthquakes occurred on an oceanic spreading center north of Iceland in late 1995 and was recorded by U.S. Navy hydrophone arrays in the Norwegian Sea. About two dozen of these events on the Mohns Ridge were detected by onshore seismic arrays. Analysis of the hydrophone array data shows that 7000 events occurred; the greatest number (40/h) took place in a 3-day period in the middle of the 70-day duration of the swarm. Recorded arrivals include P waves, water-borne T waves, PT pairs, and P waves reflected at the seasurface. Separation in arrival times of P and T waves are used to determine relative locations of events and their spatial evolution throughout the swarm. The locus of activity shifts by 30--40 km during the swarm but steady migration of activity is not apparent. This suggests that surface breaks during dike injection did not occur or, at least, did not generate T waves, or that the swarm was not associated with a simple dike emplacement along the ridge. The time history of the activity, on the other hand, is quite similar to that seen associated with two known volcanic events on the Juan de Fuca and Gorda ridges.