Long-term seismicity and ground deformation at Axial Volcano, Juan de Fuca Ridge

Seismicity along the Juan de Fuca Ridge (JdFR) has been monitored since August 1991 using U.S. Navy hydrophones in the northeast Pacific Ocean. During this time, the focus of JdFR seismicity has been Axial Volcano, where a total of 9055 earthquakes have been detected (1669 located). This activity occurred within 48 discrete swarms along the summit caldera and flank rift zones. From August 1991 to May 1996, Axial Volcano exhibited only one swarm with greater than or equal to 50 earthquakes. From May 1996 to November 1997 swarm activity increased significantly with five swarms of greater than or equal to 50 earthquakes. Activity culminated at the volcano in January 1998 with an 11-day, 8247-event swarm that was the result of a major eruption along the summit and southern flank. Seafloor deformation sensors indicated pre-eruption swarms in October 1995, May 1996, and June 1996 coincided with subsidence of the caldera floor. The 1996-1997 swarm activity increase also coincided with horizontal extension of the north rift zone, consistent with pre-eruption deformation of the volcano. Following the 1998 eruption, swarm activity dramatically decreased and no earthquakes have been detected since April 1998. This is the longest aseismic period (similar to 1 yr) observed at the volcano since acoustic monitoring began, suggesting the January 1998 swarm marked the end of an eruptive cycle at Axial Volcano.