Simultaneous earthquake swarms and eruption in Alaska, fall 1996: Statistical significance and inference of a large aseismic slip event

In the fall of 1996 the Alaska Volcano Observatory recorded an unprecedented level of seismic and volcanic activity. The following were observed: (1) a swarm at Iliamna Volcano (August 1996 to mid- 1997); (2) an eruption at Pavlof Volcano (September 1996 to December 1996); (3) a swarm at Martin/Mageik volcanoes (October 1996); (4) a swarm at Strandline Lake, which continued for several years (1996-1999); and (5) deformation of Mt. Peulik (inflation begins after October 1996 and ends in fall 1998), based on interpretation of interferometric synthetic aperture radar data. The number of monitored volcanic areas in 1996 was thirteen. We conducted two formal statistical tests to determine the likelihood of four of these events occurring randomly in the same time interval. The tests are based on different conceptual probabilistic models (classical and Bayesian) that embrace a wide range of realistic tectonic models. The first test considered only the areas in which swarms or eruptions occurred (7 of 13 if Strandline Lake is included; 6 of 12 otherwise), by comparing the real catalog with 10,000 synthetic catalogs under the assumption that the sites are independent. The second method is a hierarchical Bayesian model in which the probability of a swarm at each of the 13 (or 12) areas is different but the parent population is the same. We found that the likelihood of the swarms and eruption occurring nearly simultaneously by chance alone is small for a wide range of probabilistic schemes and, consequently, for different tectonic scenarios. Therefore, we conclude that the events may be related to a single process. We speculate that a widespread deformation pulse or strain transient occurred, mainly in the east half of the arc, and may be the most likely candidate for causing such nearly simultaneous events.