Seismicity at Augustine Volcano in south-central Alaska was monitored continuously between 1970 and 2007. Seismic instrumentation on the volcano has varied from one to two short-period instruments in the early 1970s to a complex network comprising 8 to 10 short-period, 6 broadband, and 1 strong-motion instrument in 2006. Since seismic monitoring began, the volcano has erupted four times; a relatively minor eruption in 1971 and three major eruptions in 1976, 1986, and 2006. Each of the major eruptions was preceded by 9 to 10 months of escalating volcano-tectonic (VT) earthquake activity that began near sea level. The major eruptions are characterized seismically by explosive eruptions, rock avalanches, lahars, and periods of small repetitive low-frequency seismic events often called drumbeats that are associated with periods of lava effusion, and they all followed a similar pattern, beginning with an explosive onset that was followed by several months of discontinuous effusive activity.

Earthquake hypocenters were observed to move upward from near sea level toward the volcano’s summit over a roughly 9-month period before the 1976 and 1986 eruptions. The 1976 eruption was preceded by a small number of earthquakes that ranged in depth from 2 to 5 km below sea level. Earthquakes in this depth range were also observed following the 2006 eruption. The evolution of earthquake hypocenters associated with the three major eruptions, in conjunction with other supporting geophysical and geological observations, suggests that the Augustine magmatic system consists of a deeper magma source area at about 3.5 to 5 km below sea level and a shallower system of cracks near sea level where volatiles and magma may temporally reside as they ascend to the surface. The strong similarity in seismicity and character of the 1976, 1986, and 2006 eruptions suggests that the processes responsible for magma generation, rise, and eruption at Augustine Volcano have been roughly constant since the early 1970s.