Seismic Observations of the Augustine Magmatic System: 1970 2006

Seismic activity at Augustine Volcano, Alaska, has been closely monitored since 1970. During this period three major eruptions occurred in 1976, 1986 and 2006 and a minor eruptive event took place in 1971. Seismic instrumentation at Augustine has evolved from a single station in 1970, to a network of 4 to 5 stations during the 1976 and 1986 eruptions, to 8 stations prior to the onset of unrest in 2005. An additional 6 broadband and 2 strong motion instruments were deployed prior to the onset of eruptive activity in late 2005. All three major eruptions were preceded by roughly 8.5 months of slowly escalating seismic activity centered beneath the summit. The precursory seismicity is dominantly Volcano-Tectonic (VT) earthquakes, and few low-frequency or long-period events have been observed. The largest magnitude earthquake in each precursory sequence was 2.8, 2.1 and 1.5 in 1976, 1986 and 2006 respectively. Prior to the 1976 and 1986 eruptions, hypocenters were observed to slowly migrate upwards from depths of about 0.75 km below sea level. Preliminary results suggest earthquake hypocenters followed a similar pattern prior to the 2006 eruption. Increases in steaming at the summit and phreatic explosions were observed during the later parts of all the precursory seismic sequences. Each eruption followed a similar progression beginning with a series of explosions that gave way to the effusion of lava forming a summit lava dome. Seismicity during the early portions of the eruptions is dominated by signals from explosions and pyroclastic flows. In both 1986 and 2006 periods of lava extrusion were accompanied by small regularly spaced earthquakes, many with identical waveforms. The deeper parts of the Augustine magmatic system seem to be largely aseismic or the instrumentation at Augustine between 1970 and 2006 has been insufficient to track this activity. The exception to this are earthquakes located at depths of approximately 1 to 3 km early in the 1976 precursory sequence. During inter-eruptive periods seismicity at Augustine is dominated by VT earthquakes that generally locate high in the Augustine cone, perhaps a reflection of gravitational settling or cooling of the lava domes. The similarity between eruptions and associated seismicity suggests the processes governing the production, ascent and eruption of magma and conduit geometry at Augustine are roughly constant during the 1976, 1986 and 2006 eruption seqences. Observations of seismicity suggest that during recent historical eruptions magma at Augustine begins to pressurize and ascend from an area near sea-level 8 to 9 months prior to eruption. The deeper earthquakes seen early in the 1976 precursory sequence may define a magma source zone.