Redoubt Volcano erupted for 3 months in 2009, generating spectacular ash clouds and producing a large lava dome that remains in place today. Both this eruption and the last eruption in 1989 were preceded by bursts of earthquake activity -- known as earthquake swarms-- that provided scientists with early warning of the imminent eruption.
When more earthquake swarms occurred around the vent in the year after the eruption, the alert levels were raised in anticipation of eruptive activity.
No eruption occurred, however, so the earthquakes were dismissed as having occurred either within the cooling lava dome or in the summit glaciers rather than being generated by the volcano. Helena Buurman and colleagues show that deeper earthquakes identical to those recorded during the 2009 eruption were reactivated during these earthquake swarms.
Because these deeper earthquakes were recorded during eruptive activity, their presence after the eruption suggested that the magmatic system at Redoubt was still active.
An active gas vent on the lava dome also closed after the last of the earthquake swarms, providing further evidence that although the volcano had not been erupting, the system had remained open and active for nearly a year before finally closing -- much longer than previously thought.
Helena Buurman et al., Alaska Volcano Observatory, Geophysical Institute, University of Alaska Fairbanks, DOI:10.1130/G34089.1