The Role of Multiplets in the Seismicity of the October 2007 Eruption at Bezymianny Volcano, Russia

Detailed patterns in micro-seismicity during explosive and background periods are poorly characterized at Bezymianny Volcano, Russia. Bezymianny has erupted twice-annually since 2000 and the Kamchatkan Branch of Geophysical Services (KBGS, Russian Academy of Sciences) has successfully developed the ability to forecast most of these eruptions using thermal anomalies and far-field (> 7 km from the vent) seismic observations. In 2005, a temporary seismic network was installed around Bezymianny to observe several eruptive cycles. Using 3 to 8 broadband stations from this network, we are able to better observe subtle patterns and track changes during the buildup to eruptions. Additionally, we can add better depth constraint to existing KBGS earthquake locations if we add phase picks from our stations, which are significantly closer to the vent and at higher elevations than stations in the existing KBGS network. A catalog of multiplets (repeating earthquakes) for October 2007 reveals a decline beginning October 10 in the number of multiplets occurring concurrently, and a dramatic decrease in the lifespan of active multiplets hours before the first explosion. This may indicate that the source area for the multiplets was becoming less stable as the explosion neared. Recalculated earthquake locations prior to the eruption ranged in depth from 0 to 4 km deep, however almost none of these earthquakes were part of a multiplet. The eruption itself can be divided into two distinct explosive phases separated by approximately 10 hours. Between explosions, there were several shallow low frequency events and rockfall, but little tremor and no occurrence of multiplets. The second explosive phase was nearly twice the duration of the first but less energetic. Several earthquakes with quality locations between 0 and 3.5 km occurred immediately after the second eruption. Multiplets increase in numbers and lifespan shortly after the explosions on October 15, suggesting a return to a more stable source region. Future work will track the role of multiplets in subsequent eruptions at Bezymianny and look at the role of multiplets in other eruptions at similar volcanoes, such as Mount St. Helens.