A seismological comparison of Bezymianny Volcano, Russia, and Mount St. Helens Volcano, Washington

Bezymianny Volcano, Russia and Mount St. Helens, Washington are examples of eruptions involving the sector collapse of a volcano. The study and comparison of Mount St. Helens and Bezymianny has led to a better understanding of the precursory seismicity prior to eruptions. The shallow moment release prior to an eruption is dependent on the time since the last eruption at both volcanoes. This relationship is independent of the type of eruption that ensues, suggesting that overcoming the strength of the plug in shallow conduit must occur prior to the initiation of an eruption. This finding has obvious implications for forecasting eruptions, and more importantly, for forecasting failed eruptions.;The comparison of multiplets at Bezymianny and Mount St. Helens suggests that the physical properties of magma in the shallow conduit controlled the occurrence and behavior of multiplets. When there is shallow crystallization and active degassing of the plug within the conduit, multiplets are abundant. Alternatively, when there is no shallow crystallization in a semi-open system, as at Bezymianny, multiplets were less abundant. At both volcanoes prior to an explosion, the multiplet percentage of total seismicity drops, and the average amplitude of each multiplet increases. Multiplet characteristics such as the multiplet percentage of total seismicity, multiplet durations, and the number of contemporaneous multiplets can be tracked prior to an eruption to forecast the type of eruption that will ensue.;The future volcanic activity at Mount St. Helens will likely diverge from the current explosive activity at Bezymianny. Without any additional input of mafic material or additional gas content, intermittent viscous dome building at Mount St. Helens will likely continue. In general, the high gas content of the magmas at Bezymianny compared to Mount St. Helens has driven more explosivity during eruptions despite the lower silica content and resultant lower viscosity magma at Bezymianny compared to Mount St. Helens. In future eruptions at Bezymianny and Mount St. Helens, the seismic energy release prior to eruptions will be dictated by the time since the last eruption. Stronger seismicity can be expected in cases where the conduit bends around existing dome.