Assessing the likelihood and magnitude of volcanic explosions based on seismic quiescence

Volcanic eruptions are generally forecast based on strong increases in monitoring parameters such as seismicity or gas emissions above a relatively low background level (e.g., Voight, 1988; Sparks, 2003). Because of this, forecasting individual explosions during an ongoing eruption, or at persistently restless volcanoes, is difficult as seismicity, gas emissions, and other indicators of unrest are already in a heightened state. Therefore, identification of short-term precursors to individual explosions at volcanoes already in heightened states of unrest, and an understanding of explosion trigger mechanisms, is important for the reduction of volcanic risk worldwide. Seismic and visual observations at Telica Volcano, Nicaragua, demonstrate that a) episodes of seismic quiescence reliably preceded explosions during an eruption in May 2011 and b) the duration of precursory quiescence and the energy released in the ensuing explosion were strongly correlated. Precursory seismic quiescence is interpreted as the result of sealing of shallow gas pathways, leading to pressure accumulation and eventual catastrophic failure of the system, culminating in an explosion. Longer periods of sealing and pressurization lead to greater energy release in the ensuing explosion. Near-real-time observations of seismic quiescence at restless or erupting volcanoes can thus be useful for both timely eruption warnings and for forecasting the energy of impending explosions.