Forecasting sizes and repose times of future extreme volcanic events

By using rank-order statistics, it is possible to predict the likely scale of future “extreme” volcanic events (eruptions larger than a given threshold size, or repose periods longer than a given time) on both a local and global scale. When ranked by diminishing size, the largest and most devastating volcanic eruptions in terms of volume, or consequent fatalities, can be described by power-law functions. In turn, this approach permits projection of, for example, (1) the likely size of the next volcanic eruption larger than the A.D. 1815 eruption of Tambora (∼90 km³ of magma); (2) the size of the next eruption of the Taupo volcano, New Zealand, that is larger than the eruption of A.D. 186 (∼82 km³ of tephra); and (3) the likely duration of the current repose period of the Taupo volcano (4000 ± 1200 years). This approach can also be employed on shorter time scales, for example to provide quantitative criteria to assess when a period of activity has ceased. With the establishment of procedures for making projections such as these, volcano hazard assessment may be placed onto a more rigorous and quantitative basis.