Probabilistic modelling of multi-phase eruptions found in geological records: An example from Mt. Ruapehu, New Zealand

Abstract

Abstract More than half of explosive volcanic eruptions contain multiple eruption phases with eruption dynamics (i.e., style, intensity, magnitude) varying with eruption progression. While probabilistic eruption forecast models constitute a useful tool for decision making during volcanic crises, the probabilistic forecast of likelihood, style and magnitude of potentially upcoming phase(s) within a multi-phase eruption progression remains a major challenge. Probabilistic eruption forecast models are crucially dependent on the quality of the underlying eruption records and their ability to account for inter-sequence and intra-sequence changes in eruption behaviour. Recent studies examining multi-phase eruption progression have been limited by the amount of historical data available, motivating the investigation into the feasibility of identifying and quantifying multi-phase eruption behaviour in the geological record. Mt. Ruapehu is one of New Zealand s most active volcanoes and its 1800 years-present Tufa Trig Formation comprises at least thirty-one small to moderate explosive eruptions, of which more than half contain more than one phase. An interpolated age model for the past 1800 years is presented and used to model inter-sequence eruption onsets. Four volcanic regimes, distinguished by eruption frequency and magnitude can be identified: (1) A low rate regime with an average of one eruption every 125 years, involving mostly single-phase low to mid-intensity eruptions, (2) two medium rate regimes with a 40 year average recurrence interval and (3) a high frequency regime characterised by decadal-scale reposes, and including the largest multi-phase eruptions of the past 1800 years. Tephra volume estimates are combined with a statistical model for the number of phases to characterise the intra-sequence progression. Eruption progression is consistent with existing multi-phase models developed on historical data, and suggests that tephra volume increases in later phases of an eruption.