Abstract
There is an incontrovertible link between rainfall and secondary volcanic hazards-including lahar generation and slope failure events-at many volcanoes, and there is also mounting evidence of heavy rainfall constituting a trigger mechanism for explosive eruptive activity and other primary volcanic activity. While individual case studies of rainfall-induced volcanic hazard have typically relied on sparse rainfall gauge data, the proliferation of satellite-based rainfall detection systems means that such phenomena can now be investigated on a near-global scale. We extract and analyse multi-decadal rainfall timeseries from satellite data in order to assess whether the duration and timing of rainfall plays a role in modulating eruption frequency at different volcanoes. At several volcanoes, we observe that eruptions occur more frequently during the wettest times of the year, even after accounting for the relative length of the 'wet' season. Case-by-case probability analysis distinguishes more than 30 volcanoes where the eruption record appears to be strongly correlated with the wettest parts of the year, and we propose that precipitation may be responsible for previously observed seasonality of eruptive behavior. Examples include Karkar (Papua New Guinea), Lokon-Empung (Indonesia), Mount Saint Helens (USA), and Volcan de Fuego (Guatemala). Potential trigger mechanisms for rainfall-induced volcanism, the majority of which rely on infiltration of meteoric water from the Earth's surface, operate at a range of subsurface depths and are associated with time lags from hours to months (Figure 1). Infiltration-driven increase of pore pressure at depth can reduce the effective normal stress within the edifice, in turn decreasing fault strength and promoting brittle shear failure in the vicinity of the magma chamber. In a similar vein, increasing pore pressure in the edifice can decrease the failure threshold required for magma chamber rupture, thus facilitating dike intrusions and eruptions. We propose a number of volcanic systems that may benefit from further targeted study, and argue for widespread incorporation of rainfall information into future quantitative studies, volcano monitoring programs, and probabilistic hazard assessment.