Accurate hindcasting of explosive eruptions at Whakaari, New Zealand

Phreatic eruptions are small, sudden events, commonly with few precursory signals. They are driven by interactions between magmatic and hydrothermal processes at shallow levels beneath the surface. Here we show that a sequence of banded tremor events, which occurred several weeks before the 9 December 2019 eruption of Whakaari (White Island), New Zealand, can be used to hindcast this eruption. The banded tremor sequence reveals a progressively decreasing time interval between tremor bands. Extrapolating the tremor bands to a time interval of zero provides an accurate estimate, at least one week prior to the eruption, to within 10.2 hours of when the eruption would occur, with a 2.8-day range between 95 % confidence intervals. A similar set of tremor signals appeared before the 27 April 2016 eruption, and these signals provide a very accurate hindcast of this eruption to within 2.61 hours, with a 2.2-day range between 95 % confidence intervals. Our analysis indicates that this potential forecasting approach may prove useful for successfully and accurately forecasting future eruptions at Whakaari. The approach also may be applicable to other volcanoes similar to Whakaari which experience sudden phreatic and phreatomagmatic eruptions.