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Constraints on magma ascent to eruption from patterns of unrest

Christopher Kilburn 1,3, Deni Gowen1,3 & Carmen Solana2,3

  • Affiliations: 1UCL Hazard Centre, Department of Earth Sciences, UCL, UK; 2School of the Environment and Life Sciences, University of Portsmouth, Portsmouth, UK; 3Project FEVER, UK Research and Innovation. 

  • Presentation type: Poster

  • Presentation time: Monday 16:30 - 18:30, Room Poster Hall

  • Poster Board Number: 239

  • Programme No: 2.4.24

  • Theme 2 > Session 4

Abstract

Episodes of unrest at subduction-zone volcanoes show repeatable variations with time in rates of volcano-tectonic (VT) seismicity. Two common sequences are distinguished by whether they culminate in an increasing or decreasing VT event rate. Both have been illustrated at Soufriere Hills on Montserrat.  Before its eruption in 1995, after three centuries in repose, VT rates changed from an exponential increase to a constant value and then to a hyperbolic increase. In contrast, during a non-eruptive VT sequence in 1966, the opening exponential and constant rates were followed by an inverse-time decay. The hyperbolic increase is consistent with the growth and linkage of fractures to allow magma ascent. It is thus tempting to infer that a final decay indicates conditions unfavourable to eruption. Such an inference, however, would be an over-generalisation. At Mount St Helens (USA) in 1980 and at Agung (Indonesia) in 2017, eruptions occurred at the end of declining VT event rates. Independent studies suggest that the Mount St Helens eruption was promoted by the collapse of its northern flank under the weight of shallow injections of magma, whereas the Agung eruption followed new magma entering previously opened fractures.  Apparently, therefore, the initial exponential and constant-rate trends coincide with fractures being opened wide enough to allow magma to reach shallow depths, but not to erupt without an additional factor coming into play (e.g., fracture linkage, edifice instability or injection of new magma). Incorporating such factors is thus important for enhancing forecasts and scenario planning during an emergency.