Rock fracture as a precursor to lava dome eruptions at Mount St Helens from June 1980 to October 1986 |
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Authors: | R Smith C R J Kilburn P R Sammonds |
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Institution: | (1) Benfield UCL Hazard Research Centre, Department of Earth Sciences, University College London, Gower Street, London, WC1E 6BT, UK;(2) Mineral, Ice and Rock Physics Laboratory, Department of Earth Sciences, University College London, Gower Street, London, WC1E 6BT, UK |
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Abstract: | Following its plinian eruption on 18 May 1980, Mount St Helens (Washington State, USA) entered a period of intermittent lava-dome
extrusion until 1986. Renewed extrusion was frequently preceded by accelerating rates of seismicity, with more precursory
seismicity observed prior to eruptions later in the sequence. Here the failure forecasting method (FFM) is used to investigate
changes in the observed rate of volcano–tectonic (VT) seismicity. The analysis indicates that: (1) all VT crises resulted
in an eruption within 3 weeks (usually less than 10 days), (2) the majority of eruptions had VT precursors, and (3) patterns
of precursory seismicity showed fluctuations about the ideal model trend. Thus, although these seismic events could be used
to warn of an impending eruption, specific forecasts were subject to an uncertainty of weeks or more. It is proposed that:
(1) increased seismicity prior to later eruptions is a result of a larger and more solidified dome acting as a greater impediment
to magma ascent; (2) the consistency of seismic swarms resulting in an eruption indicates that stresses high enough to initiate
fracturing in the country rock and lava dome carapace were only achieved once the approach to an eruption had already begun;
and (3) discrepancies between models of accelerating rock fracture and the observed seismicity may arise due to a significant
amount of the rocks deforming through ductile mechanisms rather than seismogenic fracture. |
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Keywords: | Seismicity Volcano– tectonic Mount St Helens Failure forecasting method Rock fracture Precursor Lava dome |
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