The 1980 pressure response and flank failure of Mount St. Helens (USA) inferred from seismic scaling exponents

S. Vinciguerra, D. Elsworth, S. Malone

Research output: Contribution to journalArticle

10 Citations (Scopus)

Abstract

The cataclysmic 18 May 1980 eruption at Mount St. Helens was preceded by intense seismic activity marking the mechanical response of the volcanic edifice to interior pressurisation. This seismicity is analysed to yield the temporal change in the seismic scaling exponent, D, inferred from the seismic b-value, that in-turn is related to the seismic moment of an earthquake. Time evolution of D preceding the eruption onset reveals: (1) a major decrease in D occurring over only a few days at the end of March; (2) a steady but stepped decrease in D (steps ∼5-10 days) occurring from the end of March to early May; (3) a sharp decrease in D in early May; and (4) steady low values of D occurring 2-3 days before the eruption onset. This response is interpreted as major ruptures, formed at the end of March, arresting and participating in, but not triggering the ultimate failure of the flank. Rather, the rate of interior fracturing slowed in the 2 months preceding the 18 May 1980 major blast, and the triggering of failure is consistent with interior gas overpressurisation. The occurrence of two swarms of low frequency seismic events and the high values of the harmonic tremor indicate the action of interior pressurisation on a cycle of ∼20-25 days. Solutions are applied to represent the harmonic interior pressurisation of the edifice by gas exsolving from the volcano core. The transient radial migration of overpressured gas may reduce the apparent strength of the edifice, and ultimately trigger failure of the flank. Importantly, this mechanism is capable of triggering flank failure both after multiple core pressurisation cycles have been sustained, and as core pressures are low and diminishing - and may be a minimum. These twin attributes are both apparent in the seismic record for Mount St. Helens, used as a proxy for the unrecorded timing and magnitude of gas pressurisation at the volcano core.

Original languageEnglish (US)
Pages (from-to)155-168
Number of pages14
JournalJournal of Volcanology and Geothermal Research
Volume144
Issue number1-4 SPEC. ISS.
DOIs
StatePublished - Jun 15 2005

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Pressurization
exponents
volcanic eruptions
scaling
Gases
volcanic eruption
gases
gas
Volcanoes
volcanoes
volcano
harmonics
tremors
cycles
fracturing
seismic moment
blasts
marking
low pressure
seismicity

All Science Journal Classification (ASJC) codes

  • Geophysics
  • Geochemistry and Petrology

Cite this

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abstract = "The cataclysmic 18 May 1980 eruption at Mount St. Helens was preceded by intense seismic activity marking the mechanical response of the volcanic edifice to interior pressurisation. This seismicity is analysed to yield the temporal change in the seismic scaling exponent, D, inferred from the seismic b-value, that in-turn is related to the seismic moment of an earthquake. Time evolution of D preceding the eruption onset reveals: (1) a major decrease in D occurring over only a few days at the end of March; (2) a steady but stepped decrease in D (steps ∼5-10 days) occurring from the end of March to early May; (3) a sharp decrease in D in early May; and (4) steady low values of D occurring 2-3 days before the eruption onset. This response is interpreted as major ruptures, formed at the end of March, arresting and participating in, but not triggering the ultimate failure of the flank. Rather, the rate of interior fracturing slowed in the 2 months preceding the 18 May 1980 major blast, and the triggering of failure is consistent with interior gas overpressurisation. The occurrence of two swarms of low frequency seismic events and the high values of the harmonic tremor indicate the action of interior pressurisation on a cycle of ∼20-25 days. Solutions are applied to represent the harmonic interior pressurisation of the edifice by gas exsolving from the volcano core. The transient radial migration of overpressured gas may reduce the apparent strength of the edifice, and ultimately trigger failure of the flank. Importantly, this mechanism is capable of triggering flank failure both after multiple core pressurisation cycles have been sustained, and as core pressures are low and diminishing - and may be a minimum. These twin attributes are both apparent in the seismic record for Mount St. Helens, used as a proxy for the unrecorded timing and magnitude of gas pressurisation at the volcano core.",
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The 1980 pressure response and flank failure of Mount St. Helens (USA) inferred from seismic scaling exponents. / Vinciguerra, S.; Elsworth, D.; Malone, S.

In: Journal of Volcanology and Geothermal Research, Vol. 144, No. 1-4 SPEC. ISS., 15.06.2005, p. 155-168.

Research output: Contribution to journalArticle

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