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| Titel |
Forecasting volcanic eruptions: the control of elastic-brittle deformation |
| VerfasserIn |
Christopher Kilburn, Robert Robertson, Richard Wall, Alexander Steele |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250125424
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| Publikation (Nr.) |
 EGU/EGU2016-5005.pdf |
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| Zusammenfassung |
| At volcanoes reawakening after long repose, patterns of unrest normally reflect the
elastic-brittle deformation of crust above a magma reservoir. Local fault movements, detected
as volcano-tectonic (VT) earthquakes, increase in number with surface deformation, at first
approximately exponentially and then linearly. The trends describe how crustal behaviour
evolves from quasi-elastic deformation under an increasing stress to inelastic deformation
under a constant stress. They have been quantified and verified against experiments for
deformation in compression [1]. We have extended the analysis to extensional deformation.
The results agree well with field data for crust being stretched by a pressurizing magmatic
system [2]. They also provide new criteria for enhancing the definitions of alert levels and
preferred times to eruption.
The VT-deformation sequence is a field proxy for changes in deformation with applied
stress. The transition from quasi-elastic to inelastic behaviour is characterised in extension by
the ratio of differential failure stress SF to tensile strength σT. Unrest data from at least
basaltic to andesitic stratovolcanoes, as well as large calderas, yield preferred values for
SF/σT ≤ 4, coinciding with the range for tensile failure expected from established
theoretical constraints (from Mohr-Coulomb-Griffiths failure). We thus associate the
transition with the approach to tensile rupture at the wall of a pressurized magma
reservoir. In particular, values of about 2 are consistent with the rupture of a cylindrical
reservoir, such as a closed conduit within a volcanic edifice, whereas values of
about 3 suggest an approximately spherical reservoir, such as may exist at deeper
levels.
The onset of inelastic behaviour reflects the emergence of self-accelerating crack growth
under a constant stress. Applied to forecasting eruptions, it provides a new and objective
criterion for raising alert levels during an emergency; it yields the classic linear
decrease in inverse-rate with time for VT seismicity, which can be extrapolated to an
expected eruption time shortly after the inverse rate becomes zero [3]; and, for
extension, it identifies preferred inverse-rate gradients of 0.001-0.01, which can
be used to distinguish between physically-meaningful and spurious inverse-rate
trends.
[1] Kilburn CRJ (2012) J Geophys Res, doi: 10.1029/2011JB008703; [2] Robertson R,
Kilburn CRJ (2016) Earth Planet Sci Lett, doi: 10.1016/j.epsl.2016.01.003; [3] Voight B
(1988) Nature. 332: 125-130. |
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