Some Challenges in Modelling Volcanic Occurrence Datambebbing/talks/volcanoes.pdf · Completeness...
Transcript of Some Challenges in Modelling Volcanic Occurrence Datambebbing/talks/volcanoes.pdf · Completeness...
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Some Challenges in Modelling Volcanic Occurrence Data
Mark BebbingtonIIST & Volcanic Risk Solutions, Massey University
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What is an Eruption?• Arrival of solid volcanic products at surface• Events ranging from an individual explosion to
eruptive periods lasting centuries– Includes quiet periods of up to 3 months– Often `stop’ dates go unrecorded.
• `Historical’ eruptions– Presence of observers, monitoring, weather
• `Pre-historical’ eruptions– Dated, radio-carbon (to 50ka, with error ~ 25 to 1000
yr), K-Ar dating (from 50ka, with error ~ 10 to 100 ka).• VEI (Volcanic Explosivity Index)
– a logarithmic `size’ assigned to historical eruptions on the basis of: explosion size; volume; column height; classification; duration; …
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CompletenessGlobally, the observance probability rises from 10% in 1500 to 100% in 1980 (assumed).
BUT
– some volcanoes are much better observed
- Big eruptions are much better observed
- what is an eruption?
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Data: The good,
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Etna flank eruptions: 1970 –Stationary behaviour?
Are time and size of next eruption `predictable’?
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Etna: Flank eruptions 1600-Non-stationary?
60 flank eruptions- “known” onset
dates, duration, and volume
- Complete since ~1600 AD
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Etna: Summit + Flank 1970-Independence?
12 summit eruptions, onset dates, durations and volumes, complete since 1970?
Any relation to the flank eruptions?
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Data: The Bad,
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A prediction?- The past behaviour of a volcano is the best predictor of its future activity.
“The repetitive nature of the eruptive activity at Mt St Helens during the last 4000 years, with dormant intervals typically of a few centuries or less, suggests that the current quiet period will not last a 1000 years. Instead, an eruption is likely within the next hundred years, possibly before the end of this century”
- Crandell et al., “Mt St Helens volcano: Recent and future behavior”. Science 187:438-441, 1975
- Mt St Helens erupted in 1980, having been quiescent since 1857
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Data: The Ugly
VEI > 3
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Vesuvius
Cycles initiated by (sub-) plinian event and terminated by repose of several centuries?
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Periodicities
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20151050
50
40
30
20
10
0
i
tau
_i
S 14.0130R-Sq 22.0%R-Sq(adj) 11.6%
Fitted Line Plottau_i = 30.32 - 5.096 i + 0.2796 i**2
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Radiocarbon Dating• Volcanic products cannot be directly
carbon dated – they were never alive.• Date organic material in (or more often just
under) an eruptive layer. • Dates are given as
N(mu,sigma)• Conversion to
calendar years • Stratigraphic
ordering
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Taveuni, Fiji
101 eruptive episodes, individually datedBUT - considerable “rounding”, large error bars
rift volcano – only one spatial dimension
120001000080006000400020000
100
80
60
40
20
0
Years BP
Co
un
t
meanup_bndlow_bnd
Variable
Eruption dates
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Taranaki 103 tephras in single core –depths on all, 10 dated individually
Stars = core dates, lines 95% confidence limits
`DODGY’DATE
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Taranaki II – 2 cores plus near source
Umutekai104 tephras, c. 1550BP –10100BP
Rotokare45 tephras, c.500BP – 6200BP
Near-source23 tephras, c.90BP – 2200BP
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Related cycles of magma-feeding pulses, large eruptions and
eruption frequencyTaranaki
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Yucca Mountain40 eruptions in last 10.5Ma. Last 80ka ago.Large error bars, and multiple eruptions with the same estimated age.Widely varying sizes- Probability of disruption of repository in 10ka?
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Auckland
49 cones in last 250ka, last two much bigger, especially last (Rangitoto) – 59% of total volume.
Is there any pattern?