Merging eruption datasets: building an integrated Holocene eruptive record for Mt Taranaki,New Zealand |
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| Authors: | Michael B Turner Mark S Bebbington Shane J Cronin Robert B Stewart |
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| Institution: | (1) Volcanic Risk Solutions, Massey University, Private Bag 11222, Palmerston North, New Zealand;(2) Institute of Fundamental Science—Statistics, Massey University, Private Bag 11222, Palmerston North, New Zealand |
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| Abstract: | Acquiring detailed eruption frequency datasets for a volcano system is essential for realistic eruption forecasts. However,
accurate datasets are inherently difficult to compile, even if one or more well-dated eruption records are available. A single
record typically under-represents the eruption frequency, while combining two or more records may result in an overrepresentation.
Although glass compositions have proven to be successful in tephrochronological studies of dominantly rhyolitic tephras; microlitic
growth and thin glass shards inhibit their application to andesitic tephras. A method consisting of a combination of two techniques
for correlating syn-eruptive deposits is demonstrated on data from the typical andesitic stratovolcano of Mt. Taranaki, New
Zealand. Firstly, tentative matches are identified using the radiocarbon age and associated error of each event. Secondly,
the compositions of titanomagnetite micro-phenocrysts are used as an independent check, and shown to be a useful correlation
tool where age data is available. Using two lake-core records containing tephra layers in an overlapping time-frame, the radiocarbon
age-correlation procedure suggested 31 tephra matches. Geochemistry data were available for 15 of these pairs. In three of
these cases, the titanomagnetite compositions did not match. Hence, these “paired” tephras were from compositionally distinct
magmas and therefore likely represent separate events. An additional three matches were reassigned within the temporal uncertainty
limits of the dating procedure, based on better geochemical pairing. The final combined dataset suggests that there have been
at least 138 separate ash fall-producing eruptions between 96 and 10 150 years B.P. from Taranaki. Using the combined dataset
the mixture of Weibulls renewal model forecasts a probability of 0.52 for an eruption occurring in the next 50 years at this
volcano. The present annual eruption probability is estimated at 1.6%. This likelihood is almost double that obtained when
relying on a single stratigraphic record. |
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