IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

3A2_3F-O8Room A3

Date/Time: July 23 11:15-11:30

The application of statistical methods in determining volcanic eruption parameters fromtephra fall deposits
Rose E Burden1 , Li Chen2 , Jeremy C Phillips1 , Jonathan C Rougier2
1

School of Earth Sciences, University of Bristol, Bristol BS8 1RJ, United Kingdom, 2 School ofMathematics, University of Bristol, Bristol BS8 1TH, United Kingdom
E-mail: j.c.phillips@bristol.ac.ukAn important methodology in determining eruption parameters for historic explosive eruptions is inversion of theproperties of tephra fall deposits. We present a range of statistical approaches for determining the uncertainty
that is inherent in these methods, in particular focussing on the estimation of erupted volume and eruption column
height. We also use statistical tools to make a more general exploration of which areas of a tephra fall deposit
contain the most information about the overall deposit structure and the eruption that formed it.

Eruption volume is typically determined by extrapolating thinning trends found from isopach maps drawn for
the areas of the deposits that are preserved, a process that can be subjective and variable. We have developed analternative statistical method to objectively determine the volume of a fall deposit without the production of isopachmaps, in which integration of a log-linear regression model for thickness measurements with distance from the ventis applied to the field measurements without any prior interpretation, and data and model uncertainty is accounted
for using Bayesian methods. We find that eruption volumes calculated using our method correspond well to thosepreviously determined by alternative approaches for eruptions including Fogo A, Askja D, Pinatubo C1 and Santa
Maria 1902.

Eruption column height and windspeed during the eruption can be inferred from measurements of the maximum
clast size found at a given distance from the vent. We have used probabilistic, maximum likelihood techniques
to determine which of the methods typically used in the field to determine maximum clast size best reconstructs
known eruptive conditions. We have simulated the Grain Size Distribution (GSD) of fall deposits for a known
volumetric flow rates, and perform inversions on maximum clast sizes from these statistically-produced GSDs
at multiple locations to determine the volumetric flow rate and confidence interval. Comparison of modelledvolumetric flow rate with the ’known’ volumetric flow rate is then used to identify which method for determining
maximum clast size gives the most accurate interpretation of eruptive parameters. Confidence intervals for theaccuracy of each method will be presented.

The number and distribution of thickness and maximum clast measurements from fall deposits can limit theamount of information gained from the deposit. We have applied a fixed point variogram to field measurements toidentify the dependence of the structure of a deposit (referenced to vent location) to different spatial distributionsof measurements. Results suggest that measurements from medial and off-axis areas of the deposit are the mostinformative.

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