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

3W_4C-P12

Date/Time: July 23 Poster

Seismic Precursors to Eruptions at Volcanoes in Extensional Stress Fields
Richard Wall1 , Christopher Kilburn1 , Philip Meredith2 , Alfonso Brancato3 , Giovanni Distefano3 , Stefano
Gresta4 , Salvatore Alparone5 , Andrea Ursino51

Aon Benfield UCL Hazard Centre, Department of Earth Sciences, University College London, WC1E
6BT, United Kingdom, 2 Rock and Ice Physics Laboratory, Department of Earth Sciences, University
College London, WC1E 6BT, United Kingdom, 3 Dipartimento di Scienze Geologiche, Universita diCatania, Corso Italia 55, 95129 Catania, Italy, 4 Istituto Nazionale di Geofisica e Vulcanologia, Via diVigna Murata, 605 - 00143 Roma, Italy, 5 Istituto Nazionale di Geofisica e Vulcanologia âĂŞ
Osservatorio Etneo, Piazza Roma 2, 95123 Catania, Italy
E-mail: richard.wall.09@ucl.ac.uk

Volcano-tectonic (VT) seismicity is one of the most common precursors detected before volcanic eruptions.
Previous studies have focused on quantitative analysis of precursory VT trends at volcanoes in compressional
background stress fields, such as subduction zones. Using data from Mt Etna, in Sicily, we here extend analysesof VT behaviour to volcanoes in extensional background stress fields.

Before andesitic-dacitic eruptions at subduction-zone volcanoes, accelerations in VT event rate appear toevolve from early exponential trends, that may develop over several months, to a change that is faster thanexponential (FTE) with time. The FTE trend tends to follow a hyperbolic increase (similar to an inverse-Omoritrend) and can develop 10-14 days before eruption. In comparison, analysis of Etna´s flank eruptions of alkali
basalt between 1977 and 2008 show that accelerations in precursory VT event rate preferentially change frominitial exponential increases, over intervals of weeks, to FTE trends less than two days before eruption. FTE trends
are thus strong indicators that an eruption may be imminent. However, although they may potentially provide
warning times of days at subduction-zone volcanoes, analogous times are reduced to hours on Etna.

From their common occurrence, we propose that the exponential trends are related to an increase in theamount of damage around a volcano´s feeding system, caused by increases in magmatic pressure or by a local
increase in tectonic extension. We further relate the abrupt nature of Etna´s FTE trend to the tensile propagationof a magma-filled fracture from a magma body to the surface. Propagation occurs when the damage accumulatedaround the magma body exceeds a critical value. The VT sequences, however, do not always culminate in aneruption. In such cases, fracture propagation must be halted underground by local stress barriers or by a decreasein the pressure gradient driving magma along the fracture. Even with a comprehensive model of precursory
fracturing, therefore, eruption forecasts will be associated with an uncertainty due at least to a lack of knowledgeof material heterogeneities and of local stress distributions in the crust along the path of a propagating fracture.

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