HR: 0800h
AN: V51D-0778    [Abstracts]
TI: The Evolution of Elastic Moduli With Increasing Crack Damage During Cyclic Stressing of Etna Basalt
AU: Heap, M J
EM: m.heap@ucl.ac.uk
AF: Mineral, Ice and Rock Physics Laboratories, Department of Earth Sciences, University College London, Gower Street, London, WC1E 6BT, United Kingdom
AU: * Meredith, P G
AF: Mineral, Ice and Rock Physics Laboratories, Department of Earth Sciences, University College London, Gower Street, London, WC1E 6BT, United Kingdom
AU: Vinciguerra, S
AF: Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Roma,Via di Vigna Murata 605, Rome, 00143, Italy
AU: Boon, S A
AF: Mineral, Ice and Rock Physics Laboratories, Department of Earth Sciences, University College London, Gower Street, London, WC1E 6BT, United Kingdom
AB: Volcanic edifices, such as Mt. Etna volcano, are commonly subject to cycles of pressurization and depressurization over extended periods of time due to repeated episodes of magma emplacement from deep reservoirs to shallow depths. Such repeated episodes of deformation can lead to an increase in the level of crack damage within the rocks of the edifice, and hence changes in their elastic properties. Importantly, a number of volcano monitoring techniques, such as seismic tomography and ground deformation modeling, rely on accurate knowledge of elastic properties. However, the effect of cyclic stressing on mechanical and elastic properties of volcanic rock remains unclear. To this end, we report results of changes in elastic moduli from stress-cycling experiments on samples of extrusive basalt from Mount Etna, Italy. The basalt contains an extensive pre-existing network of isotropic, interconnected microcracks caused by cooling. Both oven-dry and water-saturated samples were initially loaded to 20 MPa at a constant rate and then unloaded to 8 MPa. Samples were then sequentially reloaded and unloaded at the same rate with the peak stress in each subsequent cycle increased by 10 MPa. Stress-cycling was continued until each sample failed. Results from oven-dry samples showed a gradual reduction in sample stiffness with each increasing stress cycle that resulted in a total decrease in Young's modulus of approximately 30% and an increase in Poisson's ratio of approximately 60%. Results from water-saturated samples showed an almost identical trend. These changes in moduli are attributed to the growth of new cracks in each stress cycle and, hence, an increase in the total crack density. This is supported by the observation of increased acoustic emission (micro-seismic) output in each cycle. We also observed the Kaiser stress-memory effect, where acoustic emission on each cycle only occurs when the maximum stress in the previous cycle has been exceeded. During the deformation history of volcanic edifices, however, the stress in each pressurization cycle may not always exceed that of the previous cycle. In order to better understand this more realistic situation, we also report results from cyclic stressing experiments where the peak stress in each cycle has been randomly selected to be either higher or lower than that of the previous cycle. In this case we observed a more complex manifestation of the Kaiser effect, where AE output in any cycle was only observed when the stress in that cycle exceeded the maximum stress on any previous cycle.
DE: 3630 Experimental mineralogy and petrology
DE: 5102 Acoustic properties
DE: 8419 Volcano monitoring (7280)
DE: 8488 Volcanic hazards and risks
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting