Relating Seismic Velocities, Permeability and Crack Damage in Interpreting the Mechanics of Active Volcanoes. <p> HR: <span class="hr"> 1330h<br></span> AN: <span class="an"> S22A-0440 <a href="../absbook/fm03-S22A.pdf">[PDF]</a><br></span> TI: <span class="ti"> Relating Seismic Velocities, Permeability and Crack Damage in Interpreting the Mechanics of Active Volcanoes. AU: <span class="au"> * Vinciguerra, S <br></span> EM: <span class="em"> sergio.vinciguerra@ct.infn.it, vinciguerra@ov.ingv.it<br></span> AF: <span class="af"> Osservatorio Vesuviano - Istituto Nazionale di Geofisica e Vulcanologia, Via Diocleziano 328, Naples, 80124 Italy <br></span> AU: <span class="au"> * Vinciguerra, S <br></span> EM: <span class="em"> sergio.vinciguerra@ct.infn.it, vinciguerra@ov.ingv.it<br></span> AF: <span class="af"> Dipartimento di Fisica e Astronomia, Universita di Catania, Via S. Sofia 64, Catania, 95123 Italy <br></span> AU: <span class="au"> Trovato, C <br></span> EM: <span class="em"> jonxat@tin.it<br></span> AF: <span class="af"> Dipartimento di Fisica e Astronomia, Universita di Catania, Via S. Sofia 64, Catania, 95123 Italy <br></span> AU: <span class="au"> Meredith, P G<br></span> EM: <span class="em"> p.meredith@ucl.ac.uk<br></span> AF: <span class="af"> Department of Earth Sciences, University College London, Gower Street, London, WC1E 6BT United Kingdom <br></span> AU: <span class="au"> Benson, P M<br></span> EM: <span class="em"> p.benson@ucl.ac.uk<br></span> AF: <span class="af"> Department of Earth Sciences, University College London, Gower Street, London, WC1E 6BT United Kingdom <br></span> AB: <span class="ab"> Improvements in volcano monitoring techniques have resulted in interpretative models of the physical and chemical changes associated with the renewal of activity of volcanic systems. However, accurate and meaningful interpretation of the relationship between the build-up of tectonic stress and precursory deformation requires a knowledge of the changing physical and mechanical properties of the relevant rocks. We report simultaneous laboratory measurements of seismic velocities and fluid permeability on basalts from Etna (Italy) and Iceland, and tuffs from Campi Flegrei (Italy).\\ Measurements were made in a servo controlled steady state flow permeameter at effective pressures from 5 to 80MPa, during both increasing and decreasing pressure cycles. Selected samples were thermally stressed at temperatures up to $900\deg$C to induce thermal crack damage Acoustic emission output was recorded throughout each thermal stressing experiment.\\ At low pressure, the P-wave velocity for the columnar Icelandic basalt was 5.4 km/s, while for the Etnean lava flow basalt it was only 2.5 km/s. On increasing the pressure to 80 MPa, the velocity of Etnean basalt increased by 80%, whereas that of Icelandic basalt increased by less than 10%. Furthermore, the velocity of Icelandic basalt thermally stressed to $900\deg$C fell by about 2.0 km/s, whereas the decrease for Etnean basalt was negligible. A similar pattern was observed in the permeability data. Permeability of Etnean basalt fell from about 750 microD to about 150 microD over the pressure range 5 to 80 MPa, while that for Icelandic basalt varied little from its initial low value of 9nD. Again, thermal stressing significantly increased the permeability of Icelandic basalt, whilst having a negligible effect on the Etnean basalt. These results clearly show that the Etna basalts contain much more crack damage than the Icelandic material. As a consequence, their seismic velocities are much lower than those generally assumed in tomographic studies of Etna (i.e. 5-6 km/s for the basalt layer).\\ Campi Flegrei tuff is a strongly heterogeneous pyroclastic flow material, which includes cavities, pumice and crystals. It has an initial porosity of around 45%, and a P wave velocities that increases from around 2 km/s to around 3 km/s over the pressure range 5 to 80 MPa. In marked contrast to both basalts, significant velocity hysteresis was observed in the tuff, with only about 50% of the velocity change recovered during depressurization. Significantly, the overwhelmingly bulk of the reduction in both porosity and permeability occurs over the first 15 MPa of applied effective pressure. This observation, together with the hysteresis, strongly suggests that the main mechanism of deformation is inelastic pore collapse. In a similar manner to the Etna basalt, Campi Flegrei tuff exhibited significant decrease in velocity after thermal stressing. However, in this case, the main decrease occurred over the temperature range $300\deg$ to $600\deg$C. This suggests that the most likely mechanism is thermal cracking induced by dehydration of the zeolite phases present.\\ These results clearly demonstrate the importance of understanding the details of specific rock physical properties, and how they change in response to pressure and temperature, in interpreting models derived from the results of field-scale monitoring of active volcanoes. <br></span> DE: <span class="de"> 5102 Acoustic properties<br></span> DE: <span class="de"> 5112 Microstructure<br></span> DE: <span class="de"> 5114 Permeability and porosity<br></span> DE: <span class="de"> 5134 Thermal properties<br></span> SC: <span class="sc"> Seismology [S]<br></span> MN: <span class="mn"> 2003 Fall Meeting<br></span></p></body></html>