Understanding the Seismic Velocity Structure of Campi Flegrei Caldera (Italy): From the Laboratory to the Field Scale |
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Authors: | S Vinciguerra C Trovato PG Meredith PM Benson C Troise G De Natale |
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Institution: | (1) HP-HT Laboratory Istituto Nazionale di Geofisica e Vulcanologia, sezione di Roma 1, Via di Vigna Murata 605, 00143 Rome, Italy;(2) Istituto Nazionale di Geofisica e Vulcanologia, sezione di Napoli, Via Diocleziano 328, 80124 Naples, Italy;(3) Department of Earth Sciences, University College London, Gower Street, London WC1E6BT London, United Kingdom |
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Abstract: | We report laboratory measurements of P- and S-wave velocities on samples of tuff from Campi Flegrei (Italy), and a new tomographic
velocity map of the Campi Flegrei caldera. Laboratory measurements were made in a hydrostatic pressure vessel during both
increasing and decreasing effective pressure cycles. Selected samples were also thermally stressed at temperatures up to 600°C
to induce thermal crack damage. Acoustic emission output was recorded throughout each thermal stressing experiment, and velocities
were measured after thermal stressing. Laboratory P- and S-wave velocities are initially low for the tuff, which has an initial
porosity of ~45%, but both increase by between 25 and 50% over the effective pressure range of 5 to 80 MPa, corresponding
to a decrease of porosity of ~70%. Marked velocity hysteresis, due to inelastic damage processes, is also observed in samples
subjected to a pressurization-depressurization cycle. Tomographic seismic velocity distributions obtained from field recordings
are in general agreement with the laboratory measurements. Integration of the laboratory ultrasonic and seismic tomography
data indicates that the tuffs of the Campi Flegrei caldera can be water or gas saturated, and shows that inelastic pore collapse
and cracking produced by mechanical and thermal stress can significantly change the velocity properties of Campi Flegrei tuffs
at depth. These changes need to be taken into account in accurately interpreting the crustal structure from tomographic data. |
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Keywords: | Laboratory P- and S-wave velocities tomographic velocity map mechanical and thermal stresses |
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