Article,
Understanding the seismic velocity structure of Campi Flegrei caldera (Italy): From the laboratory to the field scale
Pure and Applied Geophysics, 163 (10): 2205--2221 (Oct 14, 2006)
DOI: 10.1007/s00024-006-0118-y
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 600C 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.
