European Seismological Commission, 32nd General Assembly, Sept. 6-10, 2010, Montpellier, France
N. Maercklin, M. Vassallo, G. Festa, A. Zollo, D. Dello Iacono, J. Virieux
Campi Flegrei (Phlegraean Fields) is an active, resurgent volcanic caldera that is located a few kilometres west of the city of Naples, a densely populated urban settlement in southern Italy. To image the subsurface structure of the caldera, an extensive marine seismic survey was carried out in the area in 2001 (SERAPIS experiment). Previous results from this survey include smooth 3-D velocity models for the upper 4-5 km from first-arrival tomography, and a 1-D layered model from PP and PS reflection travel times and amplitudes. The layered model shows three dominant reflectors, interpreted as the base of marine unconsolidated sediments, the top of a gas-bearing rock formation around 3 km depth, and as the top of a magma layer with high melt fraction at about 7.5 km depth.
Here we present results on the 3-D morphology of these reflectors, obtained by tomographic inversion of reflection travel times. The two shallowest reflectors are well-constrained by data and show maximum depth variations of 150 m and 300 m, respectively. The reflector around 3 km depth has a basin-like shape with a morphological high coinciding with the buried caldera rim, as previously imaged as a high-velocity anomaly. The deepest reflector is less well resolved, and it appears rather smooth with a small maximum in the western part of the submarine caldera boundary.
Rock physics modelling helps to relate the seismic velocities to lithological properties and supports our previous interpretation of the main structural discontinuities. The dominant lithological units listed above essentially extend through the entire imaged volume beneath the caldera. We discuss lateral velocity variations both in terms of their significance based on model resolution, and regarding their implications on lithology. While petrological data support the presence of melt below 7.5 km, there are no indications for larger melt reservoirs at shallower depths within the resolved crustal volume. Kinematic seismic imaging leading to the currently available velocity models is not able to resolve structures less than 1 km in diameter, such as small magma patches, which have been suggested by petrologists.