Detailed analysis of wave propagation beneath the Campi Flegrei caldera, Italy

We investigate the complex propagation of seismic waves beneath the Campi Flegrei caldera, Italy, using multichannel recordings of artificial explosions. The sources consisted of air gun explosions shot in the Gulf of Pozzuoli at offsets ranging between 3 and 7 km. A multichannel recording device was deployed in the Solfatara crater and consisted of ten vertical-component and two three-component short-period seismometers with a maximum aperture of about 150 m. The zero-lag correlation (ZLC) technique was adopted to estimate horizontal slowness and backazimuth of coherent waves crossing the array. For sources located in the northern sector of the Gulf, with maximum offset 5 km, ray parameters and backazimuths are in agreement with those predicted for the 1D velocity model used for routine locations. For sources at offsets larger than similar to 5 km, the ZLC curves depict prominent maxima associated with a secondary phase propagating with a lower velocity than the first-arrival P wave. Using finite-difference synthetic seismograms generated for a 2D realistic velocity model, we explain these late arrivals in terms of a lateral velocity variation located at depths of about 1 km. Such discontinuity would correspond to a positive V-p anomaly imaged by a recent 3D tomographic study, and interpreted as the submerged southern rim of Campi Flegrei caldera collapsed during the explosive eruption of 12 ky B.P. The small spacing among adjacent shot points allowed simultaneous wave-field decomposition at the source and receiver arrays. Using a modified version of the double-beam method, we retrieve the independent variation of horizontal slowness at both the source and receiver regions. For both cases, we found azimuthal deviations as large as 50 degrees with respect to the great circle path. At the source region, these discrepancies may be interpreted in terms of ray bending at the interface of the aforementioned positive anomaly. At the receiver array, the observed anomalies may be attributed to either velocity variations marking the Solfatara crater rim, or to a near-receiver, low-velocity body whose position would coincide with negative gravimetric anomalies and a high VP/V, ratio region inferred by independent geophysical and seismological studies.