Abstract
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 \~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
Vp 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 deg 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/Vs ratio region inferred by independent geophysical
and seismological studies. 10.1785/0120050207
Users
Please
log in to take part in the discussion (add own reviews or comments).