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P‐SV conversions at a shallow boundary beneath Campi Flegrei Caldera (Italy): Evidence for the magma chamber

1992; American Geophysical Union; Volume: 97; Issue: B11 Linguagem: Inglês

10.1029/92jb00888

2156-2202

F. Ferrucci, A. Hirn, Giuseppe De Natale, J. Virieux, L. Mirabile,

earthquake and tectonic studies

Seismograms from an active seismic experiment carried out at Campi Flegrei caldera (near Naples, Italy) show a large‐amplitude 5V‐polarized shear wave, following by less than 1.5‐s P waves reflected at wide angle from a deep crustal interface. Early arriving SV ‐polarized waves, with the same delay to direct P waves, are also observed in seismograms from a regional, 280‐km‐deep, magnitude 5.1 earthquake. Such short delays of S to P waves are consistent with a P‐SV conversion on transmission occurring at a shallow boundary beneath the receivers. The large amplitude of the converted‐ SV phase, along with that the P waves are near vertical, requires a boundary separating a very low rigidity layer from the upper caldera fill. The converted phases are interpreted as a seismic marker of a magma chamber. The top of this magma chamber is located slightly deeper than the deepest earthquakes observed during the 1982–1984 unrest of Campi Flegrei.