Аннотация
Seismograms from an active seismic experiment carried out at Campi
Flegrei caldera (near Naples, Italy) show a large-amplitude SV-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.
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