The seismicity which affects Mt. Vesuvius is, at present, the only clear in
dicator of the volcano dynamics. In the last years, two periods of increase
d seismic activity occurred (August-October 1995 and March-May 1996). This
seismicity was detected by the 10 analog stations of the Permanent Seismic
Network as well as by up to 7 three-component temporary digital stations. A
total number of about 600 events have been recorded, four of which showing
magnitude M greater than or equal to 3.0. The maximum magnitude earthquake
(M=3.4) was the strongest in the last fifty years and occurred on 25 April
1996. The use of three-component seismometers allowed us to obtain very re
liable hypocentral locations. The focal volume of the two seismic crises do
es not exceed 5-6 km of depth below the crater area. Fault plane solutions
of the most energetic events show focal planes oriented NW-SE and NE-SW, in
agreement with the regional tectonic features, indicating that at present
the seismicity of Mt. Vesuvius develops along pre-existing discontinuities.
In addition, the occurrence of a fluid-driven source mechanism suggests a
role played by the underground water on the seismic energy release. Shear w
ave splitting analyses confirmed the presence of an anisotropic volume rela
ted to a distribution of cracks and/or fractures parallely aligned to the m
ain faults system of the volcano. (C) 1999 Elsevier Science Ltd. All rights
reserved.