The seismic crises at Mt. Vesuvius during 1995 and 1996

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.