RECENT GEOPHYSICAL INVESTIGATION AT SOMMA-VESUVIO VOLCANIC COMPLEX

Activity at Somma-Vesuvio volcanic area in southern Italy is monitored by seismic stations and periodic geodetic and gravity surveys. The se ismic network, which consists at present of four vertical stations and one three-component station, recorded an increase in earthquake activ ity in 1978 and between November 1988 and March 1989. During the later activity, earthquakes were located in a cluster about 3 km beneath th e summit of the volcano. Two tide gauges, two tiltmeters and a recordi ng gravimeter are also operating at Somma-Vesuvio. Yearly levelling su rveys are conducted along several closed routes that extend from as mu ch as 6 km from the base of the volcano to the summit area. Survey res ults reveal no significant ground movement since 1959, except for a sl ight subsidence around the rim of the summit crater. Gravity changes h ave been larger than the expected 10 mu Gal uncertainty of the measure ments. The lack of contemporary elevation changes implies that the obs erved gravity changes are the result of a slight change in density str ucture. The cone of Somma-Vesuvio has been very stable for the last fe w decades, showing no indications of a buildup to activity. The lack o f surface movement should rule out a magma-supply rate to this volcano at the historic eruptive rate of 0.002 km(3)/yr.