Three decades of seismic activity at Mt. Vesuvius: 1972-2000

We analyse the seismic catalogue of the local earthquakes which occurred at Somma-Vesuvius volcano in the past three decades (1972–2000). The seismicity in this period can be described ascomposed of a background level, characterised by a low and rather uniform rate of energy release and bysporadic periods of increased seismic activity. Such relatively intense seismicity periods are characterised byenergy rates and magnitudes progressively increasing in the critical periods. The analysis of the b value in thewhole period evidences a well-defined pattern, with values of b progressively decreasing, from about 1.8 at thebeginning of the considered period, to about 1.0 at present. This steady variation indicates an increasingdynamics in the volcanic system. Within this general trend it is possible to identify a substructure in the timesequence of the seismic events, formed by the alternating episodes of quiescence and activity. The analysis ofthe source moment tensor of the largest earthquakes shows that the processes at the seismic source aregenerally not consistent with simple double-couples, but that they are compatible with isotropic components,mostly indicating volumetric expansion. These components are shown to be statistically significant for mostof the analysed events. Such focal mechanisms can be interpreted as the effect of explosion phenomena,possibly related to volatile exsolution from the crystallising magma.The availability of a reduced amount of high quality data necessary for the inversion of the sourcemoment tensor, the still limited period of systematic observation of Vesuvius micro-earthquakes and, aboveall, the absence of eruptive events during such interval of time, cannot obviously permit the outlining of anyformal premonitory signal. Nevertheless, the analysis reported in this paper indicates a progressivelyevolving dynamics, characterised by a generally increasing trend in the seismic activity in the volcanic systemand by a significant volumetric component of recent major events, thus posing serious concern for a futureevolution towards eruptive activity.