Three Decades of Seismic Activity at Mt. Vesuvius: 1972?2000

Abstract

We analyse the seismic catalogue of the local earthquakes which occurred at SommaVesuvius volcano in the past three decades . The seismicity in this period can be described as composed of a background level, characterised by a low and rather uniform rate of energy release and by sporadic periods of increased seismic activity. Such relatively intense seismicity periods are characterised by energy rates and magnitudes progressively increasing in the critical periods. The analysis of the b value in the whole period evidences a well-defined pattern, with values of b progressively decreasing, from about 1.8 at the beginning of the considered period, to about 1.0 at present. This steady variation indicates an increasing dynamics in the volcanic system. Within this general trend it is possible to identify a substructure in the time sequence of the seismic events, formed by the alternating episodes of quiescence and activity. The analysis of the source moment tensor of the largest earthquakes shows that the processes at the seismic source are generally 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 most of 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 source moment tensor, the still limited period of systematic observation of Vesuvius micro-earthquakes and, above all, the absence of eruptive events during such interval of time, cannot obviously permit the outlining of any formal premonitory signal. Nevertheless, the analysis reported in this paper indicates a progressively evolving dynamics, characterised by a generally increasing trend in the seismic activity in the volcanic system and by a significant volumetric component of recent major events, thus posing serious concern for a future evolution towards eruptive activity.

Seismological Monitoring of Mount Vesuvius (Italy): More than a Century of Observations

“…From the 1970s on, the data indicate a seismicity of the volcano-tectonic type, which consists of some hundreds of low-magnitude events per year. This seismicity has been studied by different authors (De Natale et al 2000;De Natale et al 2004;Vilardo et al 1996;Zollo et al 2002a;Zollo et al 2002b). De Natale et al (2000 interpreted it as the result of the stress perturbations concentrated around the high-rigidity anomaly in the axial part of the volcano, due to the effects of the gravitational loading.…”

Modeling of the thermal state of Mount Vesuvius from 1631 A.D. to present and the role of CO2 degassing on the volcanic conduit closure after the 1944 A.D. eruption

“…One could argue that liquidus and solidus temperatures rise as a consequence of water degassing [ De Natale et al , 2004b] due to the continuous diffusion of carbon dioxide in the melt column, thus allowing the magma to crystallize and behave as a brittle, seismogenetic structure. Upon magma dehydration, only a small decrease of temperature (20–30°K below the liquidus) is required to produce an important crystallization wave [ De Natale et al , 2004]. …”

“…and high V P /V S conversion, V S being the S wave velocity) centered along the volcanic vertical axis [e.g., De Natale et al , 2004a, 2004b; Scarpa et al , 2002; Zollo et al , 1996], it has been hypothesized the presence of a high‐rigidity body, centered beneath the crater, extending down to about 5 km of depth bsl and with a lateral extension of less than 1 km. The high‐rigidity zone is endowed with high magnetization above 2 km of depth bsl [ Fedi et al , 1998], which has been imputed to the existence of solidified magma [e.g., De Natale et al , 2004a, and reference therein], in agreement with the distribution of earthquake hypocenters concentrated along the crater axis down to 6 km of depth bsl, without significant gaps, although a lot of events clustered between 2 and 4 km bsl [ Vilardo et al , 1999]. Magnitude–frequency analysis shows that the physical conditions responsible of seismicity do not change within the seismogenetic volume.…”

Correlation between the dynamics of the Hyblean Foreland and Etnean volcanism

“…De Natale et al (2004) used another statistical methodology to distinguish formally the seismic activity of Vesuvius (CentralSouthern Italy) in periods of quiescence, characterised by seismicity background, and in periods of activity, characterised by notably higher energy releases. To assess whether this methodology can also be applied to tectonically active areas, like the Hyblean one, seismicity levels were computed using the criteria suggested by De Natale et al (2004).…”