Contents of Monthly Reports

All information contained in these reports is preliminary and subject to change.

06/1996 (BGVN 21:06) Seismicity during 1995-96 is the highest in the past 50 years

The Somma-Vesuvius volcanic complex is a central composite volcano formed by an older stratovolcano (Monte Somma) with a summit caldera partially filled by the composite cone of Vesuvius. The most noted eruption, in 79 A.D., destroyed the ancient cities of Pompeii and Herculaneum. Since the explosive sub-Plinian eruption of 1631, Vesuvius has erupted with both Strombolian and mixed effusive-explosive styles. For the past three centuries the volcanic activity has mainly focused inside the Somma caldera but occasionally lava issued outside it (i.e., 1760 eruption). The last cycle of activity ended with the 1944 eruption. Since then, the volcano has been characterized by moderate seismicity and intra-crater fumarolic activity.

The Osservatorio Vesuviano maintains an array of short-period seismographs (eight three-component and nine vertical-component instruments). Seismicity was monitored during 1995 and March-April 1996. The 1995-96 period was the most active of the past fifty years. Several hundred microearthqukes (M < 3.2) were recorded during 1995, many from sources within the volcanic edifice above sea level. An increase in strain release and in frequency of earthquakes was observed from August to October 1995. During this period 217 events were recorded. Three of these earthquakes had M > 3.0 and were felt by the local population (~600,000 people): the first event (M 3.1, focal depth 3.1 km) occurred on 2 August; the second (M 3.2, focal depth 4.2 km) on 16 September, and the third (M 3.1, focal depth 3.3 km) on 24 September.

During November 1995-February 1996 the seismicity decreased to <10 events/month, and the strongest earthquake was M 2.5. During March-April 1996, however, 296 earthquakes were recorded. A M 3.4 event at 2 km depth was the strongest recorded during the past fifty years. Seismic activity at Vesuvius decreased again after this event.

Hypocenter locations for the past two years have clustered in a small volume below the crater area, no deeper than 6 km below sea level (figure 1). Focal mechanisms of relevant events suggested that the cause of seismicity was crustal rupture. Harmonic tremor and monochromatic low-frequency events were not observed. No changes in ground deformation or fumarolic gas compositions were reported.

A Reuters news story noted that a geophysical experiment is planned at the end of June to obtain a tomographic image of the volcano. The report said that the experiment, a joint-venture of Swiss, French, and Italian scientists, includes a series of controlled explosions at 14 boreholes on the volcano's slopes and as far away as the Sorrento peninsula. The explosions will be monitored by a network of 250 seismic stations. In addition, a marine seismic prospecting survey will be carried out in the Bay of Naples to investigate the volcano's submarine flanks.

Information Contacts: Lucia Civetta, Francesca Bianco, Giuseppe Vilardo, and Mario Castellano, Osservatorio Vesuviano, Via Manzoni 249, 80123 Napoli, Italy (Email: bianco@osve.unina.it; vilardo@osve.unina.it; mario@osve.unina.it); Paul Holmes, Reuters News Service.

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08/1996 (BGVN 21:08) Ongoing sub-crater seismic activity

Non-eruptive activity at the Mt. Somma-Vesuvius volcanic complex is characterized by low seismicity both in terms of energy and numbers of events; very few episodes of intense seismicity occurred during the last twenty years.

In the last three years, however, hundreds of earthquakes have been detected with magnitudes ranging from -0.4 up to 3.4. The plot of monthly seismic events (figure 2) shows the usual fluctuating pattern observed at Vesuvius. The energy release distribution (figure 2) has a sharp peak coinciding with the 1995-96 earthquakes; the strain release curve also recorded two clear steps at these times.

After the seismic crisis of March-April 1996 (BGVN 21:06), seismic activity decreased. During May-August 1996, the permanent seismic network of the Osservatorio Vesuviano recorded 266 microearthquakes, the strongest one had M 2.7. The events belonging to this sequence affected an extremely reduced volume below the crater area at shallow depth, with hypocenters rarely exceeding 6 kilometers below the sea. No changes in ground deformation or fumarolic gas composition were reported in the last field measurements.

Information Contacts: Lucia Civetta, Francesca Bianco, Giuseppe Vilardo, and Mario Castellano, Osservatorio Vesuviano, Via Manzoni 249, 80123 Napoli, Italy (Email: bianco@osve.unina.it; vilardo@osve.unina.it; mario@ osve.unina.it).

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04/1997 (BGVN 22:04) Low seismicity prevails after March-May 1996 earthquake swarm

During late 1996 through early 1997 Somma-Vesuvius remained volcanically quiet and characterized by low seismicity both in terms of energy and number of events.

A few episodes of moderate seismic activity have occurred in the last twenty years. The crisis of March-May 1996, characterized by an M 3.4 event (BGVN 21:08) was followed by a significant decrease of the seismic activity (figure 3). After the crisis, during June 1996-April 1997, 350 microearthquakes (maximum magnitude, 2.7) were recorded at the permanent seismic network of the Osservatorio Vesuviano. As has been typical in the past, foci appeared in a small volume below the crater area, rarely at depths below 6 km.

During January 1996-April 1997 the monthly temporal distribution of both earthquakes and their energy fluctuated, as had been the case in the past. The distribution of the cumulative strain release (figure 3) had its regular trend, disturbed only by the seismic crisis of March-May 1996. Both ground deformation and fumarolic gas composition data remained stable.

Information Contacts: Lucia Civetta, Francesca Bianco, Giuseppe Vilardo, and Mario Castellano, Osservatorio Vesuviano, Via Manzoni 249, 80123 Napoli, Italy (Email: civetta@osve.unina.it; bianco@osve.unina.it; vilardo@osve.unina.it; mario@osve.unina.it).

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10/1999 (BGVN 24:10) 9 October seismic swarm includes an M ~ 3.5 event, the largest in 50 years

At the beginning of October a small cluster of seismic events was recorded by the permanent seismic network of the Osservatorio Vesuviano (figure 4). The most energetic event of the sequence occurred at 0741 on 9 October, with a duration magnitude (MD) of 3.6 and a Wood Anderson equivalent magnitude (MWA) of 3.4. This event was located in the crater area at a depth of about 3 km below sea level, with a preliminary stress-drop of 164 bar and a preliminary moment magnitude of 3.3. According to its MD value, this event was the most energetic of the last 50 years. Its fault plane solution showed a strike slip mechanism. All the events in this sequence were located below the crater area within the first 6 km of the upper crust, typical of seismicity at Vesuvius in recent years.

During the swarm no changes were observed in the temperature or the composition of the fumaroles, in the CO₂ flux from soil, or in ground deformation. Both the temperature and the level of the water-table sampled around the volcano appeared unchanged. As of mid-November seismicity seems to have returned to normal low background levels (both in terms of energy and in number of events) that have characterized Vesuvius for several years.

Information Contacts: Lucia Civetta (civetta@ osve.unina.it), Edoardo Del Pezzo (delpezzo@osve.unina.it), Francesca Bianco (bianco@osve.unina.it), Giuseppe Vilardo (vilardo@osve.unina.it), and Mario Castellano (castellano@osve.unina.it), Osservatorio Vesuviano, Via Diocleziano 328, 80124 Napoli, Italy (URL: http://www.voxneapolis.it/osservatorio.vesuviano/).

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