Journal of Volcanology and Geothermal Research
Mt. Vesuvius: a macroseismic study of the earthquake of 9 October 1999
Introduction
The seismic event of 9 October 1999, the most energetic since the 1944 eruption and one of the most energetic of those occurring in the Vesuvian area, once again drew public attention to Vesuvius since it was also felt outside the Vesuvian area and affected the city of Naples and its surroundings. As is well-known, explaining seismic source mechanisms in volcanic areas may be ambiguous owing to the complex processes involved (fracturation, fluid expansion) as well as the action of the medium, across which seismic waves spread (alternation of high acoustic impedance layers; rapid, frequent side variations in the medium’s physical characteristics). Under such conditions, interpretation is a crucial factor in forecasting eruptions and providing for public safety.
In this paper we present the results of an analysis of the macroseismic questionnaires sent out to schools in the Vesuvian and Neapolitan areas as well as surrounding towns in the provinces of Caserta and Salerno; the questionnaires were designed to mark the limits of the felt area and analyze energy spreading. Such a study makes it possible to obtain empirical formulas for magnitude intensity and seismic energy attenuation laws in quake-stricken areas. These are crucial factors in assessing seismic hazard in the Vesuvian towns and the neighboring city of Naples from seismic sources in the Vesuvian area.
Section snippets
Geological outline
An isolated symmetrical cone-shaped massif, Vesuvius stands in the Campanian Plain graben reaching a height of 1281 m. It consists of an older stratovolcano, Mount Somma, and a recent cone, Vesuvius, which has been quiescent since 1944. It is uncertain when Vesuvius began its eruptive activity. The oldest products from a 1220 m deep drilling date back about 300 000 years but the oldest outcropping ones are not older than 25 000 years. The earliest eruptive history of Vesuvius has been
Historical and present seismic activity
The oldest seismic events in the Vesuvian area that may be dated occurred in the first century A.D. when, a few days before the death of the emperor Tiberius, in 37 A.D., an earthquake brought down the lighthouse on Capri. On 5 February 62 A.D. an earthquake hit the whole Campania region causing considerable damage to Pompeii and Herculaneum and relatively minor damage to Naples and Nuceria. It was the most severe earthquake ever recorded in the Vesuvian area, M=6.1 according to Boschi et al.
The earthquake of 9 October 1999
The earthquake was felt over a very wide area within about 25 km from the crater, causing fear and anxiety among the people. Following the event, questionnaires were sent to all middle schools in the Vesuvian area, Naples and surrounding towns in the provinces of Caserta and Salerno in order to define the extent to which the earthquake had been felt. The questionnaires consisted of 18 yes/no questions (Branno et al., 1984). The students appeared to take considerable interest in the survey. The
Evaluating intensity and magnitude
The effects caused by an earthquake are measured in quantitative terms by using intensity scales (MCS, MM, MSK 64–78). Intensity degrees, up to VI, are actually the same for all scales. Higher degrees involving damage to buildings and effects on things vary as a result of different discriminating criteria, thus differing even by a degree. Macroseismic questionnaires are used for lower degrees (Karnik et al., 1976, Husebey et al., 1978, Postipishl et al., 1982, Branno et al., 1984, Marturano et
Quality factor (Q) and anomalies in the macroseismic field
The energy spreading from a seismic source decreases as distance from the hypocenter increases. In a purely homogeneous, elastic Earth, geometric spreading controls the amplitude of a seismic pulse. In actual fact, propagating waves attenuate with distance because of various energy loss mechanisms collectively described as due to anelastic processes (intrinsic attenuation). Such a decrease is quantified by parameter Q (e.g. Aki and Richards, 1980, Lay and Wallace, 1995):where ΔE/E is
Conclusions
The earthquake of 9 October 1999 was the strongest recorded in the Vesuvian area since 1944, the year in which Vesuvius’ latest eruption took place, bringing to an end a cycle that had begun with the 1631 eruption. The present study is the first to provide a reliable, extended macroseismic field. It is thus possible to assess the importance of the historical information there is on Vesuvius’ seismicity, and gain many useful insights into the level of energy released in the area and the
Acknowledgments
We are grateful to Prof. B.D. Marsh and two anonymous reviewers for their helpful suggestions. We would like to thank Prof. R. Scandone and particularly Prof. G. Luongo for helpful comments and discussion.
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