Mt. Vesuvius: a macroseismic study of the earthquake of 9 October 1999

Abstract

A macroseismic study of the earthquake of 9 October 1999, which took place at Vesuvius’ crater, was carried out. The event was not only the most energetic since the last eruption in 1944 but also one of the most energetic of those occurring in the Vesuvian area as shown by an analysis of historical seismicity. This paper contains the results obtained from an analysis of questionnaires sent out to all the schools in the Vesuvian and Neapolitan areas and the surrounding towns in the provinces of Caserta and Salerno. Over 10 000 questionnaires were administered, covering 92 sites, 18 of which were within the city of Naples. The percentage of positive reports concerning single effects was taken into account (the percentage response to Question 1 was called felt index). Thus maps of earthquake intensity and felt index were drawn up, whose isoseisms show maximum values near the volcano and radially decreasing values. The surveyed intensity plane is remarkably dense. As far as 30 km from the epicenter, the distance between two radially consecutive sites of the surveyed plane is, on average, lower than 500 m. The maximum degree turned out to be V for sites whose mean distance R V from the epicenter was 7.0±2.3 km, maximum intensity being found for the towns of Boscotrecase, Cercola and S. Sebastiano, characterized by some of the sixth degree characteristics. Typical parameters such as epicentral intensity ( I 0), attenuation coefficients ( s), and absorption factor ( b) were determined. Their values range between 6.0 and 6.4 ( I 0), 3.9 and 4.1 ( s, Blake’s formula) and 0.1 for b, while magnitude ( M) values range between 3.6 and 4.1. Furthermore, Q quality factor was determined by assuming direct proportionality between energy and felt index ( A), peak frequency f=2–3 Hz and a wave velocity β=3.3 km/s. The values obtained are Q=60–90 and, subtracting geometrical spreading, Qa=100–150. Marked attenuation areas were found in the epicentral zone E and NE of the volcano and in the Phlegraean Fields associated with the source mechanism, scattering and the high temperature of a magmatic body. Historical eruptions were taken into account, which enabled us to gain an insight into seismicity associated with them. Also, effects expected for seismic events in the Vesuvian area were estimated, their magnitude ranging from 3 to 4.5.