Statistical assessment of seismicity patterns in Italy: Are they precursors of subsequent events?

Abstract

The variations of seismicity rate in Central Apennines prior to the sequence started in September, 1997 (at 00:33 UTC, ML5.6) has been analysed by statistical methods, with the purpose of pointing up eventual periods of quiescence. The analysis was carried out on the instrumental catalogue of the Istituto Nazionale di Geofisica (ING), covering the period from January 1975 to March 1998. In a preliminary phase, the catalogue was declustered using the Reasenberg algorithm. After that, eventual magnitude shifts due to variations in the modalities of observation have been individuated and corrected. The subsequent analysis, carried out making use of the Zmap software package, has put in evidence that the sequence of September 1997 was preceded by a 2.5 year period characterised by absence of events of magnitude larger than 3.2, in an area approximately 20 x 40 km wide, including the epicentre of the main shock. The statistical methodology shows that only 1/103 of the space-time volumes analysed in this study, exhibited quiescence of the same level. The study of seismicity rate change correlated to previous main shocks in a larger area of Central Apennines shows that none of them were preceded by a seismic quiescence, specially close to the epicentre of the main shock, and lasting until the time of occurrence of the main shock as in the 1997 case. Actually, we found other patterns of precursory quiescence with different time or space distribution. We conclude that precursory quiescence is a real feature of Central Apennines seismicity, but it is difficult to define a simple hypothesis, which applies to the generality of cases and can be tested before implementation in a system of earthquake risk mitigation