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

The variations of seismicity rate in Central Apennines prior to the sequenc e started in September, 1997 (at 00:33 UTC, M(L)5.6) has been analysed by s tatistical methods, with the purpose of pointing up eventual periods of qui escence. The analysis was carried out on the instrumental catalogue of the Istituto Nazionale di Geofisica (ING), covering the period from January 197 5 to March 1998. In a preliminary phase, the catalogue was declustered usin g the Reasenberg algorithm. After that, eventual magnitude shifts due to va riations in the modalities of observation have been individuated and correc ted. The subsequent analysis, carried out making use of the Zmap software p ackage, has put in evidence that the sequence of September 1997 was precede d by a 2.5 year period characterised by absence of events of magnitude larg er than 3.2, in an area approximately 20 x 40 km wide, including the epicen tre of the main shock. The statistical methodology shows that only 1/10(3) of the space-time volumes analysed in this study, exhibited quiescence of t he same level. The study of seismicity rate change correlated to previous m ain shocks in a larger area of Central Apennines shows that none of them we re preceded by a seismic quiescence, specially close to the epicentre of th e 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 quiescen ce with different time or space distribution. We conclude that precursory q uiescence is a real feature of Central Apennines seismicity, but it is diff icult to define a simple hypothesis, which applies to the generality of cas es and can be tested before implementation in a system of earthquake risk m itigation.