Spatial and temporal distribution of seismicity before the Umbria-Marche September 26, 1997 earthquakes

Spatial and temporal distribution of seismicityoccurring prior to the Umbria-Marche earthquake ofSeptember 26, 1997, are studied. By applying the RTLprognostic parameter, a quiescence stage followed bya period of foreshock activation is observed beforethe event. The main shock occurred soon after therecovery of the RTL parameter to its normal backgroundlevel. An investigation of the clustering process isperformed on the earthquakes with M 3.5,occurred since 1989 in the area of the epicenter ofthe September 26, 1997 event. In comparison to theaverage background of the previous period, theincrease of the area of rupture activated during thetwelve months leading up to the Umbria-Marcheconcentrates in the vicinity of the main shock. Someresults of application of the time-to-failure model toseismicity before the Umbria-Marche main shock, arealso discussed.