Landslides and other surface effects induced by the 1997 Umbria-Marche seismic sequence

This paper both describes and discusses landslides and other ground effects induced by the September-October 1997 seismic sequence? which struck the U mbria and Marche regions (Central Italy). Three main events occurred on 26 September at 00:33 and 09:40 GMT and 14 October with magnitude M-w equal to 5.8, 6.0 and 5.4, respectively; furthermore hundreds of minor but signific ant events were also recorded. The authors examined an area of some 700 km( 2) around the epicentre (Colfiorito). Primary and secondary effects were ob served, including surface faulting phenomena. landslides, ground fractures, compaction and various hydrological phenomena. Surface evidence of faultin g reactivation was found along the well-known capable faults, to a total le ngth of ca. 30 km. Landslides, which were the most recurrent among the phen omena induced, consisted mainly of rock falls and subordinately of rotation al and translational slides, which were generally mobilised by the inertia forces during the seismic motion. The percentage of reactivated old landsli des decreased as the distance from the epicentral zone increased; a similar decrease had been observed for the 1980 Irpinia earthquake (Southern Italy ). The ground fracture distribution was consistent with the regional struct ural setting and the general pattern of macroseismic field. Numerous episod es of hydrological changes were observed within the most severely damaged a rea. All this evidence confirms the relevance of the study of ground surfac e effects for achieving a more complete evaluation of seismic hazard. (C) 2 000 Elsevier Science B.V. All rights reserved.