A site effect study in the Verchiano valley during the 1997 Umbria-Marche (Central Italy) earthquakes

Strong site effects were observed during the two M-W 5.7 and M-W 6.0 main s hocks of the Colfiorito seismic crisis which occured on September 26, 1997 in Umbria-Marche (Central Italy). The most obvious indications of these eff ects are the dramatic differences in damage shown by buildings of similar c onstruction in neighboring villages. Such observations were specifically ma de in the Verchiano valley in the fault area, 15 km south of Colfiorito whe re the Verchiano village and the Colle and Camino hamlets were heavily dama ged (MCS intensity IX-X) since the first main shock of 1997/09/26, while in contrast, the Curasci village located 2 km eastwards remains almost intact . In order to study the anomalous ground motion amplifications in this area , an array of 11, 3-components seismo- and accelero-meters was set up durin g the 1997/10/20-24 period, extending from the western side of the valley, up to the top of Mount San Salvatore, going accross the Colle and Curasci h amlets. During the experiment, 67 aftershocks enlightened the valley from t he Colfiorito (10 km north) and the Sellano (6 km south) active swarms. Sei smic refraction experiments were conducted at the same time in the 500 m wi de, 1500 m long Verchiano valley in order to determine the thickness and ma in characteristics of the alluvial infilling. The main results are: (i) com pared to the valley side ground motion, and for all the events, recordings in the central part of the valley (piana di Verchiano) show relative amplif ication of similar to 10 with a clear lengthening of the seismogram duratio n by a factor of similar to2 - (ii) broad band relative amplification of si milar to6-8 is also clearly identified at the top of the Mount San Salvator e overhanging the valley - (iii) any of the site effect measurements done e xplains by itself the strongly contrasted damage observed at Colle and Cura sci: i.e. the modification of the near-field radiation pattern by interacti on with the free heterogeneous surface may have induced local shadow zones that saved Curasci.