Investigations on site effects in Italy

Amplitude variations among atrong ground motions recorded at close sites and anomalies of the macroseismic fields were observed in Italy on the occasion of recent earthquakes. We present results for three sites where amplifications of ground motion larger than a factor of ten were experienced, or a difference of one intensity degree compared with nearby sites was observed. Moreover, some experiments were carried aut on sediment bodies in order to investigate site response, based on measurements of weak motions produced by teleseisms, local earthquakes and explosions. For all these study case, 1- and 2-D numerical techniques and microtremorFourier spectra were used to compare theoreticaland empirical estimations. Generally, numerical medelling allowed a good prediction of the frequency band where amplifications accur, but a satisfactory fit of amplifications observed during earthquakes was obtained in some cases only (AU)

Recent seismic activity and earthquake occurrence along the Apennines

In this paper we analyze the seismicity along the Apennines in order to relate the seismicity pattern to the seismogenic environment and to the active tectonic processes. The analysis of crustal and subcrustal seismicity show that the northern and southern Apennines are characterized by two distinct patterns of seismicity. These two domains are separated by two important lithological discontinuities (the Ancona - Anzio and the Ortona - Roccamonfina lines). The seismicity along the Apennines is mainly concentrated in a narrow belt running along the chain, with an evident geometrical offset which corresponds to the Ancona-Anzio line. We discuss these observations considering both the seismological evidence and the present tectonic regime in which the earthquakes occur. We focus on the southern Apennines, where the largest earthquakes (6 ó M ó 7.0) have occurred. We analyze the main features of some recent seismic sequences in order to discuss the seismicity patterns in terms of the hypothesis of segmentation of the southern Apennine seismogenic belt. This work aims to be a preliminary contributions to the important goals of understanding the spatial and temporal patterns of seismicity and of identifying active faults