Abstract

In the last millennium the city of Rieti, according to the most up-to-date Italian seismic catalogue (Working Group CPTI, 1999), has been affected by strong earthquakes which caused major damage to its building heritage. The Rieti urban area lies at the south-eastern border of the Rieti plain (situated around 400 m a.s.l.), one of the intermountain basins formed after the paroxysmal phases of the Apennine orogeny, and related to a still active tectonic extensive regime. Slip rates have ranged from 0.3 to 0.4 cm/year in Pleistocene and Holocene times (Michetti et alii, 1995). The geomorphologic evolution of the area has been controlled by shifting climatic conditions. These, during the Quaternary, controlling the growth and disruption of major travertine curtains, along the Velino river, strongly affected erosive and depositional processes in whole Rieti basin (terrace formation and/or disruption, sedimentation rate in the order of some mm/year during the Holocene, morphology of mountain slopes, development of the vegetation cover and the erosion of soils). The present work has been aimed at analyzing the effects of the June 27, 1898 earthquake; furthermore, by taking into consideration the distribution of damage within the urban area, it has been possible to provide clues to the seismic response of the area. It is well known that the distribution of damage in cities is hardly homogeneous, being affected by the quality and conditions of buildings, the geologic characteristics of the ground they are built on, and the morphological features of the area where the city is located. The geological survey we conducted in the Rieti urban area has allowed us to subdivide it into three sectors, each characterized by different geological formations. In particular, the southern sector is founded on the post-glacial alluvial deposits of the Velino River; the central one lies on travertines which are often covered, in the most elevated portion of the city, by thick mounds of rubble; the northern sector is built on alluvial fans from the eastern margin of the Rieti basin (Reatini mountains). By comparing the distribution of damage and the results of our geological survey, it has been possible to point out the following: -the first two damaged areas in the southern sector of the city lie on the right and left banks of the Velino River, where buildings are founded on post-glacial alluvial deposits. We believe that, for such type of events, in these areas, the presence of soft, young sediments filled with the water of an elevated, nearly emerging water table, contributes to modify strongly the incidence bedrock seismic motion and worsening the damages (this might be defined as "water mattress effect"). The third damaged area, in the highest and central portion of the city, is underlain by thick anthropic fill deposits; -in contrast, the least damaged areas are distributed around the most elevated portion of the central sector of the city (fig. 2), where houses were built on fairly solid travertine deposits. It is important to point out that, although it is not easy to infer the strong ground motion from the weak motion (because of the non linearity of the soil), the behaviour of these post-glacial alluvial sediments has also been recognized by the noise analysis that showed clear amplification peaks on the horizontal components (Barba et alii, 1995 and Milana et alii, 1996). In synthesis, the above data show that buildings founded on soft sediments suffered heavier damage compared to the ones founded on solid rock. Although we know that this phenomenon is quite common all around the world, we do not believe it could represent a general law; as a matter of fact, recent studies show that the damage patterns are significantly influenced also by the distance and the source mechanism. The aim of the present work is, therefore, to provide further data for parametric studies aimed at achieving a better understanding of the local seismic response. Although not affecting significantly the above conclusions, it is important to point out that, when analyzing damages, it is also necessary to consider the conditions of buildings, which were undoubtedly poor in the two most damaged areas along the river banks, inhabited by the least affluent citizens, and possibly in the highest portion of the city, characterized by the presence of the largest, most important and oldest buildings.

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