Page 268 - Numerical Analysis and Modelling in Geomechanics

F.PERGALANI, V.PETRINI, A.PUGLIESE AND T.SANÒ 249
damaged. The upper part of the village, Cesi Villa, suffered less damage,
reaching not more than degree VII on the MCS scale.
The lower part of the village lies partly on a toe slope formed by debris and
colluvium and partly in a valley filled by colluvial and fluvial-lacustrine fine
deposits; the upper part directly lies on bedrock made by marls and marly
limestones belonging to the Scaglia formations. An evident strong influence of
local geologic conditions governed the damage distribution and such situations
were monitored and analysed in detail (Pergalani et al., 1999).
Some aftershocks up to magnitude 5, recorded just a few days after the
mainshock on 26 September, were used to calibrate the geotechnic model
suggested by the field geologic survey. In particular, above a 35 metres depth of
fluviallacustrine silty-clayey deposits with an average shear wave velocity of 400
m/s, a thin cover of loose colluvial deposits has been hypothesized from the
analyses of the accelerometric recordings. The good agreement between the
recorded seismic signals and the results of the analysis confirmed the validity of
the geotechnic characteristics attributed to the soils.
In Figure 8.15a a schematic cross-section through Cesi Villa and Cesi valley is
shown (Sanò et al., 1998); the nodes, where the local seismic response has been
computed by the BESOIL program, are labelled with capital letters. The
corresponding response spectra are drawn in Figure 8.15b, and in Figure 8.15c
the site effects are summarised in terms of Fa amplification factors. It is worth
noting the increment in the spectral accelerations and the decrease in the
fundamental period of vibration proceeding from the centre of the valley toward
the slope toe. The response is maximum at station F, about 100 metres far from
the slope toe, for a period of about 0.3 s.

COLFIORITO VILLAGE
The village of Colfiorito is located very close to the main epicentral areas of 26
September, 00.33 and 09.40 GMT. The macroseismic intensity, updated to 20
October, is VII–VIII MCS and 38% of buildings were made unusable.
The geologic setting is that of a typical inter-mountain tectonic depression of
the central Apennines. The Umbria-Marche thrust sequence is here dissected by
normal faults whose major trend is NW-SE. The depression acted as a shallow
lake during the Quaternary and it has been filled up by a sequence of lacustrine
and fluvial deposits mainly composed of gravels, clays and silts. The local
thickness in the Colfiorito area has been estimated to be about 30–60 m from two
available bore-hole logs located in the plain (Pergalani et al., 1999).
A simplified two-dimensional section, drawn across the Colfiorito valley
(Figure 8.16a), has been analysed with the aid of the QUAD4M program. The
results in terms of acceleration spectra (Figure 8.16b) show how spectral
ordinates increase in the central part of the valley (points 8 and 17), reaching
about 3g in the period range 0.2–0.3s. Lower values of the spectral ordinates of
about 2g in the period range 0.05–0.15s are obtained in proximity of the valley