Page 242 - Numerical Analysis and Modelling in Geomechanics

Chapter 8
Seismic microzoning using numerical

modelling
The Umbria-Marche earthquake of 26 September 1997

F.Pergalani, V.Petrini, A.Pugliese and T.Sanò

Introduction
After the Umbria-Marche (Central Italy) Ms 5.9 earthquake of 26 September
1997 the Italian Government decided that the amplification due to local effects
had to be taken into account in repair and reconstruction. The area to be analysed
was quite large, including about 1000 villages, and the analysis had to be
completed in about six months to avoid excessive delay in the starting of
reconstruction; the available budget was also limited to about 1,550,000 euros.
A working group, formed by researchers of the Servizio Sismico Nazionale
(SSN) and the Istituto di Ricerca sul Rischio Sismico-Consiglio Nazionale delle
Ricerche (IRRS-CNR), was charged to define a procedure able to give reliable
results under the above-mentioned constraints. The working group was also
charged with guiding the activity.
Therefore the working group decided to proceed in two phases. In the first
phase the researchers of the working group performed the following steps:

1 Selection of 60 sample villages: the criterion was to select those showing the
highest degree of damage and representative of the main geologic and
geomorphologic features of the area struck by the earthquake, in view of the
extrapolation of the results to the entire area;
2 Collection of the basic geologic, geomorphologic and geotechnic data;
3 Field surveys, which implied a geologic and geomorphologic survey at a
detailed scale (1:5000);
4 Definition of the seismic input for the numerical analysis;
5 Computation of site amplifications through one-dimensional and two-
dimensional soil modelling, by finite and boundary element methods.

As a result a set of standard local effect situations and a table giving the values
of the amplification factors for each situation were defined.
In the second phase, a group of geologists expert in the area extended the field
surveys, again applying the geologic and geomorphologic survey at 1:5000