Page 244 - Numerical Analysis and Modelling in Geomechanics

F.PERGALANI, V.PETRINI, A.PUGLIESE AND T.SANÒ 225
f denotes the probability density function of the source to site distance; and the
R
summation of the activity rates v extends over all the seismic sources.
i
In particular, for a Poisson process, the exceedence probability that a ground
motion amplitude A is greater than a in time t is given by:
(8.2)

where λ is the yearly frequency of exceedence of the ground motion amplitude
A.
The hazard analyses were carried out with the following assumptions:
• The activity rate at each magnitude corresponds to the 50th percentile of the
frequency-magnitude relationship;
• Strong ground motion uncertainties resulting from the attenuation
relationships of Sabetta and Pugliese (1996) were taken into account;
• A threshold magnitude of 4.7 was adopted, to avoid a misleading contribution
to the seismic hazard coming from the integration in equation (8.1) of small
magnitudes of little engineering interest (McCann and Reed, 1989);
• In each seismic source zone the maximum magnitude coincides with the
maximum historical magnitude.

Seismic sources
The current Italian seismotectonic model is constituted of 80 seismic source
zones (Figure 8.1) based on a kinematic model of the most recent tectonic units
(Scandone et al., 1990; Patacca et al., 1993; Scandone et al., 1996).
The zones belong to nine main seismotectonic domains and one volcano-
tectonic domain (Figure 8.2). The mountainous Alpine arc (domain 1) is
characterized by a prevalently compressive regime, with thrust or reverse faults,
and secondary strike-slip faults. In north-eastern Italy this domain represents the
compressive margin between the Adriatic micro-plate and the Eurasian plate
(Albarello et al., 1995). The northern Apennines are divided into domains
elongated parallel to the chain (Lavecchia et al., 1994). On the western side
(domain 5—zones 27, 31, 41, 42, 49) prevails a rifting regime with a moderate
seismicity along normal faults. The eastern side of the chain (domain 3—zones
30, 35, 38, 39, 48, 53) is representative of a compressive regime due to the
residual inflection of the Adriatic micro-plate going under the Apennine chain
(roll-back mechanism; Scandone, 1996). The resulting fault mechanisms are both
compressive along the front of the Apennine chain over-thrusting the Adriatic
micro-plate and normal to transcurrent along the flexural inflection of the plate
itself. Along the chain axis (domain 4— zones 28, 29, 32, 33, 34, 36, 37, 44, 45,
46, 47, 50, 51, 52; domain 6—zones 58, 62, 63, 64 and domain 7—zones 66, 67,
69, 70, 71, 72) the uplift prevails, reaching maximum rates in the southern
Apennines (Cinque et al., 1993; Moretti et al., 1994). Focal mechanisms are