Page 247 - Numerical Analysis and Modelling in Geomechanics

228 SEISMIC MICROZONING USING NUMERICAL MODELLING
wider background area. Most of the destructive earthquakes (Ms>6) are
distributed along the Apennine chain and the eastern Alpine belt, which
represent the youngest folded areas of the peninsula. The largest earthquakes
(Ms>7) are concentrated in central and southern Italy and particularly in the
Calabrian arc (zones 65 to 71 in Figure 8.1).

Activity rates
A key question in the use of historical seismicity for seismic hazard assessments
is given by the completeness of the earthquake catalogue, usually based on
statistical analyses. Following the procedure proposed by Tinti and Mulargia
(1985), the cumulative number of earthquakes against time was plotted and the
possible completeness intervals were selected by looking at the increase in slope
or detection rate.
The activity rate of each seismic source zone was then estimated through a
least square regression analysis, assuming a Gutenberg-Richter (1954) frequency-
magnitude relation.
Source zone 47 (Umbria-Marche area) has the highest activity rate, followed
by source zone 4 (Friuli region), with at least one event above Ms 4.7 every 10
years.

Attenuation relationships

Despite the availability of much historical information, the Italian strong motion
data do not allow the development of attenuation laws for different
seismotectonic domains. Currently two attenuation models have been developed
for the whole country (Tento et al., 1992; Sabetta and Pugliese, 1996). The
attenuation equations proposed by Sabetta and Pugliese (1996) have been
developed for a wider range of seismic parameters and for both the horizontal
and the vertical components of the ground motion.
The adopted form of the ground-motion attenuation equation is the following:

(8.3)

where Y is the ground motion parameter to be predicted; M is the magnitude; R is
the distance (fault or epicentral); h is a fictitious depth determined through a non-
linear regression analysis; S , and S 2 are flags characterizing the local site
1
conditions, referring to shallow and deep alluvium sites, respectively; and σ is
the standard deviation of the logarithm of the estimated ground motion Y.