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Ambraseys (1995) which clearly underestimates the recorded accelerations for all
distances less than 50 km. This significant variation up to 50 km can be interpreted
as due to the amplification effects but it requires further field investigations. The site
amplification, however, is clearly highlighted by Laouami et al. (2004) at Kaddara
site in which a significant PGA variation is observed between two stations of 150 m
distance (0.34 g and 0.58 g) suggesting site effect phenomena. Unfortunately, there
are no ground acceleration records in the epicentral zone of intensity IX and X in
which the ground motion was certainly strong. The plot on Fig. 4f suggests that the
peak ground horizontal acceleration at the macroseismic intensity IX and X loca-
tions is probably more than 0.70 g. On the basis of the strong-motion and intensity
databases (I
4) of the 2003 Zemmouri earthquake (Laouami et al. 2004, Harbi
et al. 2007b, respectively), we investigated the correlation between the available
strong ground-motion and earthquake damage through a regression analysis. The
peak ground acceleration (PGA) well correlates with the earthquake damage. The
empirical relationship between PGA and the intensity (
I
) is determined in this study
as follow:
≥
(PGA)
=
0
.
403
+
0
.
292(I)
−
2
.
554 Log(I)
(5)
This PGA-intensity correlation is particularly useful in real-time applications for
damage prediction and assessment. This empirical relationship shows (Fig. 4g) that
the PGA value could be higher for I
8 particularly in the zones (the grey area on
Fig. 4g) close to the epicentre (Zemmouri (I
>
=
10) and at Boumerdes (I
=
10) for
which PGA records are not available.
6 Comparison with Historical Damaging Events
The Djidjelli earthquake of 22 August 1856 and its foreshock and aftershocks as
well as the Villebourg earthquake of 15 January 1891 and its following seismic
sequence caused the largest catastrophes affecting respectively the eastern and the
central Algeria coastal area before the 2003 Zemmouri earthquake. They are also
considered as among the most well documented historical seismic events. The
detailed re-appraisal of the damage and surface effects of these historical events
allowed us to obtain a complete isoseismals map for each one of them. The compar-
ison with the recent Zemmouri earthquake is thus pertinent since these earthquakes,
being the largest that occurred along the coastal area, contributes considerably to
the reduction of seismic risk in northern Algeria.
6.1 The Djidjelli Earthquake
The 1856 Djidjelli earthquake produced damage effects in a large area along the
Algerian coast and was felt at several points of the northern Mediterranean coast
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