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(VIII and IX). The decrease of intensity with the epicentral distance is not homo-
geneous and depends strongly on the azimuth at a regional scale. The area with
intensity greater than or equal to VI is elongated in the NE-SW direction with a
length of 160 km and a width of 50 km. A regression analysis performed in two
different azimuths conduced to eliminate the effect of the fault itself and the low
attenuation around the faulted area. The results obtained, using the average radii of
isoseismals, show a clear difference between the fault azimuth and its perpendicular
direction. This difference is related to the geological conditions which are marked
by the thrust-and-fold Atlas belt and the Mitidja intermountain basins.
As shown in diverse attenuation curves related to the Zemmouri mainshock, this
study suggest a low attenuation in the affected area. The intensity attenuation is
clearly stronger along the NW-SE direction with an abrupt decrease to the South
at the Blida Atlas Ranges. This highly fractured E-W zone could have played the
role of a screen for the seismic waves propagating to the South. The occurrence of
an earthquake with epicentre further to the west part along the tectonically active
zone, at Blida for example, will have a strong impact on the Algiers capital city.
The different attenuation relationships deduced for the Zemmouri earthquake can be
inferred to calculate the probability of damage due to a future earthquake occurring
in the same area including Algiers and its surroundings (Table 3).
The Djidjelli 1856 and Villebourg 1891 earthquakes are smaller than the Zem-
mouri event which produced significant surface effects and deformation. However,
the three earthquakes are almost comparable in the extent of the affected area as well
as in some of their characteristics (Table 4). As we know, the magnitude of historical
events may be assessed roughly from the area of perceptibility. By using the rela-
tionships derived by Benouar (1994) for Algeria: Ms
56 log(r
3
) (where
r
3
corresponds to the mean epicentral distance of an area within which the shaking
was felt with intensity III (MSK or EMS)), we calculated the surface-wave mag-
nitude of both historical events (Table 4). It is worthwhile noting that the Djidjelli
and Villebourg earthquakes were also felt far from the shore by sailors of the
Aviso
Ta r t a re
located at 15 mi at North 7
◦
of Djidjelli and the ship
Porro
located at 6 mi
of Cherchell, respectively. The maximum intensities VIII+ and IX EMS have been
estimated, respectively, for the 1856 and 1891 historical events. For both of them,
the intensity could easily exceed these estimations in the case of an epicentre closer
to the coast. In the same way, the respective surface-wave magnitude may differ
from those calculated (Table 4). Regarding the Djidjelli earthquake if we consider
an area of perceptibility including the localities of the north-Mediterranean coast
=−
0
.
04
+
2
.
Ta b l e 3
The different attenuation relationships deduced for the Zemmouri earthquake
Attenuation relationship
All data
I
=
I
0
+
1
.
31
−
0
.
001D
−
2
.
95 log(D)
Perpendicular to the fault azimuth
I
=
I
0
+
0
.
38
−
0
.
0004D
−
1
.
167 log(D)
Parallel to the fault azimuth
I
=
I
0
+
0
.
137
−
0
.
0032D
−
1
.
193 log(D)
Parallel to the fault azimuth using isoseismals
I
=
I
0
+
1
.
22
−
0
.
01D
−
1
.
820 log(D)
Perpendicular to the fault azimuth using isoseismals
I
=
I
0
+
0
.
159
−
0
.
004D
−
2
.
325 log(D)
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