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Ta b l e 4 Similarities of the characteristics of three destructive coastal earthquakes
Zemmouri 2003
Djidjelli 1856
Villebourg 1891
Type of location
Coastal
Offshore*
Coastal
Magnitude
Mw 6.8
Ms
6.0
Ms
6.0
Intensity I 0
X (EMS)
VIII + MSK
IX EMS
Source
Offshore
Offshore
Offshore
Mean radius of
I
40 km
40 km
15 km
=
VIII
Mean radius of
I
350 km
230 km
260 km
=
III
Direction of the
isoseismals
NE-SW
NE-SW
NE-SW to E-W
Other parameters
- Faulted area:
50 km
- Coastal uplift:
50 cm
- The sea
retreated and
flooded the
Balearic coasts
- Tsunami
- The sea
retreated by
- Coastal uplift
of 30 cm
- The sea
retreated by
35 m and a
sea-wave of
3 m flooded
the Algerian
coast a number
of times.
- Tsunami
- No evidence of
coastal uplift
30 m and
flooded the
Algerian coast.
-Nostrong
evidence of
tsunami
* Even if the macroseismic epicenter is coastal, we think that the instrumental one could be
offshore.
=
.
where the shock was felt (Fig. 5a), we obtain Ms
6 when Ms is equal to 6.0 if
only the Algerian part is taken into account (r 3 on Fig. 5b). On the other hand, the
lack of intensity points offshore determines the shift of the epicentre onshore. We
assume that the strongest earthquake, which hit Djidjelli in the past, may be due to
an offshore causative fault (Harbi et al. 2003b). The comparison of the isoseismal
maps of the three earthquakes presented in this work show quite similar attenuation
laws with a slight shift for the 2003 Zemmouri event. This difference is due to the
size of shock (Mw
6
8, greater than the magnitude of the two others events). In
order to develop a curve that predicts the intensity for the Algerian offshore events,
we used the equation in the form of:
=
6
.
b D
c log(D)
I
=
a
+
+
(6)
Where D 2
8km),
r is the epicentral distance in km. The results of the fitting are presented in Fig. 8
from which, we can see that the relation fit the used data with a certain degree of
reliability and thus, they can be used in seismic hazard analysis for Algerian offshore
events.
Destructive earthquakes occur very infrequently along the Algerian coastline
but the Zemmouri 2003 event (Mw 6.8) warns us against possible strong events
=
(r 2
+
h 2 ), h is the focal depth (taken at an optimum value of h
=
 
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