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This paper is two-fold: (1) Using the dataset of the isoseismal map produced by
Harbi et al. (2007b) and taking into account previous field investigations and macro-
seismic distribution in the Algiers-Zemmouri region, we estimate the attenuation of
intensity with distance from assigned European Macroseismic Scale intensity value
to 600 localities; and (2) we compare the results to those obtained for the 1856
(Djidjelli) and 1891 (Villebourg) coastal earthquakes and show similarities from the
damage distribution, seismological effects and geological structures viewpoints.
2 The 21 May 2003 Earthquake
The Mw 6.8 coastal mainshock generated strong and damaging effects within
150 km radius as well as significant ground deformation with uplifted marine ter-
races, liquefaction, minor landslides, rockfalls, ground fissures and anomalies in
the flow of springs (Harbi et al. 2007b). The most impressive phenomenon induced
by the earthquake corresponds to the large coastal uplift of marine terraces which
implied an important continental deformation related to a SE dipping and 55-km-
long thrust fault (Meghraoui et al. 2004). The seismic event has been a subject of
several studies. Using a simple double difference method, Bounif et al. (2004) re-
located the mainshock epicentre on the coastline (36.83
◦
N, 3.65
◦
E, Fig. 1) with
8-10 km hypocentral depth and analysed the distribution of the aftershocks se-
quence which shows a
40
◦
-50
◦
south dipping fault plane and two distinct clusters
of seismic events along strike. From the inversion of the teleseismic body waves,
joined with GPS and uplift data, Delouis et al. (2004) calculated the effective 12s
rupture duration, the 2
∼
10
19
N-m seismic moment and pointed up that the
Zemmouri earthquake involved a bilateral rupture propagation from the hypocen-
tre: the south-westward slip with 11-2 km depth range, and the north-eastward
slip zone that extends from 6 km depth to the surface. Meghraoui et al. (2004)
measured coseismic shoreline changes of emerged algae jointly with kinematic
GPS and conventional levelling lines. The obtained dataset allowed them to model
the surface deformation along about 60 km coastline and suggest two rupture
patches along a 50
◦
SE dipping planar reverse fault geometry located between 5
and 10 km offshore. Using modelling GPS data from 5 stations located west of
the epicentral area, Yelles et al. (2004) infer a uniform model on a plane dipping
42
◦
to the south. Semmane et al. (2005) combined geodetic data and accelero-
grams to model the fault location at 15-22 km offshore and showed two large
slip zones on the fault with the largest located west of the hypocenter. Alasset
et al. (2006) modelled the initiation and propagation of the tsunami wave trig-
gered by the earthquake and compared synthetic results with the 2 m high waves
of tide gauge records of the Balearic Islands, whereas no similar effect was re-
ported along the Algerian coast. Their analysis and modelling lead to the conclu-
sion that an earthquake larger than Mw
.
86
×
7 followed by tsunami could produce
a possible run-up along the Algerian coast (the Zemmouri earthquake did not) and
large wave-amplitudes (more than 3 m) could reach the Balearic Islands. Laouami
=
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