Geoscience Reference
In-Depth Information
Fig. 1
The most damaging earthquakes in Algeria with estimated intensities (as cited in Table 1).
Circle
:city,
square
: seismic event
east of Algiers (Ayadi et al. 2003, Bounif et al. 2004). Therefore, it offers an op-
portunity to study in detail the spatial variation of damage distribution and evaluate
the related attenuation of intensity necessary for the earthquake engineering and
seismic hazard assessment near the capital city of Algeria. The macroseismic survey
was carried out a few days after the 2003 mainshock by means of a thorough field
investigation using a detailed questionnaire and official reports in the damaged area
(Harbi et al. 2007b). The detailed macroseismic study has provided us with the
most complete intensity dataset ever obtained from field investigations of previous
earthquakes. For comparison, we revisited two destructive historical offshore seis-
mic events, namely the Djidjelli earthquake of 22 August 1856 (Jijel, Io VIII MKS;
Ambraseys 1982) and the Villebourg earthquake of 15 January 1891 (Larhat, Io X
MM; Rothe 1950).
The crustal attenuation in northern Algeria has been poorly studied due to the
lack of strong motion records. The damage distribution of moderate and large earth-
quakes along the Tell Atlas provides, however, a wealth of macroseismic informa-
tion. The intensity distribution has been the subject of several studies that allowed
determining an attenuation law in Europe (Ambraseys 1995). The decay of body
waves may have a direct relationship with the source dimension as represented by
the seismic moment and fault rupture size (Frankel 1991). The tsunamigenic 1856
Djidjelli earthquake was studied by Ambraseys and Vogt (1988) who prepared an
isoseismals map, and by Harbi et al. (1999, 2003b) who discussed the possible seis-
mic source from the interpretation of seismic profiles. The 1891 Villebourg earth-
quake presents favourable conditions for a comparison with the 2003 Zemmouri
earthquake. Indeed, the coastal location between the seismogenic Cheliff and the
Mitidja Plio-Quaternary basins confers to this seismic event and causative source
a great interest for the understanding of the earthquake hazard in northern Algeria.
These results, in connection with the soil conditions and the building vulnerabil-
ity suggest that the coastal area, which extends along 1,200 km in the E-W direc-
tion, is exposed to a relatively high seismic risk sometimes caused by tsunamigenic
sources.
Search WWH ::
Custom Search