ALBERT

Description: As large destructive seismic events are not frequent in Algeria, an exhaustive knowledge of the historical seismicity is required to have a realistic view of seismic hazard in this part of the world. This research work presents a critical reappraisal of seismicity in the north-eastern Algeria for seismotectonic and seismic hazard purposes. This part of work focuses on the seismicity of pre-1900 period for the area under consideration [33°N-38°N, 4°E-9.5°E]. By going back to the available documentary sources and evaluating and analysing the events in geographical, cultural and historical context, it has been possible to identify 111 events, from 1850-1899, which are not reported in the recent Algerian catalogue. Several spurious events, reported in standard listings, have been deleted and nine unknown events have been discovered. It is quite clear that macroseismic information derived from press reports and published documents in Algeria, under certain conditions, is very incomplete, even for destructive earthquakes, located in the countryside away from communication centres. One of the reasons for this is censorship, noticeable during the colonisation period. Critical analysis of newly collected information has allowed the determination and/or the improvement of the macroseismic parameters of each event, such as location, maximum epicentral intensity and magnitude to produce an earthquake catalogue as homogeneous and complete as the available data, for the zone under study. The criteria used in this research are explained and eight historical earthquakes have been the subject of retrospective macroseismic field construction. The investigation of historical earthquakes is one of the most important task in studying seismotectonic for seismic hazard evaluation purposes.

Description: The main objective of the research work is the homogenisation of the two recent Algerian earthquake catalogues for the common covered period of time, from 1900 to 1990, for the region under consideration limited by [33°N-38°N, 4°E-9.5°E] and the updating of the catalogue for the twentieth century (1900-2000). To mitigate the deficiency of the incompleteness of catalogue, an effort was made to establish a correlation between surface-wave magnitude Ms and body-wave magnitude mb in the form of Ms = a + b (mb). A complete, exact and homogeneous earthquake catalogue as much as possible, comprising 870 seismic events, has been compiled. Seismicity analysis of the region shows a strong concentration of seismicity along a band of no more than 400 km width oriented mainly in the east-west direction parallel to the coast. Moreover, earthquakes in this zone are rather associated to strike-slip mechanism. The focal mechanism show a regional stress regime that corresponds to horizontal compression in NW-SE to N-S direction. As a result of the review of the seismicity of north-eastern Algeria from the compilation of checked and corrected data and its correlation with other geologic and geophysical investigations based on documentary sources, it was possible to construct a most complete seismotectonic map. It leads also to delineate four seismogenic zones in the Tellian Atlas, a less important zone in the Saharan Atlas, a sixth zone at the boundary of both Atlas and finally, a seventh one along the coastal zone. The seismicity-active faults correlation of some of these defined zones is examined in details with a special attention to the Saharan Atlas zone, the Hodna and Biban zone as well as Soumman and Babor zone where further research work allowed to find some neotectonic features considered as a significant sign of recent tectonic activity.