Bulletin du Service Géologique de l'Algérie
Volume 17, Numéro 2, Pages 183-211
2006-07-01
Large Scale Tectonic And Mega-scale Seismic Zoning. Implications On Civil Engineering Constructions. The Case Of The Maghreb
Authors : Beghoul Noureddine . Benhallou Hadj .
Abstract
This study presents substantial arguments that the effect of the regional stress field associated with the mega-scale lateral tectonic variations out weights by far the usually considered effect of sediment amplification and seismic source rupture complexities. Indeed, most of civil engineering hazard assessment studies either use statistical methods, or speak most of the time in term of micro-zon ?tion at thé scale of a city ( sediment amplification etc. ), and over- look the large scale deformation and the tectonic structures in general. Therefore, their results are not real 1> useful practically. This study considers the case of the Maghreb and the overall influence of the regional stress field associated with the existence of the currently active Atlas faults system on the seismic safety of the constructions. From the engineering perspective we argue that the deformation of the Maghreb could be characterised by six mega seismic zoning. Those mega-seismic zoning have been determined on the basis of 1 ) the nature and styles of deformation and displacements deduced from seismic focal mechanisms; 2) the observed time recurrence of large destructive earthquakes and their spatial-temporal variations; 3) on the basis of the recent satellite and field mapping of mega scale tectonic provinces and structural features such as folds, quaternary faulting, basins etc. It is shown that a correct exposure of low elevated constructions orthogonal to the main components of the regional stress field has a weight four order of magnitudes larger than the usually considered sediment amplification and three orders of magnitudes larger than the effect of the accelerations caused by source complexities along faults planes; and an order of magnitude larger than the effect of the three dimensional resonance phenomena of a valley in a specific geodynamical context. On the basis of those rough but certainly meaningful evaluations a mega-seismic zoning of the Maghreb is provided based on regional stress field and the geodynamical context of the regions. Those results are largely fulfilled by the field observations after the EI-Asnam earth-quake of October 1980. Therefore, any construction planning should take in account those simple but crucial megastructural and seismic zonings differences whose effects surprisingly out weight by far the usually considered site effect or sediment amplification. This study is not intended to address the issues of the art associated to constructions which is very important in regions at high seismic risk but we think that the modelling of the structures responses in those seismic prone areas remain incorrectly modelled because the seismotectonic excitation signal remain either poorly understood or poorly quantified . Therefore, the practicability of their results remains expensive or inefficient.
Keywords
Atlas faults system - Seismic zoning - Sediments amplification - Resonance of a valley - Faults ruptures characteristics - Maximal acceleration- Seismic empirical relations for the Maghreb - Main components of the regional stress tensor
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