Sismicité en algerie de 1790 à 2000 : mécanismes aux foyers et vitesse de rapprochement inter-plaques

Plus de huit cents (800) événements sismiques de magnitude supérieure ou égale à 4, ont permis de dresser une carte des épicentres pour le Nord de l'Algérie et les régions limitrophes (31°.-38°N; 3°W-9°E), pour la période 1790-2000. Ce document qui fait ressortir le caractère hautement sismique des chaînes littorales de l'Algérie, est assorti d'une représentation des solutions focales de vingt séismf'o 1,,~ plus significatifs, associés à la période 1954-2000. Lt' régime eompressif auquel est soumise la région d'étude, induit par la collision des Plaques Afrique et Eurasie, est caractérisé par une direction NNW-SSE très proche de l'horizontale, donnée par l'orientation des axes P des solutions focales. Un calcul théorique des vecteursvitesses inter-plaques fournit un azimut de 330°, compatible avec la direction de convergence obtenue à partir des axes P. Le taux moyen de rapprochement calculé est très voisin de 6mmja

The Oran January 9th (Mw 4.7) and June 6th, 2008 (Mw 5.4) earthquakes: Seismological study and seismotectonic implication

The Oran region (Western Tell Atlas) was hit by two earthquakes on January 9th, 2008 (Mw = 4.7) and on June 6th, 2008 (Mw = 5.4). The two events were recorded by the national accelerographs network maintained by National Center of Applied Research in Earthquakes Engineering (CGS, Algiers). This work aims to characterize these two earthquakes in order to contribute to understand the seismotectonic context of the study region. We located the events using Grid Search Method based on P and S waves arrival time differences. The obtained hypocenter locations are, respectively, (0.457°W, 35.689°N, 11 km) and (0.576°W, 35.689°N, 8 km). We perform waveforms inversion to estimate the moment magnitudes and the seismic moments which are, respectively, Mw=4.7,M0 =0.13E+17NmandMw=5.4,M0 =0.1810E+18Nmforthetwoevents.Concerningthe focal mechanism solutions both earthquakes yield reverse faulting with a compressional axes oriented NW-SE which is consistent with the active fault system trend in the study region. These two events occurred in an area which has been seismically quiet, since the event of December 1959 (Ms 4.7) and the historical major event of October 9th, 1790 (I0= IX-X; likely Ms ≥ 6.0).This work is carried out as a part of research activities of the National Center of Applied Research in Earthquakes Engineering (CGS, Algiers)

Seismic empirical relations for the Tellian Atlas, North Africa, and their usefulness for seismic risk assessment

Seismic events that occurred during the past half century in the Tellian Atlas, North Africa, are used to establish fundamental seismic empirical relations, tying earthquake magnitude to source parameters (seismic moment, fault plane area, maximal displacement along the fault, and fault plane length). Those empirical relations applied to the overall seismicity from 1716 to present are used to transform the magnitude (or intensity) versus time distribution into (1) cumulative seismic moment versus time, and (2) cumulative displacements versus time. Both of those parameters as well as the computed seismic moment rate, the strain rate along the Tellian Atlas strike, and various other geological observations are consistent with the existence, in the Tellian Atlas, of three distinct active tectonic blocks. These blocks are seismically decoupled from each other, thus allowing consideration of the seismicity as occurring in three different distinct seismotectonic blocks. The cumulative displacement versus time from 1900 to present for each of these tectonic blocks presents a remarkable pattern of recurrence time intervals and precursors associated with major earthquakes. Indeed, most major earthquakes that occurred in these three blocks might have been predicted in time. The Tellian Atlas historical seismicity from the year 881 to the present more substantially confirms these observations, in particular for the western block of the Tellian Atlas. Theoretical determination of recurrence time intervals for the Tellian Atlas large earthquakes using Molnar and Kostrov formalisms is also consistent with these observations. Substantial observations support the fact that the western and central Tellian Atlas are currently at very high seismic risk, in particular the central part. Indeed, most of the accumulated seismic energy in the central Tellian Atlas crust has yet to be released, despite the occurrence of the recent destructive May 2003 Boumerdes earthquake (M (w) = 6.8). The accumulated seismic energy is equivalent to a magnitude 7.6 earthquake. In situ stress and geodetic measurements, as well as other geophysical field data measurements, are now required to practically check the validity of those observations