ZusammenfassungSprengseismische Untersuchungen wurden 1986 l~ings einer etwa 350 km langen Traverse durchgeftihrt, die NNW-SSO verl~iuft und den Mittleren und den Hohen Atlas quert. Zwei weitere Profile verlaufen E-W durch den Mittleren und NW-SO durch den Hohen Atlas.Der Hohe und der Mittlere Atlas besitzen keine ausgepr~igte Gebirgswurzel. Die maximale Krustendicke, wird mit 38-39 km unter dem n6rd-lichen Rand des Hohen Atlas angetroffen. Stidlich und n6rdlich vom Hohen Atlas betr~igt die Krustenm~ichtigkeit 35 km. Sowohl die obere als auch die untere Kruste sind in vertikaler Richtung stark strukturiert. Auf allen beobachteten Profilen ist ein Wechsel zwischen hohen und niedrigen Geschwindigkeiten ftir die gesamte Kruste charakteristisch. Die Geschwindigkeiten im obersten Mantel sind mit Werten von 7.7-7.9 km/s relativ niedrig. Die Durchschnittsgeschwindigkeiten ftir die gesamte Kruste liegen bei 6.1-6.2 km/s. AbstractIn 1986 explosion seismic investigations have been carried out along a traverse of about 350 km length running from NNW to SSE crossing the High and Middle Atlas. Two further profiles run E/W through the Middle and NW-SE through the High Atlas.Neither the High nor the Middle Atlas mountains have a significant root. The maximum thickness of crust with 38-39 km is found under the northern border of the High Atlas. South and north of the High Atlas the crustal thickness amounts to 35 km. The upper as well as the lower crust are stongly structurized in the vertical direction. Significant for the profiles observed is the change between high and low velocities in the whole crust. The velocities at the uppermost mantle are relatively low with values of 7.7-7.9 km/s. The average velocities for the whole crust are 6.1-6.2 km/s. R6sum6Durant l'ann6e 1986 des sondages sismiques de tirs ont 6t6 entrepris le long d'une traverse d'enviton 350 km de longueur se dirigeant du nord nordouest au sud sud-est et parcourant le Haut et Moyen Atlas. Dettx autres profils parcourent le Moyen Atlas de l'est ~t l'ouest et le Haut Atlas du nord-ouest au sud-est.Le Haut et Moyen Atlas ne poss6dent pas de racine montagneuse. I26paisseur maximale de 38-39 km de la crofite a 6t6 localisde sous le bord nord du Haut Atlas. Au sud et au nord du Haut Atlas l'6paisseur de la crofite s'dl6ve ?a 35 kin. La crofite sup6rieure ainsi clue la crofite inf6rieure est tr6s structur6e en direction verticale.
The present paper focuses on the moderate earthquake that occurred on May, 26, 1994, at 8 h 27 min, which caused great damage and two deaths. The epicentre of the main shock was located at 35.280N, 3.990W. The focal depth was 13 km, and the magnitude (Md) attained 5.6. A field survey of the earthquake effects showed that the maximal intensity (VIII-IX EMS) follows an elongated corridor trending NNE-SSW, where 80% of the constructions were destroyed. During the 14-day survey carried out with the help of the temporary network established in the area, 512 shocks were located. The best constrained epicentres (68) are shallow and are largely distributed over a NNE-SSW trending cluster along an almost-vertical plane. Focal mechanisms determined for the main shock and for the 7 largest aftershocks, correspond to strike-slip faulting with a reverse (main shock and one aftershock) or normal component (6 events). The P axes have a NNW-SSE trend, with variable plunge, whereas the T axes are ENE-WSW with a slight plunge. The state of stress determined with the help of these mechanisms corresponds to a strike-slip regime with s1 oriented NNW-SSE and s3 ENE-WSW, which is in conformity with previous studies. The present study also shows that the Nekor fault remained inactive during the seismic crisis of 1994, as during the previous surveys, and this casts some doubt on the present-day role of this major fault. Instead, as proposed by some authors, a seismic zone trending NNE-SSW may be related to the faults of the same trend that appear to cross the Al Hoceima area towards the Alboran Sea.
A seismic data file of 3,740 earthquakes from January 1987 to December 1994 has been elaborated for Morocco and the border regions, with 10 main events registering magnitudes from 5 to 5.6. Such seismicity is particularly important for Morocco as the released seismic energy constitutes a considerable part of the total energy radiated during the 20th century. Relative seismicity maps confirm the persistence of the major features of the seismicity of Morocco. An important seismic activity is observed in the Alboran region continental crust, which absorbs the maximum deformation resulting from the convergence of the African and Iberian plates. However, in the longitude window 3.5°-6°W at depths of 25 to 50 km, a seismic gap zone seems to take place. An explanation of this phenomenon may be provided by the slab breakoff model. Even if the seismicity of Morocco remains moderate, heavy damage is observed when the magnitude of earthquakes exceeds 4.5, especially in the case of traditional buildings.
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