The Al Hoceima Mw 6.4 earthquake of 24 February 2004 and its aftershocks sequence |
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| Institution: | 1. Institut de Physique du Globe de Strasbourg, UMR 7516, CNRS – Université de Strasbourg, 5, rue Descartes, 67084 Strasbourg Cedex, France;2. Institut de Recherches pour le Développement, Unité de Recherches 154, France;3. Centre de Recherche en Astronomie Astrophysique et Géophysique (CRAAG), Bouzareah, 16340 Alger, Algeria;4. GeoAzur, Observatoire de la Côte d’Azur, CNRS, Université de Nice Sophia Antipolis, 250 Rue Albert Einstein, Sophia Antipolis, 06560 Valbonne, France;5. Institut de Physique du Globe de Paris, UMR7154, 1 rue Jussieu, 75238 Paris Cedex 05, France;6. Earth Observatory of Singapore, Nanyang Technological Universit N2-01A-09, 50 Nanyang Avenue, 639798, Singapore;7. Department of Geology, University Mohammed V, Faculty of Sciences, Rabat, Morocco;8. Geophysics Laboratory, CNRST, Rabat, Morocco;1. Géosciences Montpellier, UMR5243, Université de Montpellier, Montpellier, France;2. School of Earth and Ocean Sciences, University of Victoria, Victoria BC, Canada;3. Scientific Institute, Mohammed V University, Rabat, Morocco;4. International Centre for Theoretical Physics, Trieste, Italy;5. Faculté des Sciences et Techniques de Tanger, Tanger, Morocco |
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| Abstract: | The Al Hoceima Mw 6.4 earthquake of 24 February 2004 that occurred in the eastern Rif region of Morocco already hit by a large event in May 1994 (Mw 5.9) has been followed by numerous aftershocks in the months following the event. The aftershock sequence has been monitored by a temporary network of 17 autonomous seismic stations during 15 days (28 March–10 April) in addition to 5 permanent stations of the Moroccan seismic network (CNRST, SPG, Rabat). This network allowed locating accurately about 650 aftershocks that are aligned in two directions, about N10-20E and N110-120E, in rough agreement with the two nodal planes of the focal mechanism (Harvard). The aftershock alignments are long enough, about 20 km or more, to correspond both to the main rupture plane. To further constrain the source of the earthquake main shock and aftershocks (mb > 3.5) have been relocated thanks to regional seismic data from Morocco and Spain. While the main shock is located at the intersection of the aftershock clouds, most of the aftershocks are aligned along the N10-20E direction. This direction together with normal sinistral slip implied by the focal mechanism is similar with the direction and mechanisms of active faults in the region, particularly the N10E Trougout oblique normal fault. Indeed, the Al Hoceima region is dominated by an approximate ENE-SSW direction of extension, with oblique normal faults. Three major 10–30 km-long faults, oriented NNE-SSW to NW-SE are particularly clear in the morphology, the Ajdir and Trougout faults, west and east of the Al Hoceima basin, respectively, and the NS Rouadi fault 20 km to the west. These faults show clear evidence of recent vertical displacements during the late Quaternary such as uplifted alluvial terraces along Oued Rihs, offset fan surfaces by the Rouadi fault and also uplifted and tilted abandoned marine terraces on both sides of the Al Hoceima bay.However, the N20E direction is in contrast with seismic sources identified from geodetic inversions, which favour but not exclusively the N110-120E rupture directions, suggesting that the 1994 and 2004 events occurred on conjugate faults. In any event, the recent seismicity is thus concentrated on sinistral N10-20E or N110-120E dextral strike-slip faults, which surface expressions remain hidden below the 3–5 km-thick Rif nappes, as shown by the tomographic images build from the aftershock sequence and the concentration of the seismicity below 3 km. These observations may suggest that strain decoupling between the thrusted cover and the underlying bedrock and highlights the difficulty to determine the source properties of moderate events with blind faults even in the case of good quality recorded data. |
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| Keywords: | Al Hoceima Seismicity Aftershocks sequence Focal mechanisms Tomography Teleseismic inversion |
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