The 1994-2004 Al Hoceima (Morocco) Earthquake Sequence: Conjugate fault ruptures deduced from InSAR

Ahmet M. Akoglu(1) , Ziyadin Cakir(2) , Mustapha Meghraoui(3) , Nasser Jabour(4) , Lahsen Ait-Brahim(5) , and Semih Ergintav(6)

(1) ITU, Maslak, 34469 Istanbul, Turkey
(2) IPG Strasbourg, UMR 7516, 5, rue Rene Descartes, 67084 Strasbourg, France
(3) EOST, 5 rue Rene Descartes, 67084 Strasbourg, France
(4) CNRST, 4 Avenue Ibn Battouta, 1014 Rabat, Morocco
(5) Universite Mohammed V, 52, Charii Omar Ibn El Khattab, 10102 Rabat, Morocco
(6) TUBITAK MRC, Gebze, 41470 Izmit, Turkey

Abstract

The Al Hoceima region of northern Morocco was struck by two moderate-to-large earthquakes, first on May 26, 1994 (Mw=6.0) and later on February 24, 2004 (Mw=6.4). The focal mechanism solutions indicate that they are associated with strike-slip faults trending either NE-SW with left-lateral slip or NW-SE with right-lateral slip. However, the exact location and kinematics of these earthquakes are not known in detail because neither of them produced surface ruptures. Field observations show that the two earthquakes took place on nearby blind strike-slip faults. In the absence of surface faulting and complex aftershocks distribution in a region like northern Morocco where morphology does not provide clear signals of active strike-slip faults, InSAR is among the best methods used to characterize the seismic source parameters accurately and in detail. We therefore calculated interferograms that capture the surface displacement field of the two earthquakes both in the ascending and descending geometry of the ERS and Envisat satellites. The analysis of the interferograms and subsequent elastic modeling suggest that the two mainshocks occurred on conjugate strike-slip faults; the 1994 earthquake being associated with N20°E trending left-lateral fault and the 2004 earthquake with N50°W trending right-lateral fault. This result contradicts previous inferences on the kinematics, location and rupture geometry of the earthquakes deduced from conventional analyses of seismic waveforms and aftershocks distribution. Thrust-and-fold structures of the Rif Mountains developed during the Tertiary period but the recent significant seismic events and late Quaternary deformation indicate east-west extension accommodated by NS trending normal and NW-SE and NE-SW trending conjugate strike-slip faults. The InSAR analysis reveals the fragmentation of the Rif Mountain throughout a complex network of blind faults. This active deformation illustrates the Africa-Iberia collision and west to southwestward escape tectonics along the plate boundary.

 

Workshop presentation

 

  Higher level                 Last modified: 07.10.03