The 1994–2004 Al Hoceima (Morocco) earthquake sequence: Conjugate fault ruptures deduced from InSAR

We study the May 26, 1994 (Mw = 6.0) and February 24, 2004 (Mw = 6.4) earthquakes that affected the Al Hoceima region of northern Morocco. These events are the two strongest earthquakes recorded in this region. Yet, the exact location, kinematics and relationships between these earthquakes are poorly known since neither of them produced surface ruptures. Using Synthetic Aperture Radar interferometry (InSAR) we mapped the surface displacement field of the two earthquakes to characterize their seismic source parameters. Analysis of the interferograms constructed from ascending and descending orbits for both earthquakes and subsequent elastic modeling using slip inversions on triangular fault patches suggest that the two mainshocks occurred on blind conjugate strike–slip faults; the 1994 earthquake being associated with N23° E trending left-lateral fault and the 2004 earthquake with N45° 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. The InSAR analysis reveals the fragmentation of the Rif Mountain throughout a complex network of conjugate blind faults consistent with the transpression tectonics along the plate boundary in North Africa. Although the two earthquakes took place in the Rif thrust-and-fold belt, the late Quaternary deformation indicates E–W extension in agreement with the NW–SE and NE–SW trending conjugate strike–slip faulting.