The Al Hoceima Mw 6.4 earthquake of 24 February 2004 and its aftershocks sequence

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.

Introduction

The region of Al Hoceima, Morocco, was shaken by a strong Mw 6.4 earthquake (CSEM, USGS, Harvard) on February 24th, 2004 at 2h27mn (UTC) (Jabour et al., 2004). It is the largest earthquake since at least two centuries (Fig. 1; El Mrabet, 2005a, El Mrabet, 2005b, Pelaez et al., 2007) in the Al Hoceima region considered as the most seismically active of Morocco (Cherkaoui et al., 1990, El Alami et al., 1998, Bezzeghoud and Buforn, 1999). An earthquake of magnitude Mw 6.0 occurred on 2nd May 1994 in the same area about 10 km to the west-northwest of the 2004 event in the morning of a spring day, when most of the local people were in the fields and only few casualties were to deplore. The 2004 earthquake occurred in the night, in the winter season, and caused more than 800 fatalities. While the magnitude of the 2004 earthquake was larger, this difference alone does not explain the difference in human loss. Traditional houses made of stones and adobe and heavy roofs suffered considerable destructions while the modern building in Al Hoceima experienced weak damages. However, recent buildings suffered heavy damages, as far as complete collapse in Imzouren, some 12 km south-east of Al Hoceima (Fig. 2) resulting mainly from bad design (Murphy Corella, 2005) though without significant site effect (Goula et al., 2005). An accelerometer installed close to a dam on the Nekor river, about 10 km southeast from the epicentre recorded a maximum horizontal acceleration of 0.24 g, a relatively high value (CNRST, 2004).

Despite several geophysical studies since the earthquake (Stich et al., 2005, Cakir et al., 2006, Akoglu et al., 2006, Biggs et al., 2006, Tahayt et al., 2009), the source of the 2004 event remains debated and puzzling (Galindo-Zaldivar et al., 2009). Indeed, similarly to the 1994 event, the 2004 event did not rupture to the surface and occurred along a fault that has no cumulative surface expression, to the contrary of well expressed normal faults in the Nekor basin (Trougout and Ajdir faults) and to the west (Rouadi fault), or such as the Nekor strike-slip fault (Morel, 1988, Van der Woerd et al., 2005) (Fig. 2, Fig. 3). Surface deformations (open cracks, ground ruptures, landslides) were observed mainly along a 10 km-wide strip running from Ajdir to Beni Abdellah, in a roughly NNE-SSW direction (Fig. 2, Fig. 4; Ait Brahim et al., 2004), but modelling of radar interferograms (InSAR) suggest a WNW-ESE fault plane (Cakir et al., 2006, Akoglu et al., 2006, Tahayt et al., 2009, Biggs et al., 2006). In addition, the aftershock clouds extend along two almost perpendicular directions similarly to the directions of the nodal planes of the main shock focal mechanism (this study; Harvard catalogue; USGS) with the main shock at the intersection of the two clouds (Fig. 2b) making any attempt to link the seismicity to a preferential fault plane difficult.

The Rif region of northern Morocco belongs to the complex plate boundary between western Africa and western Europe, which extends for more than 1500 km E-W from Portugal to Tunisia and 1000 km N-S from France to south Morocco, and comprises several collisional belts, the Pyrenees, the Betics, the Rif, the Middle Atlas, the Atlas, and the Tellian Atlas (Fig. 1). North of the Rif, the Alboran Sea, characterized by a thinned continental crust, resulted from extension in a back arc setting during the Oligo-Miocene (Watts et al., 1993, Seber et al., 1996). The knowledge of the present day tectonics in the Rif has strong implications in the understanding of strain pattern in the western Mediterranean region (Ait Brahim and Chotin, 1984, Ait Brahim et al., 1990, Morel and Meghraoui, 1996, Lopez Casado et al., 2001). While the convergence between western Africa and western Europe can be assessed by plate tectonic models or GPS velocity fields (DeMets et al., 1990, DeMets et al., 1994, Nocquet and Calais, 2003, Nocquet, 2012), the manner and where the strain is taken into account remains debated, mostly because the rates of deformation are slow and need to be averaged over long time intervals.

Both earthquakes of 1994 and 2004 are characterized by blind ruptures that have not propagated to the surface making their seismotectonic characterizations challenging. The challenge is re-enforced as these two earthquakes are the largest known in this part of the Rif since several centuries (El Mrabet, 2005a, Pelaez et al., 2007). It has been proposed that the ruptures remained located below a structural level made of shallow dipping décollement, remnants of the Miocene Rif nappes (Galindo-Zaldivar et al., 2009, Negro et al., 2007, Mattauer, 1964). However, the nearby occurrence of active faults shaping the landscape of the lower Nekor basin as well as the well-known Nekor and Jebbha strike-slip-faults remains to be explained (Leblanc and Olivier, 1984).

To better document the deformation of the Al Hoceima region, we present here an aftershocks study of the 2004 event together with a review of recent seismological and geological data.

The main shock of February 24, 2004 was followed by a large number of aftershocks, some of them with magnitude greater than 5 between February 25th and 28th, and on March 20th (Table S3). The main shock and the aftershocks were located independently by the national seismological network of Spain and Morocco and by the European-Mediterranean Seismological Center. The analysis of the aftershocks we present here results from 15 days of recording with a temporary network installed in the epicentral area from March 29 to April 11 2004 (Dorbath et al., 2005; Table 1). The main shock and the largest aftershocks were relocated. The P and SH waveforms recorded at teleseismic distances were then inverted. We then compare these results with the main tectonic features of the region.