Stress field variations along the Maghreb region derived from inversion of major seismic crisis fault plane solutions
Introduction
The continental tectonic deformation observed along the Ibero-Maghreb region is the expression of the convergence since the Cretaceous between the Eurasia and Africa plates (Le Pichon et al., 1988). The boundary between the two last plates is well delimited from the Azores triple junction to the strait of Gibraltar. To the east, the boundary becomes more diffused and forms a larger region of complex deformation (Buforn et al., 2004). The Maghreb area is part of this diffuse boundary and consequently displays a complex pattern of active deformation, which is partly taken up seismically. Deep basins and fault-and-thrust belts characterize the morphology of this area.
The main tectonic domain in the Maghreb region is the Atlas Mountains, consisting of the Moroccan Middle and High Atlas, the Algerian Sahara and Tellian Atlas and the Tunisian Atlas, which were uplifted during the Cenozoic (Fig. 1). To explain the complex tectonics of this region, based on the spatial distribution of seismicity, several authors have suggested the existence of several micro-plates and relative motion between them (Anderson and Jackson, 1987, Dewey et al., 1989, McKenzie, 1972, Sengör and Canitez, 1982). The kinematic of the Africa and Eurasia plates defined by the position of rotation poles and the angular velocities have been determined using slip vectors of large earthquakes, sea floor spreading rates and transform fault trends (Anderson, 1985, Argus et al., 1989, DeMetz et al., 1990, DeMetz et al., 1994, McKenzie, 1972, Minster and Jordan, 1978). More recently, GPS data were added to the same information (McClusky et al., 2003, Nocquet, 2012, Nocquet and Calais, 2004, Sella et al., 2002, Serpelloni et al., 2007). As a conclusion, all these studies indicate that these poles are situated in the central Atlantic, however, with large discrepancies in the latitude, from 20°N to 20°S. Along the plate boundary, from Tunisia to Gibraltar, the convergence direction presents an anti-clockwise rotation and the velocity decreases.
The general stress pattern shows extension in the Azores region, right-lateral strike slip motion in the Gibraltar strait and Alboran Sea, and compression in the Maghreb Area (Buforn et al., 2004). More recently, de Vicente et al. (2008) performed an active stress inversion around and within the Iberian peninsula. They concluded that from the Terceira Ridge to the Gulf of Cadiz, the stress progressively changes from triaxial extension to uniaxial compression while in the Betics–Alboran–Rift zone and in northern Algeria uniaxial extension and uniaxial compression respectively predominate.
The recent seismic activity gives us the opportunity to determine the stress field in the Maghreb region more accurately. In this work, we present results obtained from the inversion of the focal mechanism of the 5 strongest seismic events that occurred in the region and their aftershocks during the time span of 1980–2004. These are the only earthquakes that were the object of aftershock studies conducted with a dense network of seismic stations that allow a precise determination of the focal mechanisms. The selected events are those of El Asnam 1980, Ms 7.3; Constantine 1985, Ms 6.0; Chenoua–Tipasa 1989, Ms 5.8; Zemmouri 2003, Mw 6.8 and Al Hoceima 2004, Mw 6.4.
Section snippets
Tectonic setting and seismicity
During the Mesozoic and Cenozoic periods, the Mediterranean region experienced, complex tectonic events and associated collisions that resulted in mountain building, plateau formation, foreland and hinterland deformation, foreland flexure and sedimentary basin evolution (Dilek, 2006). The Maghreb region is commonly divided into specific structural domains: the Atlas Mountains, the High Plateau rigid cores and the Tell–Rif system bordering the Mediterranean Sea (Fig. 1). The Atlas Mountain
Focal mechanism solution database
As mentioned in the introduction, we focused on the area where large earthquakes occurred and where a dense seismic station network was deployed afterwards, which allowed the construction of numerous well constrained focal mechanism solutions (FMSs). Our database consists of these FMSs together with the CMT solutions of the main events and few early large aftershocks. The database encompasses 632 FMSs, our main contribution consisting of 433 new solutions for the period 1980–2004, constructed
Stress inversion method
The focal mechanism represents the fracture that occurred at depth under the local stress regime (Arthaud, 1969). The determination of the tectonic regime in an area is obtained under two assumptions: the stress field is homogeneous over the spatial and temporal extent of the event, and the focal mechanisms are adequately diverse (Hardebeck and Hauksson, 2001). Several methods exist to retrieve the stress tensor that brings the shear tractions resolved on the fault planes into alignment with
Stress inversion results
The results of the stress tensors for the selected major earthquake source regions are presented in Table 6 and Fig. 8. The mechanisms that are rejected by the inversion process are differentiated by color on the figures related for each sequence (Fig. 3, Fig. 4, Fig. 5, Fig. 6, Fig. 7). These events are spread in the aftershock cloud within any clear organization.
Discussion and conclusion
Within this study we have constructed a FMS database containing 632 solutions. Part of it consist of a compilation of focal solutions built from P-wave polarities from studies by others (Ouyed et al., 1983, Yielding et al., 1989), by inversion of broadband seismograms (Braunmiller and Bernardi, 2005), and CMT solutions. We have constructed the remaining by processing the first P-arrival onsets of aftershocks following large earthquakes, recorded and located by dense temporary seismic networks.
Acknowledgments
This study has been supported by the CRAAG/Algiers, the EOST/University of Strasbourg, and the TASSILI CMEP project (11 MDU 847). We thank Said Maouche for fruitful discussion and comments on tectonic framework and Seid Bourouis for his help. We are grateful to Rob Govers and the two anonymous reviewers for their fruitful comments. The figures were generated by the Generic Mapping Tool (GMT) code developed by Wessel and Smith (1998).
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