Stress field variations along the Maghreb region derived from inversion of fault plane solutions.

International audienceThis study is devoted to the analysis of the stress state along the Maghreb region based on the inversion of focalsolutions.Wehave inverted themain shock and aftershock focalmechanisms of the strongest seismic events thatoccurred in five seismogenic zones, from west to east: Al Hoceima (2004), Cheliff (1980), Tipasa–Chenoua(1989), Zemmouri (2003) and Constantine (1985). Most of the focal mechanisms of the aftershock sequenceshave been constructed within this study. Compressive stress regime is observed in the central part of Algeria betweenCheliff and Zemmouri. On both edges of the Maghreb region, the stress regime becomes strike-slip in theConstantine region and in the Moroccan Rift. These different regimes seem to be linked to the free-edge effect(Ionian slab subduction) and to the dynamics of the Alboran Sea in the eastern and western parts of the studyarea respectively. The σ1 directions experience an anticlockwise rotation of about 20° from eastern to centralAlgeria. We observe that the direction of σ1 and the direction of convergence are the closest in central Algeria,where the collision is not perturbed by edge effect

The Constantine (Algeria) seismic sequence of 27 October 1985: a new rupture model from aftershock relocation, focal mechanisms, and stress tensors

International audienceThe October 27, 1985 Constantine earthquake of magnitude MS 5.9 (NEIC) although moderate is the strongest earthquake recorded in the eastern Tellian Atlas (northeast Algeria) since the beginning of instrumental seismology. The main shock locations given by different institutions are scattered and up to 10 km away northwest from the NE-SW 30 km long elongated aftershocks cloud localized by a dedicated temporary portable network. The focal mechanism indicates left-lateral strike-slip on an almost vertical fault with a small reverse component on the northwest dipping plane. This paper presents relocations of the main shock and aftershocks using TomoDD. One hundred thirty-eight individual focal mechanisms have been built allowing the determination of the stres

Revisiting the Laalam (Eastern Algeria) March 20, 2006 (Mw 5.1) Earthquake and its Seismotectonic Implication

Hypocenters of main shock and aftershocks of the March 20, 2006 Laalam earthquake are relocated using HypoDD double-difference technique. We combined accelerogram and seismogram data of the National Center of Applied Research in Earthquake Engineering (CGS). Among about 191 aftershocks, recorded at least by 4 stations, 141 aftershocks of Md 1.2–2.7 were relocated using HypoDD. The obtained swarm of epicenters occupying a crustal volume of 5 km 9 3 km 9 5 km and the focal mechanism corresponds to an unknown pure left lateral strike slip, trending N174 E. We were able to calculate focal mechanisms for only seven aftershocks with more than seven polarity readings, which give a P axis oriented NNW–SSE. The waveform inversion also provided values of Mw 5.1, M0 5.9 10-16 Nm and the depth 4.9 km. The dislocation and the stress drop were estimated to 90 cm and 16.5 bars, respectively

Algerian’s Seismic Catalogue Completeness from Historical Instrumental Monitoring, Archeoseismological and Paleoseismological Studies.

For any seismic hazard study, a reliable, homogenized and complete seismic catalogue is required. The Algerian seismicity catalogue has been recently updated by retrieving and reappraising many historical events. The Algerian seismic network has also been densified up to about 80 seismic stations covering the Tell Atlas which is the most active area of northern Algeria for monitoring of the seismic activity reducing the magnitude threshold. Recently, we have launched archeoseismological studies to retrieve past strong earthquakes that have affected Roman sites located along the Tell Atlas. Here, we proceed with tectonic investigations around selected sites where significant observed damage was identified. On the other hand, paleoseismological investigations were con- ducted along the El Asnam fault (now Chlef) following the large Ms 7.3 earthquake of 1980. Paleoseismic studies combined with archeoseismological results provide the dating of past earthquakes and contribute to the com- pleteness of the seismicity catalogue

The Zemmouri-Boumerdes Earthquake of May 21st, 2003, Mw=6.8: Source Parameters and Rupture Propagation Study from Teleseismic Data.

On May 21, 2003, occurred the Zemmouri-Boumerdes earth- quake (Mw=6.8, depth 7km) in a zone characterized by rela- tively moderate and diffuse seismicity. The main shock have been relocated at Zemmouri el Bahri (36.83N,3.65E) close to the continent and the aftershocks sequence (CRAAG mobile stations) gives a distribution in the NE-SW direction with most of the epicenters located on the continent or near the coast. Source mechanism of this event, including fault plane solutions, waveform inversion, spectral analysis, fault slip distribution and displacement field model, is analysed. The results show pure thrusting motion (plane striking 64o and dipping 50o to the SSE) with a STF formed by two main asperities at shallow depth (7 and 4km respectively) and Mo=1.3x1019Nm. Slip distribution show an E-W asymmetric bilateral rupture process. The rupture front through the region (Zemmouri-Boumerdes) of high slip (206 cm) and maximum moment released (50% of the total Mo), in direction of Algiers, produced a pulse of energy about 8s after the start of the earthquake. This strong subevent located at SW of the hypocenter started at 4.0km depth and probably broke the surface. The vertical displacements estimated from the fault slip distribution model, the main shock relocation and the aftershock distribution suggest a fault close to the coastline as origin of the Zemmouri-Boumerdes earthquake

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

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

Seismicity of the Algerian tell atlas and the impacts of major earthquakes

The seismicity of the Tell Atlas, which extends from the Algerian margin to the South Atlasic fault system, is related to the dynamics of Quaternary basins under an oblique NW–SE convergent stress regime, including the basins of Mleta and L’Habra in the west, Cheliff and Mitidja in the centre, and Soummam, Hodna and Guelma in the east. This seismicity is characterized by moderate to low magnitudes with strong events occurring generally once a decade. Over the last six decades, several moderate, strong and major events occurred that were associated with extensive and severe damage, such as those of El Asnam (1954, Ms 6.7; 1980, Ms 7.3), Constantine (1985, Ms 6.0), Tipasa–Chenoua (1989, Ms 6.0), Mascara (1994, Ms 6.0), Ain Temouchent (1999, Ms 5.8), Beni Ouartilane (2001, Ms 5.6), Zemmouri—Boumerdes (2003, Mw 6.8) and Laalam (2005, Ms 5.8), in addition to numerous large historical seismic events, including those that occurred in Algiers (1365 and 1716, Io = X), Oran (1790, Io = X), Mascara (1819, Io = X), Djidjelli (1856, Io = VIII) and M’sila (1885, Io = IX). This chapter presents a review of the seismicity of North Algeria and a detailed analysis of the main earthquakes that have occurred in the Tell Atlas since 1980. Finally, the impacts of several signif- icant earthquakes that occurred during the period between 1364 and 2015 are presented and discussed in terms of seismic energy.Acknowledgements This study was conducted within the scope of the MEDYNA FP7-PEOPLE-2013-IRSES project, WP-1: Present-day Kinematics and seismic hazards, funded by the Seventh Framework European Programme. The authors acknowledge the support provided by the Instituto de Ciências da Terra da Universidade de Évora (Portugal) and the Centre de Recherche en Astronomie, Astrophysique et Géophysique (Algiers, Algeria), under contract with the Portuguese Science and Technology Foundation (FCT, Portugal), PEst-OE/CTE/UI0078/2011. We would like to thank the Editors, Prof. Abderrahmane Bendaoud and Prof. Mohamed Hamoudi (Algiers University, USTHB), for their help

Zemmouri earthquake rupture zone (Mw 6.8, Algeria) : aftershocks sequence relocation and 3D velocity model

International audienceWe analyze the aftershocks sequence of the Zemmouri thrust faulting earthquake (21 May 2003, M w 6.8) located east of Algiers in the Tell Atlas. The seismic sequence located during ~2 months following the mainshock is made of more than 1500 earthquakes and extends NE-SW along a ~60-km fault rupture zone crossing the coastline. The earthquake relocation was performed using handpicked P and S phases located with the tomoDD in a detailed 3D velocity structure of the epicentral area. Contrasts between velocity patches seem to correlate with contacts between granitic-volcanic basement rocks and the sedimentary formation of the eastern Mitidja basin. The aftershock sequence exhibits at least three seismic clouds and a well-defined SE-dipping main fault geometry that reflects the complex rupture. The distribution of seismic events presents a clear contrast between a dense SW zone and a NE zone with scattered aftershocks. We observe that the mainshock locates between the SW and NE seismic zones; it also lies at the NNS-SSE contact that separates a basement block to the east and sedimentary formations to the west. The aftershock distribution also suggests fault bifurcation at the SW end of the fault rupture, with a 20-km-long ~N 100° trending seismic cluster, with a vertical fault geometry parallel to the coastline juxtaposed. Another aftershock cloud may correspond to 75° SE dipping fault. The fault geometry and related SW branches may illustrate the interference between pre-existing fault structures and the SW rupture propagation. The rupture zone, related kinematics, and velocity contrasts obtained from the aftershocks distribution are in agreement with the coastal uplift and reflect the characteristics of an active zone controlled by convergent movements at a plate boundary