CRAAG

Algiers, Algeria
Algiers, Algeria

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Sawires R.,Assiut University | Pelaez J.A.,University of Jaén | Fat-Helbary R.E.,Aswan Regional Earthquake Research Center | Panzera F.,University of Catania | And 2 more authors.
Pure and Applied Geophysics | Year: 2017

A probabilistic seismic hazard analysis in terms of peak ground acceleration (PGA) and spectral acceleration (SA) values has been performed for the Egyptian territory. Eighty-eight potential seismic sources (for shallow- and intermediate-depth seismicity) in and around Egypt were identified and characterized based on an updated and unified earthquake catalog spanning the time period from 2200 B.C. until 2013 A.D. A logic-tree approach was followed, after a sensitivity analysis, to consider the epistemic uncertainty in the different input parameters, including the selected ground-motion attenuation models to predict the ground motion for the different tectonic environments. Then the seismic hazard deaggregation results, in terms of distance and magnitude, for the most important cities in Egypt have been computed to help understanding the relative contributions of the different seismic sources. Seismic hazard deaggregation, in particular, was computed for PGA and SA at periods of 0.2, 1.0 and 2.0 s for rock-site conditions, and for 10% probability of exceedance in 50 years. In general, the results at most of the cities indicate that the distance to the seismic sources which mostly contribute to the seismic hazard is mainly controlled by the nearby seismic sources (especially for PGA). However, distant events contribute more to the hazard for larger spectral periods (for 1.0 and 2.0 s). A significant result of this type of work is that seismic hazard deaggregation provides useful data on the distance and magnitude of the contributing seismic sources to the hazard in a certain place, which can be applied to generate scenario earthquakes and select acceleration records for seismic design. © 2017, Springer International Publishing.


Nouar O.,CRAAG | Henry B.,University of Paris Pantheon Sorbonne | Liegeois J.P.,Royal Museum for Central Africa | Derder M.E.M.,CRAAG | And 6 more authors.
Journal of African Earth Sciences | Year: 2011

In the Arokam Ténéré, the three studied massifs of granitoids are located around the N-S oriented Raghane mega-shear zone, which separates two of the main domains of the Tuareg shield, the Saharan metacraton and the Central Hoggar. The field observations, AMS analyses and U-Pb zircon dating were completed by a study of Landsat images because of the scarcity of outcrops in several parts of the studied area. The image analysis allows to distinguish Arokam-East and Abdou granitoids in the eastern plutonic complex. It has also shown that the western plutonic complex corresponds to two different intrusions: the Yvonne granite and granodiorite. This is confirmed by the magnetic fabric that presents different characteristics in the different granitoids. U-Pb zircon dating and field observations show that the Arokam-West basement granite is much older (1915. Ma) than the Hanane granodiorite (582. Ma) in the central plutonic complex. Arokam-East and -West granites then belong to the Eburnean basement. The magnetic fabric of these granites is mainly associated with a post-magmatic deformation, probably of Pan-African age. Yvonne granodiorite is likely contemporaneous of the main displacement along the Raghane shear zone. The Yvonne granite (594. Ma) and Hanane granodiorite have a fabric similar to that previously obtained in most plutons of the Tiririne area. This fabric is related to the regional stress field, associated with the activity of the Raghane shear zone, during the late-magmatic phase in the plutons. On the contrary, the magnetic fabric of the Abdou pluton is still reflecting only the magma flow. © 2011 Elsevier Ltd.


Maouche S.,CRAAG | Maouche S.,University of Science and Technology Houari Boumediene | Meghraoui M.,Institute Of Physique Du Globe | Morhange C.,CEREGE | And 3 more authors.
Tectonophysics | Year: 2011

Major uplifts of late Quaternary marine terraces are visible along the coastline of the Tell Atlas of Algeria located along the Africa-Eurasia convergent plate boundary. The active tectonics of this region is associated with large shallow earthquakes (M ≥ 6.5), numerous thrust mechanisms and surface fault-related fold. We conducted a detailed levelling survey of late Pleistocene and Holocene marine notches in the Algiers region that experienced 0.50. m coastal uplift during the 2003 Zemmouri earthquake (Mw 6.8). East of Algiers, Holocene marine indicators show three pre-2003 main notch levels formed in the last 21.9. ka. West of Algiers on the Sahel anticline, the levelling of uplifted marine terraces shows a distinct staircase morphology with successive notches that document the incremental folding uplift during the late Pleistocene and Holocene. The timing of successive uplifts related to past coseismic movements along this coastal region indicates episodic activity during the late Holocene. Modelling of surface deformation in the Zemmouri earthquake area implies a 50-km-long, 20-km-wide, NE-SW trending, SE dipping fault rupture and an average 1.3 m coseismic slip at depth. Further west, the 70-km-long Sahel fold is subdivided in 3 sub-segments and shows ~0.84-1.2 mm/yr uplift rate in the last 120-140. ka. The homogeneous Holocene uplift of marine terraces and the anticline dimensions imply the possible occurrence of large earthquakes with Mw ≥ 7 in the past. The surface deformation and related successive uplifts are modelled to infer the size and characteristics of probable future earthquakes and their seismic hazard implications for the Algiers region. © 2011 Elsevier B.V.


Derder M.E.M.,C.R.A.A.G. | Henry B.,University Paris Diderot | Maouche S.,C.R.A.A.G. | Bayou B.,C.R.A.A.G. | And 6 more authors.
Tectonophysics | Year: 2013

The present paleomagnetic study has been conducted on volcanic rocks of Miocene age outcropping on the northern border of the Neogene Chelif basin, northwest Algeria. The results show the existence of numerous small tectonic blocks, of probable size around 0.5 to 0.6. km, which underwent clockwise rotations. The magnitude of these rotations is often important and of different strengths according to the studied sites. That evidences the effect of a major narrow dextral E-W crustal shear structure. The location of this structure on the northern border of Chelif basin is coherent with the geodynamical context of this basin, which corresponds to a wide zone affected by clockwise rotations of large blocks limited by such major faults. The present paleomagnetic results confirm that the relative convergence motion between the Africa and Eurasia plates could be interpreted as a transpressional tectonic deformation model with block rotations along the Algerian continental margin. © 2013 Elsevier B.V.


Sardou M.,University Sidi Mohammed Ben Abdellah | Maouche S.,CRAAG | Missoum H.,University Sidi Mohammed Ben Abdellah
Arabian Journal of Geosciences | Year: 2016

Floods are among the significant natural hazards in Algeria. They cause severe casualties, damage to building and destruction of roads, public works, and infrastructures. Northwestern Algeria has experienced devastating floods in the past that caused considerable damage, e.g., Mohammadia 1881, Mostaganem 1927, and El Asnam 1966. Analysis of historical floods is one of the major tools in flood hazard assessment that enables predicting future flood events. One of the objectives of this work is to address the gap of available information regarding the floods that happened in Algeria, particularly during historical times. The first historical data analysis that we performed on the basis of an intensive survey shows that the study area is subject to flooding. This inventory is the first step towards flood zonation and construction of an atlas of extreme floods in northwestern Algeria. The catalog of historical floods contains more than 127 documented events. As results, this paper presents a method of compiling historical flooding data and the analysis of the obtained database which represents a major contribution to flood risk assessment. © 2016, Saudi Society for Geosciences.


Giresse P.,CNRS Training and Research Center on Mediterranean Environments | Bassetti M.-A.,CNRS Training and Research Center on Mediterranean Environments | Pauc H.,CNRS Training and Research Center on Mediterranean Environments | Gaullier V.,CNRS Training and Research Center on Mediterranean Environments | And 3 more authors.
Sedimentary Geology | Year: 2013

From the analysis of seven new sediment piston-cores sampled in 2005 (MARADJA-2 French-Algerian cruise), this study aims to identify for the first time possible late Pleistocene to recent sedimentary instabilities controlled by seismicity off or close to the eastern coast of Algeria. The detailed lithologic study allows us to identify the frequency of the gravity events (turbidites, debrites) and to discuss their geographical sources and triggering mechanisms. Based on a chronostratigraphy of 24 14C AMS datings, sediment accumulation rates in zones extending off Bejaia and Annaba and, in particular, semi-quantitative analysis of the microfossils and lithogenic tracers of the origin of gravity flows was discussed. Two sediment cores, here considered as reference cores, enabled the estimation of palaeoenvironmental parameters that controlled sedimentation: (1) in the prodelta of Soummam Oued, after 2215calyrBP, floods were less frequent and sediment accumulation rates decreased because of a drier climate; (2) in the middle slope to the NE of Annaba, a location shielded from gravity flows, an increased sedimentation rate coincided with the passage of warmer waters leading to maxima of carbonate biogenic fluxes (particularly pteropods). Off Bejaia, two deep sediment cores show a spectacular increase in sediment accumulation rate between 2200 and 1000calyrBP while turbidites become more frequent. According to the eustatic and climatic stability of this interval, an episode of strong slope instability of the slope is suggested. Both sediment cores on the slope of Annaba indicate an increase in gravity flows during the same last thousand years, which is tentatively related to a regional increase of seismicity during this interval. This spatial distribution of gravity events is clearly different to that of the western margin where the sedimentation is less perturbed.© 2013 Elsevier B.V.


Abtout A.,C.R.A.A.G. | Boukerbout H.,C.R.A.A.G. | Bouyahiaoui B.,C.R.A.A.G. | Derder M.,C.R.A.A.G.
Near Surface Geoscience 2012 | Year: 2012

The gravity data were collected in different surveys carried out in this area. All these data were in first step homogenised. The stations are irregularly distributed. This distribution however, is generally sufficient for our purposes. The data were uniformly reduced to Bouguer anomaly at mean sea level, and topographic reductions were computed. The Bouguer gravity map shows ENE-WSW oriented isoanomalies increasing from SSE to NNW. The positive oriented EW anomalies near the cost reach to 70 mgals, are mainly due to the effect of the oceanic nature of the Mediterranean crust. In the South we note the positive effect of the Ouarsenis Mountain. The central part of the map is characterized by closed low anomaly correlated with Chellif basin. The results based on various filtered maps (horizontal derivative, upward continuation) and continuous wavelet transform of the gravity data were used to make a structural map of the study area; this map confirms the existence of several faults, localised or inferred, from former geological studies. It complements information on some of them and outlines a great number of deep or near-surface faults that had remained unknown until the present time.


Soula S.,CNRS Laboratory for Aerology | Iacovella F.,CNRS Laboratory for Aerology | van der Velde O.,Polytechnic University of Catalonia | Montanya J.,Polytechnic University of Catalonia | And 6 more authors.
Atmospheric Research | Year: 2014

During the night of 01-02 September, 2009, seventeen distinct sprite events including 3 halos were observed above a storm in north-western Mediterranean Sea, with a video camera at Pic du Midi (42.93N; 0.14E; 2877m). The sprites occurred at distances between 280 and 390km which are estimated based on their parent CG location. The MCS-type storm was characterized by a trailing-stratiform structure and a very circular shape with a size of about 70,000km2 (cloud top temperature lower than -35°C) when the TLEs were observed. The cloud to ground (CG) flash rate was large (45min-1) one hour before the TLE observation and very low (<5min-1) during it. Out of the 17 sprite events, 15 parent+CG (P+CG) strokes have been identified and their average peak current is 87kA (67kA for the 14 events without halo), while the associated charge moment changes (CMC) that could be determined, range from 424 to 2088±20%Ckm. Several 2-second videos contain multiple sprite events: one with four events, one with three events and three with two events. Column and carrot type sprites are identified, either together or separately. All P+CG strokes are clearly located within the stratiform region of the storm and the second P+CG stroke of a multiple event is back within the stratiform region. Groups of large and bright carrots reach ~70km height and ~80km horizontal extent. These groups are associated with a second pulse of electric field radiation in the ELF range which occurs ~5ms after the P+CG stroke and exhibits the same polarity, which is evidence for current in the sprite body. VLF perturbations associated with the sprite events were recorded with a station in Algiers. © 2012 Elsevier B.V.


Naitamor S.,CRAAG | Cohen M.B.,Stanford University | Cotts B.R.T.,Exponent, Inc. | Ghalila H.,University of Tunis | And 2 more authors.
Journal of Geophysical Research: Space Physics | Year: 2013

Lightning strokes are capable of initiating disturbances in the lower ionosphere, whose recoveries persist for many minutes. These events are remotely sensed via monitoring subionospherically propagating very low frequency (VLF) transmitter signals, which are perturbed as they pass through the region above the lightning stroke. In this paper we describe the properties and characteristics of the early VLF signal perturbations, which exhibit long recovery times using subionospheric VLF transmitter data from three identical receivers located at Algiers (Algeria), Tunis (Tunisia), and Sebha (Libya). The results indicate that the observation of long recovery events depends strongly on the modal structure of the signal electromagnetic field and the distance from the disturbed region and the receiver or transmitter locations. Comparison of simultaneously collected data at the three sites indicates that the role of the causative lightning stroke properties (e.g., peak current and polarity), or that of transient luminous events may be much less important. The dominant parameter which determines the duration of the recovery time and amplitude appears to be the modal structure of the subionospheric VLF probe signal at the ionospheric disturbance, where scattering occurs, and the subsequent modal structure that propagates to the receiver location. Key Points Signal Mode Composition Long Recovery Early events Lightning peak current role ©2013. American Geophysical Union. All Rights Reserved.


Sawires R.,Assiut University | Sawires R.,University of Jaén | Pelaez J.A.,University of Jaén | Ibrahim H.A.,Assiut University | And 3 more authors.
Natural Hazards | Year: 2016

In the present study, a new seismic source model for the Egyptian territory and its surroundings is proposed. This model can be readily used for seismic hazard assessment and seismic forecasting studies. Seismicity data, focal mechanism solutions, as well as all available geological and tectonic information (e.g. active faults) were taken into account during the definition of this model, in an attempt to define zones which do not show only a rather homogeneous seismicity release, but also exhibit similar seismotectonic characteristics. This work presents a comprehensive description of the different tectonic features and their associated seismicity to define the possible seismic sources in and around Egypt. The proposed seismic source model comprises 28 seismic sources covering the shallow seismicity (h ≤ 35 km) for the Egyptian territory and its surroundings. In addition, for the Eastern Mediterranean region, we considered the shallow seismic source zones (h ≤ 20 km), used in the SHARE project for estimating the seismic hazard for Europe. Furthermore, to cover the intermediate-depth seismicity (20 ≤ h ≤ 100 km), seven intermediate seismic source zones were delineated in the Eastern Mediterranean region. Following the determination of zone boundaries, a separate earthquake and focal mechanism sub-catalogue for each seismic zone was created. Seismicity parameters (b-value, activity “a-value” and maximum expected magnitude) have been computed for each source. In addition, the predominant focal mechanism solution was assigned for each source zone using the stress field inversion approach. The proposed seismic source model and its related seismicity parameters can be employed directly in seismic hazard assessment studies for Egypt. © 2015, Springer Science+Business Media Dordrecht.

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