Seismic risk assessment for the downtown of the city of Blida, Algeria

Blida (Algeria) is characterized by a high level of seismic exposure and vulnerability due to its dense population and the presence of aging buildings. The historical earthquake that occurred in 1825, with a moment magnitude (Mw7.1), underscored the urgent need for a thorough assessment of seismic risk in the area. Here, an extensive study conducted in downtown of the city of Blida to evaluate seismic risk and its consequences is presented. Geounits 141 and 148 emerged as the most severely affected in all the simulated earthquake scenarios indicating severe damage and casualties mainly for closest earthquakes (Blida and Bounaian, both with moment magnitude Mw7.1) but also for furthest earthquakes as Mouzaia El Affroun (Mw6.6), and Hammam Melouane (Mw.5). The sensitivity analysis demonstrated the importance of the selection of the performance point computation method (improved displacement coefficient method -IDCM, modified capacity spectrum-MADRS, and nonlinear analysis method-N2) and the choice of the ground motion prediction equation. IDCM results are less influenced by the choice of the GMPE, but they provide higher damage results expressed as a mean damage ratio. Moreover, the study estimated potential human impacts in the Blida region, highlighting varying levels of impact on different geounits under different earthquake scenarios. The study's primary findings from seismic risk assessments in the studied region highlight its high susceptibility to earthquakes and can be summarized as follows: The mean damage ratio will be 52.6% ± 1.4%, 50.9% ± 1.6%; 31.8% ± 3.4% and 21.4% ± 3.1% for the Blida, Bounaian, Mouzaia El Affroun and Hammam Melouane earthquakes respectively.We would like to acknowledge the support of the Directorate General for Scientific Research and Technological Development [ N°01/CRAAG/DGRSDT] for their financial assistance in conducting this study

Modern seismicity in Northern Algeria: performance of local networks and evolution compared to the historical seismicity.

During the past decades, there was a remarkable rebound in seismicity studies in northern Algeria, where almost all moderate earthquakes were thoroughly studied and result published. This fact was made possible by the deployment, extension and modernization of local seismological networks. In this work, 20 earthquakes (4.5 ≤ Mw ≤ 5.9) occurred in the Tell Atlas of Algeria for the period (2006–2021) and recorded by many digital stations of the national accelerograph network are considered. First, we located hypocenters of the events. Second, we performed a waveforms inversion of accelerograms to describe the seismic source (seismic moment, moment magnitude and focal mechanisms of the corresponding earthquake). The hypocenters are well constrained better than those provided by international organisms, while the magnitudes and focal mechanisms are comparable. In the western Tell Atlas, the focal mechanisms often yielded reverse faulting, while in eastern Algeria, they showed often strike slip solutions. The instrumental seismicity often occurred inside or around Neogene basins, known already to have experienced historical seismicity.National Center of Applied Research in Earthquakes Engineering (CGS, Algiers

Tectonic and Hydrothermal Activities in Debagh, Guelma Basin (Algeria)

International audienceQuaternary and Pliocene travertines, deposited from hot springs, can reveal much about neotectonic and hydrothermal activity. Theaimof this work is the understanding of the actual tectonic activity in the Guelma Basin and in one of its spa structures. Gravity data were collected during a field study in the Hammam Debagh (HD) area and then analyzed to better highlight the architecture of its subsurface underlying structures.This analysis was performed by means of a Bouguer anomaly, upward continuations, and residual and derivative maps. Comparison of gravity maps, field geology, geomorphic observations, and structural maps allowed us to identify the major structural features in the Hammam Debagh. As a result, we confirm the position of the Hammam Debagh active fault which is superimposed to the hydrothermal active source in the NW-SE direction characterized by a negative gravity anomaly

Active Coastal Thrusting and Folding, and Uplift Rate of the Sahel Anticline and Zemmouri Earthquake area (Tell Atlas, Algeria)

International audienceMajor 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 mcoseismic 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

Mediterranean extreme floods and flash floods

The Mediterranean area is particularly exposed to flash floods. Floods are weather-related hazards and their patterns are likely to be significantly affected by climate change. Floods are already the most frequent and among the costliest and deadliest natural disasters worldwide (Munich RE, NatCat Service; Swiss RE, 2015). This is also true in the Mediterranean area. The EMDAT international disaster database (http://www.emdat.be/) lists for instance 200 billion Euros damages related to various disasters since 1900 in the countries surrounding the Mediterranean Sea, out of which 85 billion are related to river flooding. Disastrous flash-floods1 are much more frequent in some parts of the Mediterranean region than in the rest of Europe (Gaume et al. 2009; Llassat et al. 2010). This is due to the local climate, which is prone to short intense bursts of rainfall. The reliefs surrounding the Mediterranean Sea force the convergence of low-level atmospheric flows and the uplift of warm wet air masses that drift from the Mediterranean Sea to the coasts, thereby creating active convection. In addition, population growth is particularly high along the Mediterranean coasts, leading to a rapid increase in urban settlements and populations exposed to flooding

Sub-chapter 1.3.4. Mediterranean extreme floods and flash floods

The Mediterranean area is particularly exposed to flash floods Floods are weather-related hazards and their patterns are likely to be significantly affected by climate change. Floods are already the most frequent and among the costliest and deadliest natural disasters worldwide (Munich RE, NatCat Service; Swiss RE, 2015). This is also true in the Mediterranean area. The EM-DAT international disaster database (http://www.emdat.be/) lists for instance 200 billion Euros damages related to variou..

Paleotsunami deposits along the coast of Egypt correlate with historical earthquake records of eastern Mediterranean

International audienceWe study the sedimentary record of past tsunamis along the coastal area west of Alexandria (NW Egypt) taking into account the occurrence of major historical earthquakes in the eastern Mediterranean. The two selected sites at Kefr Saber (∼ 32 km west of Marsa-Matrouh city) and ∼ 10 km northwest of El Alamein village are coastal lagoons protected by 2-20 m-high dunes parallel to the shoreline. Field data were collected by (1) coastal geomorphol-ogy along estuaries, wedge-protected and dune-protected lagoons ; and (2) identification and spatial distribution of pale-otsunamis deposits using five trenches (1.5 m-depth) at Kefr Saber and twelve cores (1 to 2.5 m-depth) at El Alamein. Detailed logging of sedimentary sections was conducted using X-rays, grain size and sorting, total organic and inorganic matter, bulk mineralogy, magnetic susceptibility, and radio-carbon dating to identify past tsunamis records. Generally of low energy, the stratigraphic succession made of coastal lagoon and alluvial deposits includes intercalated high-energy deposits made of mixed fine and coarse sand with broken shells, interpreted as catastrophic layers correlated with tsunami deposits. Radiocarbon dating of 46 samples consist in mixed old (>13 000 BP) and young (<5500 BP), dated charcoal and shells in sedimentary units correlate with the 24 June AD 1870 (M w 7.5), 8 August AD 1303 (M w ∼ 8) and 21 July AD 365 (M w 8-8.5) large tsunamigenic earthquakes that caused inundation along the Alexandria and northern Egyptian shoreline. Our results point out the size and recurrence of past tsunamis and the potential for future tsunami hazards on the Egyptian coastline and the eastern Mediter-ranean regions

An update of Algerian’s seismic catalog from historical seismicity, archeoseismological, and paleoseismological studies

For any seismic hazard study, a reliable, homogenized, and complete seismic catalog is required. The Algerian seismicity catalog 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 were identified. On the other hand, paleoseismological investigations were conducted 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 completeness of the seismicity catalog.This work was prepared with the support of UNESCO-IGCP-659 Project “Seismic Hazard and Risk in Africa”

The Beni Haoua, Algeria, Mw 4.9 earthquake: source parameters, engineering, and seismotectonic implications

International audienc

Paleomagnetic and magnetostructural study of Jurassic formations of Gara Djebilet (Tindouf Basin, Southwestern Algeria)

International audienceRecent geochemical analyzes and 40Ar/39Ar datings of dolerite sills and dykes and basalticlava flows from Southwestern Algeria (Tindouf, Reggane, Bechar and Hank basins) haveshown that these rocks are related to the Central Atlantic Magmatic Province (CAMP). TheCAMP is one of the largest among the Mesozoic basaltic provinces identified and formed about200 Ma ago as a preamble to the breakup of Pangea. These data were solid arguments for undertakinggeological observations and sampling for paleomagnetic studies and magnetic fabrics onthe CAMP formations. A network of three long doleritic dykes (198.9 ± 1.8 Ma) located in theTindouf Basin were targeted to clarify the structural context of their emplacement (magneticfabrics) and to determine a new reliable Mesozoic pole.The magnetic fabric, in almost all the sections sampled, is defined mainly through groupingof k1 and k2 axes on the dyke plane whereas axis k3 is almost perpendicular to the dykeplane. This fabric can therefore be interpreted as the magma flow direction. The new Jurassicpaleomagnetic pole, of excellent quality, is very close to those obtained on detrital formationsof the Algerian Sahara and close to those recently determined on igneous formations of Morocco.This new pole is also very close to the North African pole at 200 Ma. These results representa new contribution for a better knowledge of the geodynamic context during this period