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The establishment of a unified earthquake catalog is a basic requirement for attaining a detailed seismological analysis and an improved seismic hazard assessment. Accordingly, the current research paper aims to present a reliable instrumental unified earthquake catalog for the northern part of Algeria. This unified catalog is compiled using regional empirical relationships derived from converting different magnitude scales to an advanced physical-based moment magnitude scale suggested in the recent literature and denoted by Mwg. The regression methodology used in this study is called the New General Orthogonal Regression approach (N.GOR), which was selected since it is the most trustworthy procedure for scaling the magnitudes to Mwg. To highlight the significance of using the N.GOR, the obtained outcomes of the scaling relations were compared with those issued from the conventional general orthogonal regression (GOR) method, which is deemed to be the most dependable methodology used worldwide for magnitude conversion problems. The unified earthquake catalog was assembled from 1954 to 2022. Reliably converting the original magnitudes into homogenized moment magnitude leads to a complete and consistent unified earthquake catalog. Indeed, the unified moment magnitude catalog presented in this paper could provide reliable data for studying earthquakes distribution and the assessment of seismic hazards in the north of Algeria. [ABSTRACT FROM AUTHOR]

The existence of several magnitude scales in the compiled earthquake catalogs of a specific region has made the development of regional relations connecting magnitude scales a necessity, especially for creating a homogeneous seismic catalog in which all magnitudes should be converted to a preferred magnitude scale. To consistently estimate all magnitude ranges and avoid the saturation problem for large earthquakes, the preference is always given to the moment magnitude Mw scale, wherein the most complete and reliable catalog used worldwide is the Global Centroid Moment Tensor (GCMT). However, to our knowledge, no study has yielded regional (in Algeria) relationships for converting different magnitudes to the moment magnitude (Mw,GCMT). The main reason is typically due to the lack of data pairs of different magnitude scales with Mw (GCMT). To overcome this issue, in this research paper, the moment magnitudes data used for northern Algeria (the area bounded by 32° to 39° N and 3° W to 10° E) have been taken principally from the GCMT catalog and enhanced with the European–Mediterranean Regional Centroid Moment Tensor catalogs RCMT and ZUR-CMT. Regarding this latter, it has been demonstrated in the literature that for the Mediterranean regions, a minor correction should be addressed before merging its data with the GCMT and RCMT catalogs, which are perfectly correlated. To accomplish this task, the magnitude scales tested against Mw are the surface wave magnitude, MS, and the body wave magnitude mb issued from the international seismological sources of ISC and NEIC for the same boundaries. As long as the earthquake magnitudes, in general, are affected by errors of comparable size, the best and most reliable regression method that considers the errors in both dependent and independent variables for linear conversion problems is the General Orthogonal Regression, which is adopted and applied herein to develop the regional relations. [ABSTRACT FROM AUTHOR]

Algeria is a country with a high seismic activity. During the last decade, many destructive earthquakes occurred, particularly in the northern part, causing enormous losses in human lives, buildings, and equipments. In order to reduce this risk in the capital and avoid serious damages to the strategic existing buildings, the government decided to invest in seismic upgrade, strengthening, and retrofitting of these buildings. To do so, seismic vulnerability study of this category of buildings has been considered. Structural analysis is performed based on a site investigation (inspection of the building, collecting data, materials characteristics, general conditions of the building, etc.) and existing drawings (architectural plans, structural design, etc.). The aim of these seismic vulnerability studies is to develop guidelines and a methodology for rehabilitation of existing buildings. This paper presents the methodology followed in our study and summarizes the vulnerability assessment and strengthening of one of the strategic buildings according to the new Algerian Seismic Design Code RPA 99/version 2003. As a direct application of this methodology, both static equivalent method and nonlinear dynamic analysis are performed and presented in this paper. [ABSTRACT FROM AUTHOR]

The city of Boumerdès, located in Northern Algeria, was badly affected during the May 21, 2003 Zemmouri (Mw = 6.8) earthquake where extensive liquefaction has been reported. The aim of this paper is to assess and to map the liquefaction potential for Boumerdès. We collected and used data from 154 boreholes, 10 down-hole tests, 56 standard penetrations tests (SPT), and inventory of 35 water level points. This data has been analyzed in the framework of geographical information systems (GIS). We assessed the liquefaction potential index (LPI) by considering a seismic hazard scenario corresponding to amax = 0.48 g calculated, using a probabilistic approach, for a return period of 500 years. LPI values have been correlated to compile the liquefaction hazard map that indicates the quantitative characteristics of the liquefiable layers and the induced disruption probability area. Results show that the main part of the city of Boumerdès belongs to a low liquefaction potential area except for a narrow corridor along the Corso waterway, where the liquefaction potential is moderate to high. The obtained results are compatible with the geological, geotechnical and hydrogeological susceptibility to liquefaction of the area. The results also show a good agreement with the observations made after the May 21, 2003 Zemmouri earthquake. The obtained liquefaction hazard maps may serve as useful tools for land management and planning in the city of Boumerdès and as an example of liquefaction hazard assessment that may be applied in other populated cities in northern Algeria’s seismic prone areas. [ABSTRACT FROM AUTHOR]

Two strong earthquakes occurred in the region of Chlef (north western part of Algeria) during the last century. From the geological context, there were several great masses of sandy soil ejections on the ground surface level and severe damages to civil and hydraulic structures. These damages were due to the soil liquefaction phenomenon. The objective of this paper is to study the resistance to static and cyclic liquefaction of Chlef sand through triaxial tests with focus on the effect of the initial state. For this purpose, a series of undrained monotonic and cyclic triaxial tests were carried out on saturated silty sand at two initial relative densities (Dr = 29 and 80 %). The initial confining pressure was kept at 200 kPa. Moist tamping and dry funnel Pluviation were used to prepare sand specimens. Test results indicated that the monotonic and cyclic resistance of soil mainly depends on initial conditions. Samples prepared by dry funnel Pluviation method have a greater resistance to liquefaction while those by moist tamping have been found to be potentially liquefiable. Further, liquefaction resistance increases with increase in initial relative density and confining pressure. [ABSTRACT FROM AUTHOR]

A great number of local newspapers and periodicals of the nineteenth and twentieth centuries contributed to the survival of information related to the earthquakes that occurred in Algeria during those periods. These documents which often relay the information from official institutions provide fruitful seismological accounts allowing a sound reappraisal of Algeria seismicity in terms of the estimation of macroseismic parameters. A recent systematic documentation on the Algeria seismicity revealed the occurrence of more than 650 earthquakes that were not reported in the previous seismological compilations, from 1844 onward, despite their significant number. In this paper we provide a quick overview on the newspapers and periodicals that existed at that time and from which fruitful information were retrieved and focus on the most significant events that we newly found. A careful analysis of the historical records allowed the estimation of the intensity of these earthquakes, which globally ranges from V to VIII EMS. These new results augment the seismological knowledge in Algeria and will certainly have implications for seismic hazard assessment. [ABSTRACT FROM AUTHOR]

The Northern part of Algeria is considered to be the most active seismogenic area in the Western Mediterranean region. This area has a rich history of seismicity and had experienced many destructive earthquakes such as the Chlef (1954), El-Asnam (1980), Beni-Chograne (1994), Aïn-Temouchent (1999) and recently Boumerdes (2003) earthquakes. The earthquake of Aïn-Temouchent on December 22, 1999, was of magnitude 5.7, killed at least 28 people and made thousands of families homeless. Consequent damage was seen in all the structures located in a radius of 30 km. In the city of El-Maleh, located 12 km northeast of Aïn-Temouchent, the 'The National Bank Branch' of El-Maleh suffered moderate damage, but enough to justify questions about its safety. The project of rehabilitating this building required a broad analysis of its static and dynamic, past and present behaviors. The study reported in this paper was a necessary preliminary step toward the development of an optimal retrofit solution. [ABSTRACT FROM AUTHOR]

Seismic hazard in terms of spectral acceleration ( SA) has been estimated for the first time in northern Algeria. For this purpose, we have used the spatially-smoothed seismicity approach. The present paper is intended to be a continuation of previous work in which we have evaluated the seismic hazard in terms of peak ground acceleration ( PGA) using the same methodology. To perform these evaluations, four complete and Poissonian seismic models have been used. One of them considers earthquakes with magnitudes above M S 6.5 in the last 300 years, that is, the most energetic seismicity in the region. Firstly, seismic hazard maps in terms of SA, at periods of 0.1, 0.2, 0.3, 0.4, 0.5, 1.0, 1.5 and 2.0 sec, with 39.3% and 10% probability of exceedance in 50 years, have been obtained. Therefore, uniform hazard spectra ( UHS) are computed and examined in detail for twelve of the most industrial and populated cities in northern Algeria. All the reported results in this study are for rock soil and 5% of damping. It is noteworthy that, in the seismic hazard maps as well as in the UHS plots, we observe maximum SA values in the central area of the Tell. The higher values are reached in the Chleff region (previously El Asnam), specifically around the location of the destructive earthquakes of September 9, 1954 ( M S 6.8), and October 10, 1980 ( M S 7.3). These maximum values, 0.4 g and 1.0 g, are associated with periods of about 0.2 and 0.3 sec for return periods of 100 and 475 years, respectively. [ABSTRACT FROM AUTHOR]

The occurrence of the Algiers earthquake ( M 6.8) of May 21, 2003, has motivated the necessity to reassess the probabilistic seismic hazard of northern Algeria. The fact that this destructive earthquake took place in an area where there was no evidence of previous significant earthquakes, neither instrumental nor historical, strongly encourages us to review the seismic hazard map of this region. Recently, the probabilistic seismic hazard of northern Algeria was computed using the spatially smoothed seismicity methodology. The catalog used in the previous computation was updated for this review, and not only includes information until June 2003, but also considers a recent re-evaluation of several historical earthquakes. In this paper, the same methodology and seismicity models are utilized in an effort to compare this methodology against an improved and updated seismic catalog. The largest mean peak ground acceleration (PGA) values are obtained in northernmost Algeria, specifically in the central area of the Tell Atlas. These values are of the order of 0.48 g for a return period of 475 years. In the City of Algiers, the capital of Algeria, and approximately 50 km from the reported epicenter of this latest destructive earthquake, a new mean PGA value of 0.23 g is obtained for the same return period. This value is 0.07 g greater than that obtained in the previous computation. In general, we receive greater seismic hazard results in the surrounding area of Algiers, especially to the southwest. The main reason is not this recent earthquake by itself, but the significant increase in the mmax magnitude in the seismic source where the city and the epicenter are included. [ABSTRACT FROM AUTHOR]

On January 24, 2020, a Mw 5.0 earthquake occurred in the El Aouana region, northeastern Algeria. This region is located at the western end of the Lesser Kabylian Block (LKB), a rigid body that was weakly deformed during the late Cenozoic tectonic phase, and it is characterized by a lower seismic activity than that in its bounding regions. The mainshock focal mechanism was estimated via both the P-wave first motion and waveform modeling methods. The earthquake was associated with the rupture of a NW–SE-oriented right-lateral strike-slip fault, as revealed by a 6 km long and 2 km wide aftershock cluster. The seismic moment estimated through waveform modeling was 3.6 × 1016 Nm, while spectral analysis yielded a value of 3.9 × 1016 Nm corresponding to a magnitude of Mw 5.0, a source radius of 1.6 km, and a stress drop of 4 MPa. The spatiotemporal evolution of the aftershock sequence, as modeled using a restricted epidemic-type aftershock sequence (RETAS) stochastic model, yielded a slope p = 1, indicating that the earthquake was generated by tectonic forces and that the aftershock sequence included many subsequences. The calculated stress tensor suggested N–S compression, rotated clockwise relative to NW–SE Eurasia–Africa convergence. Finally, the recent seismic activity (2012–2021) and geological observations in the area led to the suggestion of a new NW–SE right-lateral shear zone, namely, the Bejaia-Babors shear zone, which was incorporated into a seismotectonic growth model involving slip along inherited E–W structures. The pattern of stepover structures throughout this wide shear zone was enhanced during the recent seismic evolution. [ABSTRACT FROM AUTHOR]

On August 7th, 2020, a magnitude Mw = 5.0 earthquake shook 5 km north of Mila city center, northeast of Algeria, causing substantial damage directly to structures, and indirectly from induced impacts of landslides and rock falls, ultimately disrupt to everyday civilian life. Given the recent significant seismic occurrences in the region, a detailed and comprehensive examination and assessment of post-earthquake damage is critical to Algeria. This is primarily because masonry, concrete, and colonial-era structures are sensitive to horizontal motions caused by seismic waves, and because masonry and concrete structures constitute a substantial portion of today's Algeria's build environment. We present a post-earthquake investigation of the Mila earthquake, starting from the earthquake source, and a catalogue of buildings type, damage categorization, and failure patterns of residential structures in Mila's historic old town, where colonial-era brick buildings prevail. We find that structures that represent notable architectural achievements were severely damaged as a result of the earthquake. Data acquired during the immediate post-earthquake analysis was also evaluated and discussed. The graphical representations of the damages are detailed and complemented by photos. This seismic event has shown the fragility of Algeria's building stock, which must be addressed properly in future years. This study reports on an overall estimate of residential buildings in Mila's lower city, as well as an evaluation of the seismic vulnerability of three neighborhood towns (El-Kherba, Grareme-Gouga, and Azzeba). A generic database for graphical surveys and geometric research was developed and implemented making it possible to evaluate the shear strength on-site. The broad observations, collated data, and consequences were then loaded into the 3Muri structural verification program. Nonlinear static analysis was conducted to analyze probable failure paths and compare the real damage to the software results. [ABSTRACT FROM AUTHOR]

Seismic vulnerability analysis of historical masonry buildings, addressed to earthquake emergency management and protection strategy planning, requires large-scale vulnerability and damage assessment. In this sense, two seismic vulnerability index methods and damage estimation have been adapted and applied to a pilot-scale urban area located in Annaba city (Algeria). The first method is based on the EMS-98 building typologies, and the second is based on the Italian GNDT approach. To accomplish this task, the authors employed an existing data survey, which, however, was not originally developed for seismic purposes. It was used herein to provide input to the vulnerability methods. The main goal of this research is not only to assess the seismic vulnerability and expected damage within Annaba, but also to adapt the two mentioned approaches to the existing data survey, aiming to use such nonspecific building database and to study the possibility of its application for seismic risk estimation in similar regions. The results obtained from the application of these two methods were integrated and compared using a geographic information system, proving that the vulnerability of the buildings surveyed in Annaba is significant, and therefore risk mitigation efforts and public awareness are required. [ABSTRACT FROM AUTHOR]

When dealing with structural damages, under the effect of natural hazards such as earthquakes, it is still a scientific challenge to predict the potential damages, before occurrence of a given hazard, as well as to evaluate the damages once the earthquake has occurred. In the present study, two distinct methods addressing these topics are developed. Thousands (∼54,000) of existing buildings damaged during the Boumerdes earthquake that occurred in Algeria (M = 6.8, May 21, 2003) are considered in order to study their accuracy and sensitivity. Once an earthquake has occurred, quick evaluations of the damages are required in order to distinguish which structures should be demolished or evacuated immediately from those which can be kept in service without evacuation of its inhabitants. For this purpose, visual inspections are performed by trained and qualified engineers. For the case of Algeria, an evaluation form has been developed and is still in use since the early 80s: Five categories of damages are considered (no damage or very slight, slight, moderate, major, and very severe/collapse). This paper develops a theoretical methodology that processes the observed damages caused to the structural and nonstructural components (foundations, roofs, slabs, walls, beams, columns, fillings, partition walls, stairways, balconies, etc.), in order to help the evaluator to derive the global damage evaluation. This theoretical methodology transforms the damage category into a corresponding 'residual' risk of failure ranging from zero (no damage) to one (complete damage). The global failure risk, in fact its corresponding damage category, is then derived according to given combinations of probabilistic events in order to express the influence of any component on the global damage and behavior. The method is calibrated on a set of ∼54,000 buildings inspected after Boumerdes earthquake. Almost 80 % of accordance (same damage category) is obtained, when comparing the theoretical results to the observed damages. For pre-earthquake analysis, the methodology widely used around the world relies on the prior calibration of the seismic response of the structures under given expected scenarios. As the structural response is governed by the constitutive materials and structural typology as well as the seismic input and soil conditions, the damage prediction depends intimately on the accuracy of the so-called fragility curve and response spectrum established for each type of structure (RC framed structures, confined or unconfined masonry, etc.) and soil (hard rock, soft soil, etc.). In the present study, the adaptation to Algerian buildings concerns the specific soil conditions as well as the structural dynamic response. The theoretical prediction of the expected damages is helpful for the calibration of the methodology. Thousands (∼3,700) of real structures and the damages caused by the earthquake (Algeria, Boumerdes: M = 6.8, May 21, 2003) are considered for the a posteriori calibration and validation process. The theoretical predictions show the importance of the elastic response spectrum, the local soil conditions, and the structural typology. Although the observed and predicted categories of damage are close, it appears that the existing form used for the visual damage inspection would still require further improvements, in order to allow easy evaluation and identification of the damage level. These methods coupled to databases, and GIS tools could be helpful for the local and technical authorities during the post-earthquake evaluation process: real time information on the damage extent at urban or regional scales as well as the extent of losses and the required resources for reconstruction, evacuation, strengthening, etc. [ABSTRACT FROM AUTHOR]

The seismicity of Algeria since the nineteenth century is relatively well documented. However, compared with the numerous damaging earthquakes that are documented since 1850, fewer than a dozen reports of earthquakes are listed for the pre-1850 ad period, suggesting that the historical record is missing a substantial number of earthquakes. This paper examines the use of literary and epigraphic sources relevant to the investigation of seismicity in Algeria during Roman times. We provide examples where the meager written literary record may be supplemented with appropriate archaeological and epigraphic data describing damage to ancient Roman sites. The examples show that collaboration between earth scientists and archeologists is of utility in improving the seismic record and highlights the need for further study of data sources and repositories located both inside and outside of Algeria. [ABSTRACT FROM AUTHOR]

The 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 stress tensor at different scales. A rupture model has been suggested, which explains the different observations of aftershock distribution and stress tensor rotation. [ABSTRACT FROM AUTHOR]

A moderate earthquake (ML 5.0) occurred in the Oued Djer region of the Tell Atlas, Algiers Province, Central Algeria, on 02 January 2018 (20:59:00 UTC), 10 km southwest of Mitidja Basin (36.39° N, 2.54° E), at a depth of 4 km. The earthquake is significant because it occurred immediately south of the epicentral area of the 1988 MS 5.4 Oued Djer Earthquake and a few kilometers west of the 1867 Mouzaïa-El Afroun Earthquake, a large historic event (I0 = IX EMS-98). In this study seismological, morphotectonic, and geological field data were collected to identify the likely seismogenic structure and give new insight to better understand the seismotectonic configuration of the study area. The mainshock–aftershock sequence was recorded by the Algerian Digital Seismic Network, and a temporary seismic network installed 1 day after the mainshock. By analyzing 97 relocated aftershocks (1.0 < Md < 4.3) recorded during the first 2 months after the mainshock, we demonstrate that the mainshock occurred on a N80° E-striking, 65° S-dipping reverse fault with significant right-lateral component, this in consistence with the ENE–WSW nodal plane of the mainshock focal mechanism. The vertical distribution of the aftershocks is between 3 and 5 km which is shallower than the seismic sequences happened recently in the Tell Atlas. The main aftershocks present reverse and strike-slip focal mechanisms. Our field investigations yield evidence of N80° E-striking, north-vergent reverse faulting, which affects the northeastern boundary of Boumedfaa Basin, a small Miocene depression situated adjacent to the northwestern edge of the Mitidja Basin, in agreement with the results of the microseismic study. In the epicentral area, we highlight Quaternary faults striking N120° E that cut the alluvial terraces of Bouroumi River and are related to NW–SE-striking dextral strike-slip fault. These results show that active deformation in the southwestern edge of the Mitidja basin is complicated and implies more than a fault segment, the combination of strike slip and reverse tectonics is proven here. [ABSTRACT FROM AUTHOR]

In this study, loss estimation models were developed for reasonably accurate assessment of economic and human losses from seismic events in the Mediterranean region, based on damage assessment at an urban scale. Data were compiled from existing worldwide databases, and completed with earthquake information from regional studies. Economic data were converted to a single common currency unit (2015 USD value) and the wealth of the areas affected by 65 earthquakes of the region from 1900 to 2015 was assessed. Reduced-form models were used to determine economic and human losses, with earthquake magnitude and intensity as hazard-related variables, and gross domestic product of the affected area and the affected population as exposure-related variables. Damage to buildings was also used as a hazard-related variable to predict economic and human losses. Finally, site-specific regression models were proposed for economic and human losses due to earthquakes in the Mediterranean region, and more specifically, in Algeria. We show that by introducing the damage variable into the models, prediction error can be reduced, and that accuracy of loss model estimation is site dependent and requires regional data on earthquake losses to improve. A case study for Constantine, Algeria shows the improvements needed for increased accuracy. [ABSTRACT FROM AUTHOR]

Northern Algeria has experienced many destructive earthquakes throughout its history. The largest recent events occurred in El Asnam on October 10, 1980 (moment magnitude; Mw = 7.3), in Constantine on October 27, 1985 (surface-wave magnitude; Ms = 6.0), and in Zemmouri-Boumerdes on May 21, 2003 (Mw = 6.8). Because of the high population density and industrialization in these regions, the earthquakes had disastrous consequences and hence highlighted the vulnerability of Algeria to seismic events. To reduce seismic risk in Constantine, the capital city of East Algeria, we present a seismic risk scenario for this city, focusing on the vulnerability of the key historic areas of Coudia, Bellevue-Ciloc, and the Old City. This scenario allows us to assess the maximum ground acceleration using empirical attenuation laws, based on the following considerations: (a) the 1985 Constantine seismic event as an earthquake reference; (b) site effects related to regional geology; (c) damage to buildings, and (d) seismic vulnerability. This study shows the map of peak ground acceleration taking into account the effects of site lithology (Avib). We observe the strongest vibrations along the two rivers 'Boumerzoug and Rhumel' and also, we note that the EC8 gives a good estimate acceleration in the image of the three studied areas (Bellevue-Ciloc, Coudia, and Old Town). By correlating with the geology, we observe an acceleration of 0.13 g in the neritic limestone of the rock (Old Town) something that fits with the value obtained 0.14 g (PGA) without taking into consideration the lithology. Moreover, according to the Algerian Earthquake Engineering Code (2003) (RPA), the Wilaya of Constantine is classified in the zone IIa (medium seismicity) with an acceleration data of 0.25 g. This study integrates geographic information system (GIS) data into risk models. [ABSTRACT FROM AUTHOR]

On August 1st, 2014, a moderate-sized earthquake struck the capital city of Algiers at 05:11:17.6 (GMT+1). The earthquake caused the death of six peoples and injured 420, mainly following a panic movement among the population. Following the main shock, we surveyed the aftershock activity using a portable seismological network (short period), installed from August 2nd, 2014 to August 21st, 2015. In this work, first, we determined the main shock epicenter using the accelerograms recorded by the Algerian accelerograph network (under the coordination of the National Center of Applied Research in Earthquake Engineering-CGS). We calculated the focal mechanism of the main shock, using the inversion of the accelerograph waveforms in displacement that provides a reverse fault with a slight right-lateral component of slip and a compression axis striking NNW-SSE. The obtained scalar seismic moment ( M = 1.25 × 10 Nm) corresponds to a moment magnitude of M = 5.3. Second, the analysis of the obtained aftershock swarm, of the survey, suggests an offshore ENE-WSW, trending and NNW dipping, causative active fault in the bay of Algiers, which may likely correspond to an offshore unknown segment of the Sahel active fault. [ABSTRACT FROM AUTHOR]

Spatial variability of ground motions has significant influence on dynamic response of extended structures such as bridges and tunnels. In this study, the widely used finite-source ground motion simulation approach, the so-called Empirical Green's Function (EGF) method, is extended to synthesize seismic motions across an array of stations located at bedrock in the epicentral region of the 1980 El-Asnam region (North-West Algeria). The target event being simulated is the October 10 1980 $$ M_{s} = 7.2 $$ Earthquake, and the EGF is obtained from the ground motion recorded at Sogedia Factory station during the 8 November 1980 $$ M_{L} = 5.6 $$ aftershock. Coherency functions are then estimated from the simulated ground accelerations. A parametric study investigating the influence of shear wave velocity, earthquake magnitude, and epicentral distance is conducted by simulating ground acceleration for different scenarios using the Hybrid Green's Function method. The main finding of the study is that finite source effects can cause significant loss in coherency at bedrock in the near-field. In the far-field, the source effect alone does not seem to produce incoherent motion, which implies that scattering and local site effects could be dominating there. Furthermore, coherency functions are found to be more sensitive to inter-station separation in the near-field than in the far-field. Increasing shear wave velocity seems to increase coherency functions, and larger earthquakes seem to produce more incoherent motion than smaller ones. The simulation method presented here produces incoherent motion mainly due to the finite source effect, while path effects are partially accounted for through the EGF, and local site effects are not considered. In this sense, the estimated coherency functions represent that of plane waves. A parametric model of plane wave coherency is calibrated and presented based on the simulation results. The results indicate that the parametric model can be used as a first approximation, and at least an upper bound of lagged coherency in the near-field region of the El-Asnam Earthquake scenario. This model could be useful in random vibration analysis or generation of spatially variable ground motion for time history analysis of lifeline structures in the study area. [ABSTRACT FROM AUTHOR]

Starting on 14 May 2010 and lasting several months, the village of Beni-Ilmane (Msila District, North-Central Algeria) and its surroundings were struck by an important seismic crisis marked by three successive moderate shocks (5.0 ≤ M ≤ 5.2). This sequence of events caused severe damage in the Beni-Ilmane village and in the epicentral area. The poor quality of masonry construction and the cumulative effects of the large number of aftershock events played a key role in the destruction. To follow this earthquake sequence, 11 temporary seismic stations, in addition to the permanent stations of the Algerian seismic network, were deployed in the region. A representative set of well located aftershocks in the period of maximum activity (lasting 18 days) were selected. The horizontal distribution of the aftershocks shows two main earthquake clusters located near Beni-Ilmane village, one cluster oriented E-W and the other oriented NNE-SSW, crossing the first cluster at its eastern tip. The aftershocks distribution suggests that the three main shocks ruptured two distinct and adjacent fault segments of about 8 km length. The focal mechanisms of the first and third events, located in the NNE-SSW cluster, show near-vertical left-lateral strike-slip fault planes. In the second cluster, oriented E-W, focal mechanisms show a high-angle reverse fault. A field survey, initiated immediately after the first main shock, identified surface fissures generated by the three largest events in the sequence. The fissures, concentrated in a narrow area at the western termination of the NE-SW Jebel Choukchot anticline (location of Beni-Ilmane village), showed several orientations which were mainly related to gravity instabilities. The 2010 Beni-Ilmane earthquake sequence, located in the Bibans-Hodna Mountains transition zone, demonstrates that the Tellian Atlas-High Plateaus border region is an active seismic zone marked by moderate and possibly strong earthquakes; thus, a reevaluation of the seismic hazard in the region is needed. [ABSTRACT FROM AUTHOR]

Based on the moment magnitude scale, a probabilistic model was developed to predict the occurrences of strong earthquakes in the seismoactive area of Zemmouri, Algeria. Firstly, the distributions of earthquake magnitudes M were described using the distribution function F( m), which adjusts the magnitudes considered as independent random variables. Secondly, the obtained result, i.e., the distribution function F( m) of the variables M was used to deduce the distribution functions G( x) and H( y) of the variables Y = Log M and Z = M, where ( Y) and ( Z) are independent. Thirdly, some forecast for moments of the future earthquakes in the studied area is given. [ABSTRACT FROM AUTHOR]

In December 2007, the Mila region of northeastern Algeria experienced thousands of microearthquakes (0.8 ≤ Md ≤ 3.9) recorded by eight temporary stations, in addition to permanent stations. Most of the events were too small to be located precisely, but a set of 122 precisely located events shows an alignment of epicenters, extending mainly in a horizontal band at about 1-2 km depth in a NNW-SSE direction and concentrated in a small area, 3 km southeast of Jebel Akhal, a small rocky hill between the Beni Haroun dam/reservoir and the Oued Athmania reservoir. The reservoirs are connected by pipelines, and a pumping station ensures water can be transferred between them at transient pressures of up to 80 bars. During the pumping in 2007, only 45 % of the transferred water (~600,000 m per day) was recovered at the Oued Athmania reservoir, and a large amount of the slightly pressurized water leaked through defective joints in a tunnel that passes through the Jebel. This water penetrated deeply into the soil with the assistance of preexisting fractures, faults, and karsts. Nine days after the first pumping started, a local increase in pore fluid pressures at shallow depths triggered seismicity southeast of Jebel Akhal, where the faults were probably close to failure. The focal mechanisms show a near vertical N-S strike-slip fault plane under regional NW-SE tectonic compression. One of the fault plane solutions is consistent with the NNW-SSE direction along which the seismic events are aligned. Furthermore, a long-term comparison of the seismic activity in the region versus water levels behind the dam and the pumping of water shows that the earthquake swarm was a one-off event related to the pumping operation. [ABSTRACT FROM AUTHOR]

Two strong earthquakes occurred in the region of Chlef (north western part of Algeria) during the last century. From the geological context, there were several great masses of sandy soil ejections on to the ground surface level and severe damages to civil and hydraulic structures. These damages were due to the soil liquefaction phenomenon. The objective of this laboratory investigation is to study the effect of low plastic fines and gradation characteristics on the undrained shear strength (liquefaction resistance) response of sand-silt mixture samples. For this purpose, a series of undrained monotonic triaxial tests were carried out on reconstituted saturated silty sand samples with different fines content ranging from 0 to 50 % at two initial relative densities (D = 20 and 91 %). The initial confining pressure was kept at 100 kPa. The evaluation of the data indicates that the undrained shear strength at the peak (q) can be correlated to the undrained residual strength (S), the excess pore pressure (Δu), the fines content (F) and the intergranular void ratio (e). The test results indicate also that the undrained shear strength at the peak decreases with the increment of the coefficient of uniformity and fines content as well as with the decrement of the mean grain size in the range of 0-50 % fines content for both relative densities (D = 20 and 91 %). [ABSTRACT FROM AUTHOR]

The aim of this study is the comparison between the fundamental periods identified experimentally and those calculated using the formulas given in the Algerian Seismic Code (RPA 99) for vulnerability assessment and for experimental data collection of selected sample of old buildings. The results obtained for vulnerability assessment will then be extrapolated to buildings of the same typology built during the 1949 to 1954 period in the northern part of Algeria. From 1949 to 1954, the reinforced concrete constructions in Algeria were built before the first generation of the Algerian Seismic Code. These buildings being old are certainly weakened by the occupancy activities and seismic event loads. Hence, the evaluation of their vulnerability with respect to the regional seismic hazard requires the knowledge of their structure on a site capacity. The empirical formulas to calculate the fundamental period of a building are based on the Algerian Earthquake Code (RPA 99) .These formulas consider only the geometrical dimension (length, width and height) and the structural design of the buildings. The fundamental periods of vibration of twenty-two buildings, located in Algiers, calculated using the empirical formulas given in the RPA 99 are lower than those identified experimentally. A question then rises, do these tested buildings present any damage or not? As five of these buildings were tested before the 21 May 2003 earthquake, the experimental testing highlighted a decrease in the fundamental frequency which means that these buildings are damaged. Hence, for vulnerability assessment, the empirical formulas given in the Algerian Seismic Code (RPA 99) may not be appropriate for vulnerability assessment of the old buildings built during the 1949 to 1954 period. [ABSTRACT FROM AUTHOR]

The Cheliff region has experienced some significant earthquakes in the last century (1937, 1954, and 1980). The most destructive one is that of El Asnam on October 10, 1980, Ms = 7.3 (Io = IX), which destroyed the Chlef city (formerly El Asnam) and its surrounding villages. On December 16, 2006 a moderate earthquake (Mw = 5.0) hit the Cheliff region. The maximum observed intensity (Io = V: MSK-scale) was observed at Abou El Hassen, Benaria, Bouzghaïa and Tadjena. No damages or human losses were recorded. Nevertheless, minor cracks on walls of the old school at Tadjena were observed. The point source focal mechanism of the event was determined by inverting the waveforms of three regional broadband stations of the ADSN (Algerian Digital Seismic Network). It corresponds to thrust-reverse faulting with a strike-slip component. The stress tensor obtained by the inversion of the 15 focal mechanisms available in the Cheliff region exhibits a well constrained compression axis σ1 horizontal and trending N145°. The NW dipping nodal plane indicating a NE-SW thrust fault with a right-lateral component (strike, dip, rake = 249, 38, 137) is more compatible with the regional stress tensor than the steep dipping NNE-SSW nodal plane showing reverse faulting with a left-lateral component (strike, dip, rake = 15, 65, 60). Accordingly, the Tadjena moderate size earthquake can be related to the Boukadir active fault bordering the lower Cheliff basin to the north, a situation similar to that of the El Asnam fault bordering the middle Cheliff basin to the north. [ABSTRACT FROM AUTHOR]

On August 21st and 22nd 1856, two strong earthquakes occurred off the seaport of Djidjelli, a small city of 1000 inhabitants, located 300 km east of Algiers (capital of Algeria). In relation to these two earthquakes, an important tsunami (at least one) affected the western Mediterranean region and the eastern Algerian coastline between Algiers and La Calle (Algero-Tunisian border). Based on historical information as well as on data recently collected during the Maradja 2 survey conducted in 2005 over the Algerian margin, we show that the tsunami could have been generated by the simultaneous rupture of a set of three en echelon faults evidenced off Djidjelli. From synthetic models, we point out that the area affected along the Algerian coast extended from Bejaia to Annaba. The maximum height of waves reached 1.5 m near the harbor of Djidjelli. [ABSTRACT FROM AUTHOR]