Seismicity and seismic hazard mapping of northern Algeria: Map of Maximum Calculated Intensities (MCI)

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1 Journal of Seismology 8: 1 10, Kluwer Academic Publishers. Printed in the Netherlands. 1 Seismicity and seismic hazard mapping of northern Algeria: Map of Maximum Calculated Intensities (MCI) M.S. Boughacha 1,2, M. Ouyed 1,2,A.Ayadi 1,3, & H. Benhallou 2 1 Dépt. Etudes et Surveillance Sismique, Centre de Recherche en Astronomie Astrophysique et Géophysique, BP 63 Bouzaréah, Algiers, Algeria; 2 Dépt. de Géophysique, Faculté des Sciences de la Terre, de la Géographie et de l Aménagement du Territoire, BP 32, El Alia Bab Ezzouar, Algiers, Algeria; 3 The Abdus Salam International Centre for Theoretical Physics- SAND Group- Strada Costiera, , Trieste, Italy; Author for correspondence, tel: , fax: , ayadi63@hotmail.com Received 7 September 2001; accepted in revised form 13 August 2003 Key words: Algeria, earthquake catalogue, intensity attenuation law, macroseismic maps, maximum calculated intensities map Abstract An earthquake catalogue covering the period , comprising 2430 events, has been compiled for the region lying between 3 W-9 E and 31 N-38 N. It results from raw data of IGN, ISC, USGS and Algerian sources, enabling an input consisting of origin time H, geographical coordinates (longitude λ and latitude ϕ) and at least one of the following parameters: surface wave magnitude Ms, body wave magnitude Mb, epicentral intensities Io. Empirical relations permit transformations of Mb and Ms into Io. The output consists of H, λ, ϕ, Ms,Mb,Io,and focal depth h which is fixed to 10 km. The number of events falls to 1458 characterised by Ms 3.3 and Mb 3.6, or Io III. The fixed depth is suggested by the best documented Algerian macroseismic maps that also lead to an empirical intensity attenuation law. A first application of this catalogue allows the drawing up of an updated Seismicity and a Maximal Calculated Intensities (MCI) Map of Algeria. The MCI map is obtained by using the empirical attenuation law: the intensities inferred by the whole events constituting the catalogue are computed at nodes of a 5x5-km grid covering the area of study. The corresponding maximum value is assigned to each node. The MCI map produced that way gives precise spatial information in comparison with Maximum Observed Intensities (MOI) maps obtained in previous macroseismic studies. This document may be useful in mapping the seismic hazard in Northern Algeria, without attaching probabilities to ground-motion parameters. Introduction The first study of Algeria s seismicity was carried out by Perrey (1847) who described in detail the damages caused by seismic events, by drawing the corresponding isoseismal curves. However, the low population density and the weak urbanisation level in the Algerian provinces, made difficult the evaluation of the earthquake effects. The absence of seismological instruments confined the seismological studies to their macroseismic aspect during a long time, until 1910 that coincides with the installation of the first Algerian seismological station, opening the period of instrumental seismicity without relegating the macroseismicity. Through this study, we aim at obtaining the map of maximal calculated intensity reached at any point of the region defined by (3 W-9 E) and (31 N-38 N), corresponding mainly to the northern part of Algeria, during the period This requires an adequate information about the seismic sources and an intensity attenuation law with the epicentral distance: locations and origin time will be extracted from earthquake catalogues; sometimes, to have access to intensities or magnitudes, empirical laws are necessary. In order to get the attenuation versus distance, we shall have recourse to macroseismic maps.

2 2 To understand the soil and structures behaviour, in term of macroseismic intensity and caused damages, during an earthquake in the northern part of Algeria, two documents: Maximum Observed Intensities map (MOI, modified from Bezzeghoud et al., 1996) and Maximum Calculated Intensities map (MCI, this study) have been drawn using different approaches. The produced map, based on the calculation of the maximum macroseismic intensity at any point of the region under study, emphasizes zones of high or moderate seismicity where the potential seismic hazard is assigned without attaching probabilities to the groundmotion parameters. The seismic risk is related to the maximum intensities of occurred seismic events. Both maps are intended to distinguish the areas of high, medium and low seismic risk. They could be used together with a seismotectonic map in studies relative to the land-use and urbanisation. Geological setting of Algeria Four morphostructural domains form the northern Algeria (Aoudia et al., 2000), namely: the Tellian Atlas, the High Plateaus, the Saharan Atlas and the Saharan Platform (Figure 1). The Tellian Atlas consists of a succession of mountain ranges and valleys parallel to the coastline with juxtaposed platforms (alluvial basins) and high topography relief with a maximum of about 2000 m in the Djurdjura chain. Besides the presence of alluvial basins trending E-W to NE-SW, a system of faults (reverse and thrust) as well as folds is shown by many authors (Harbi et al., 1999; Aoudia et al., 2000). This geological domain is located within the active zone generated by the collision between the Eurasian and African plates. In this area, the tectonic regime is compressional since Early Cenozoic, with a Late Quaternary N-S to NW-SE convergence (Aoudia et al., 2000). Due to this convergence, neogene and quaternary basins show E-W to NE-SW striking folds and related reverse faults. The causal relationship has been well established between theseismic activityand geological structures since 1980 following the El Asnam earthquake (Ouyed, 1981; Ouyed et al., 1983; Philip and Meghraoui, 1983). The crustal shortening and dextral shearing (Meghraoui, 1988; Meghraoui and Ponderelli, 1996) is responsible for the present day seismicity (quaternary active faulting). The High Plateaus domain between the Tellian and the Saharan Atlas is an elevated region (1000 m) of relatively tabular topography. This area seems to be seismically less active than the rest of northern Algeria (Figure 3). The Saharan Atlas domain is a mountain range with a folded mesozoic-cenozoic cover and a scattered seismic activity. This uplifted domain has been generated also by the shortening between the African and the Eurasian plates. Only these three entities constitute earthquake prone areas. Geologically, they define northern Algeria, by opposition to the Saharan Platform extending south and considered as seismically non-active. The abrupt transition from northern Algeria to the Saharan Platform is materialised by the South Atlasic Fault that extends from Morocco to Tunisia. Earthquake catalogue and related seismicity The main purpose to be attained in our study is to draw up a map of macroseismic intensities generated during the period , at any point of the region extending from 3 Wto9 E, and 31 Nto38 N. In order to get exhaustive information, the starting point consisted in a compilation of the earthquake material published by different sources (Rothé, 1950; Benhallou, 1985; Mezcua and Solares, 1983; López Marinas and Salord,1990; Ambraseys et al., 1991a, 1991b; ISC Bulletin, 1994; Mokrane et al., 1994; Benouar, 1994), consisting mainly of origin time H, epicentral coordinates (longitude λ and latitude ϕ) and possibly macroseismic intensity Io, magnitude, depth h, site. The type of magnitude reported may be Ms (surface wave magnitude), Mb (body wave magnitude), local or duration magnitude assumed to be Mb. Instrumental values of the parameter h are ignored because of the uncertainties inherent to the determination of depth, and h will be fixed in an empirical manner. The set (H, λ, ϕ, Io, Ms, Mb, site, reference) where some cells corresponding to Io, Ms, Mb may be zero, constitutes the input catalogue with 2430 events characterized by Io III and Mb 2.0. A selected set of events from ISC file containing Ms and Mb, provides the following empirical relation: Ms = 1.18 Mb 0.96 illustrated in Figure 2a. The couples (Io-Mb) taken from the input provide a second empirical relation: and Io = 4.76 Mb 14.27, Io VI or Mb 4.3

3 3 Figure 1. Geological setting of Algeria showing the four morphostructural domains. The + symbol stands for the south atlasic flexure. Figure 2. (a) Derived empirical relation between surface wave magnitude Ms and body wave magnitude Mb. Data are from ISC Bulletin (1994). (b) Derived empirical relation between epicentral intensity Io and body wave magnitude Mb (averaged over the number of events indicated by the labels). Computations are made without weighting. Data are from Mezcua and Martinez Solares (1983), Benhallou (1985) and Mokrane et al. (1994). Io = 1.59 Mb 0.62, Io > VI or Mb > 4.3 illustrated in Figure 2b. The recourse to the macroseismic maps of Algerian events (Benhallou, 1985; Mokrane et al., 1994) suggests an averaged value of 10 km to the depth h, that will be attributed to all events. The set (H, λ, ϕ, Ms, Mb, Io, h, site, reference), where each cell is attributed a non-zero value, constitutes the output catalogue, with 1458 events characterised by Ms 3.3, Mb 3.6 and Io III. Table 1 presents a sample of events of magnitude Ms 6. The intensities are related to Mercalli or MSK scale (Benhallou et al., 1971; López Marinas and Salord, 1990; Ambraseys et al., 1991a, 1991b; Mokrane et al., 1994) except for few events related to Rossi- Forel scale. The difference between the two scales is not important, and will not introduce any serious uncertainties affecting the accuracy of the database.this

4 4 Figure 3. Epicentre map of northern Algeria for historical ( ) and instrumental ( ) events, and the four seismogenic sub-regions: Oran (1), El Asnam (2), Algiers (3), Bibans-Babors-Constantine (4). data file may constitute a database for spatiotemporal analysis of seismicity. The seismicity, as resulting from the catalogue obtained this way and concerning the whole region under study, is exhibited through Figure 3, for events Ms 4.0. In Algeria, outside the scattered few (and generally weak) events associated to the High Plateaus and the Saharan Atlas, the epicentres are mainly confined along the northern part, through the Tellian Atlas structures where the sources are concentrated in swarms through the north and northwest, and become more sparsely distributed through the east. This distribution suggests four seismogenic sub-regions namely Oran, El-Asnam, Algiers, Bibans- Babors-Constantine, as shown in Figure 3. The Tellian Atlas is an integral delimited part of the plate boundary between Africa and Eurasia, which extends from Azores to Aegean Sea (Buforn et al., 1988). The associated seismicity is controlled by the convergence along this boundary, illustrated by a crust shortening in the Atlas structures confirmed as the most earthquake prone areas in the Maghreb. Particularly, the Tellian Atlas in Algeria is known as the most active zone where, during the last two decades, the biggest earthquakes occurred: El Asnam, (10/10/1980, Ms = 7.3, Ouyed, 1981; Ouyed et al., 1983; Yielding et al., 1989), Constantine (27/10/1985, Ms = 6.0, Bounif, 1990; Deschamps et al., 1991), Mont Chenoua-Tipasa (29/10/1989, Ms = 6.0, Yahia Ouahmed, 1997), Mascara (18/08/1994, Ms = 6.0, Ayadi et al., 2002), Algiers (04/09/1996, Ms = 5.8) and recently Ain Temouchent (22/12/1999, Ms = 5.5). All these events have generated many damages and casualties most of which, resulted in great loss of life. These earthquakes are associated with the above four seismogenic zones represented by a system of thrust and strike slip faults. Intensity attenuation law The lack of adequate instrumental observations of strong motion in northern Algeria is the reason why we focused on macroseismic data. Many studies have produced macroseismic maps on large and moderate earthquakes, for both historical and recent periods (Hée, 1925, 1933, 1950, 1953; Benhallou et al., 1971; Roussel, 1973a, 1973b; Benhallou, 1985; Mokrane et al., 1994). As intensity attenuation law, for a given seismic event, we adopt the following form derived from Sponhauer (in Muñoz, 1989): Io I=aLn(r/h)+b(r h) wherei,ioandhare,respectively, the intensity at hypocentral distance r, the epicentral intensity, and

5 5 Table 1. An example, in chronological order, of the input (from column Y to column hm) and output catalogue (from column Y to column reference), consisting in origin time (Y: year, Mo: month, D: day, H: hour, Mn: minute, S: second), epicentre coordinates (longitude λ and latitude ϕ in degrees), Ms (surface wave magnitude), Mb (body wave magnitude), Io (epicentral intensity), hm (macroseismic focal depth), hf (fixed focal depth), site and reference Y Mo D H Mn S ϕ λ Ms Mb Io hm Ms Mb Io hf Site Reference ,7 0, ,9 6, Oran, Algeria López Marinas and Salord, ,57 2, ,2 6, Mediterranean Sea Mezcua and Solares, ,5 2, ,2 6, Blida, Algeria Mezcua and Solares, ,3 1,8 0 6, ,7 6, Kherba, Algeria Ambraseys et al., 1991a ,9 5, ,2 6, Aures, Algeria Mokrane et al., ,5 1, ,2 6, Dupleix, Algeria Mezcua and Solares, ,05 3,42 0 6, ,6 6, Masqueray Ambraseys et al., 1991b ,3 1, ,2 6, Fromentin, Algeria Mezcua and Solares, ,31 1,47 6, , Orléansville, Algeria Benouar, ,6 1,3 6, , Ténès, Algeria Benouar, ,48 1,3 6 6, , Montenotte, Algeria Benouar, ,16 1,4 7,2 6, ,2 6, El Asnam, Algeria ISC, ,25 1, Les Attafs, Algeria ISC, 1994; Mezcua and Solares, 1983 the focal depth. The coefficients a and b are to be determined. The first one represents the geometrical expansion of the wave front; the second one is related to the anelastic absorption of the medium of propagation. The adopted attenuation law implies that the isoseismal curves are represented by circles. In order to compute the intensity attenuation law versus epicentral distance, we had recourse to a careful examination of macroseismic maps (Benhallou, 1985; Mokrane et al., 1994) which led to 97 selected ones. The criterion of selection consisted in retaining the best documented ones and covering the above seismogenic sub-regions. The inferred mean value of focal depth is 10 km. For each sub-region, the first step concerned computing, for each event, the couples (Io I, ) along the eight directions (when possible) defined by the four cardinal points and the four bisectors. The second step however, consisted in averaging these couples. As for the third step, it dealt with computing, by least mean squares method, the coefficients a and b. The global number of couples amounts to 618. The examination of the plots concerning the attenuation laws (Figure 4) does not indicate significant regional variations, and consequently, we adopt a unique law, that corresponds to the whole of Algeria: Io I = 2.26 Ln (r / 10) (r 10) The Maximum Observed Intensities Map In the absence of accelerometers network, an easy way to have a view of the distribution of the seismic risk in Algeria is given by the distribution of maximum observed intensities (MOI). The first map drawn by Roussel (1973b) using the available data for the period shows three major regions (Figure 5a): The northern part of Algeria, which appears to be the most active zone with an observed maximum intensity of X degree. The southern region, in the northern part of the south atlasic flexure which presents very sparse and moderate seismicity, except for Biskra and Batna s regions where few events with intensity IX were observed The High Plateaus with a low seismic activity: the observed intensities are less than VI degree. Compiling the seismicity covering the period , especially the three main events of El Asnam (10/10/1980, Ms = 7.3), Constantine (27/10/1985,

6 6 Figure 4. Regional intensity attenuation laws versus epicentral intensity, computed for the four seismogenic sub-regions Oran, El Asnam, Algiers, Bibans-Babors-Constantine (exhibited in Figure 3), using selected macroseismic maps. The plot is for an averaged focal depth h = 10 km, obtained from the used macroseismic maps. The labels beside the graph indicate the number of observed points used to compute the averaged epicentral distance. Computations are made without weighting. Data are from Benhallou (1985) and Mokrane et al. (1994). Ms = 6.0) and Mont-Chenoua-Tipasa (29/10/1989, Ms = 6.0), Bezzeghoud et al. (1996), have updated the Roussel s map. The resulting MOI map (Figure 5b) shows notable changes induced by the activity where four regions are displayed. The first one corresponds to the east, surrounding the districts of Constantine, Guelma and Souk Ahras. The second one includes a part of the Bibans-Babors chain, the Aures Mountains and the Hodna district. The third region however, includes the district of Algiers, Cherchell and El Asnam with an extension to the south of the Ouarsenis Massif. As for the last one, it concerns the Oran region including Oran, Relizane and Sidi Bel-Abbes. All these areas are surrounded by

7 Figure 5. (a) The Roussel s Maximum Observed Intensities Map (1973 b). (b) The Maximum Observed Intensities Map (modified from Bezzeghoud et al., 1996). 7

8 8 Figure 6. The Maximum Calculated Intensities map (MCI). the VII degree intensity curve. A zone of intensity VIII is shown near Annaba, in the northeastern part of Algeria. The Maximum Calculated Intensities Map (MCI) Taking into account the whole seismic activity resulting from the established catalogue for the period , we intend to draw the Map of Maximum Calculated Intensities (MCI). The whole studied area is covered by a 5 5-km grid: using the derived attenuation law for Algeria, each node of the grid is assigned the maximum value of the intensities generated by all the events of the catalogue. The obtained document is the Map of Maximum Calculated Intensities, drawn in Figure 6. This new approach is advantageous in the risk mapping because it takes into account the contribution of all the events that occurred in the studied area. The results depend strongly on the reliability of the events contained in the earthquake catalogue (location, intensity, and focal depth), the validity of the attenuation law and the exhaustiveness of the seismicity. Three major trends are emphasized by the MCI map: The first one coincides with the Tellian Atlas extending west-east, from Oran to Guelma. The second one originates around Algiers-Blida and terminates at Biskra, in a N140 direction. These two branches are assigned the highest intensities (degree X at Oran and El-Asnam, IX around Algiers and Blida) and surrounded by an envelope defined by the degree V of the isoseismal curve. Inside this envelope we notice different small zones with variable intensities with respect to the size of the earthquakes. The third one takes the shape of the Atlasic flexure, extending from about the S-W of Ain-Sefra to Tozeur in Tunisia. The assigned intensities, associated with sparse sources, may exceed the VII degree. From the MCI map, it emerges that the Saharan Atlas, the High Plateaus, the Tell Atlas have experienced an intensity I III at least once during the period The maximum intensities calculated for the whole area are located at El Asnam (10/10/1980, Io = X), Oran (09/10/1790, Io = X), Algiers (03/02/1716, Io = IX), Biskra (16/11/1869, Io = IX) and Constantine (27/10/1985, Io = VIII). Comparing the Roussel s MOI map (1973b), the map modified from Bezzeghoud et al. (1996), and the MCI map presented in this study, we notice that the general trend of the maximum observed or calculated intensities is verified by the three studies. The MCI map could represent the most precise information on the spatial distribution of the maximum intensit-

9 9 ies at any point of the studied area, enabling us to identify the prone earthquake areas. On the other hand, it could be a required document in the designing of structures in seismic areas, in order to provide suitable earthquake resistance to civil engineering works. Conclusion The MCI map produced through this study could be a possible means to assess the seismic hazard in northern Algeria in the absence of probabilistic studies based on strong-motion database. This document which represents an aspect of the deterministic seismic hazard assessment, has been obtained after a compilation of data constituting our earthquake catalogue. Our results are compared with those obtained in previous studies in order to find out some possible notable differences according to the methodology followed by the different authors. The present map is less scattered in comparison with that obtained by Roussel (1973b) and Bezzeghoud et al. (1996). Instead of having a lot of many envelopes as shown in the previous ones, it presents a unique and continuous isoseismal which covers the whole Tellien Atlas. The most important seismogenic zones are found within this defined area. The bifurcation of the isoseismal curves (trend Blida-Batna) may be significant, reflecting possible change in the geological context when passing to the eastern part of Algeria. The same remark could be made on the MOI map (Bezzeghoud et al., 1996) but it is more evident on the MCI map, especially when we consider the branch which goes off shore showing the links with the off shore system of faults (Harbi et al., 1999). The elaborated earthquake catalogue which is at the root of this work, is derived from raw data, and therefore determines strongly the reliability of results. A thorough analysis, especially by checking all the sources of the important historical events, will improve this hazard map by taking into account the acceleration parameter using a probabilistic analysis. Combined geological and seismological data would be very interesting to be introduced in the next study to enhance the estimation of seismic hazard. Acknowledgements We are grateful to two anonymous reviewers for their critical comments. This project has been supported by the Houari Boumediene University of Science and Technology, Algiers. References Ambraseys, N.N., Vogt, J. and Adams, R.D., 1991a, Seismicity of the Central Cheliff Valley in Algeria, In: J. Mezcua and A. 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10 10 Hée, A., 1953, La fréquence des tremblements de terre en Algérie, 2ème thèse, Université de Strasbourg. ISC Bulletin 1994, International Seismological Centre, ISC Catalogue CD-ROM, ISC ed., Berkshire, U.K. López Marinas, J.M. and Salord, R., 1990, El período sísmico oranés de 1790 a la luz de la documentación de los archivos españoles. I.G.N Publication, 6, Madrid, 64 pp. Meghraoui, M., 1988, Géologie des zones sismiques du nord de l Algérie, Tectonique active, Paléosismologie et synthèse sismotectonique, PhD Thesis, Univ. Paris-Sud Orsay, 350 pp. Meghraoui, M. and Ponderelli, S., 1996, Transpression and block rotation along the plate boundary in north Africa, Abstract, Journées Luxemb. Geoldy, 80 th session. Mezcua, J. and Martinez Solares, J.M., 1983, Sismicidad del Area Ibero-Mogrebi, 203, I.G.N, Madrid, 303 pp. Mokrane, A., Ait Messaoud, A., Sebai, A., Menia, N., Ayadi, A., Bezzeghoud, M. and Benhallou, H., 1994, Les séismes en Algérie de 1365 à 1992, CRAAG, Algiers, 227 pp. Muñoz, D., 1989, Conceptos básicos en riesgo sísmico. Física de la Tierra, 1, Ouyed, M., 1981, Le tremblement de terre d El Asnam du 10 octobre Etude des répliques. Thèse de 3 e cycle, Université Scientifique et Médicale de Grenoble, 227 pp. Ouyed, M., Yielding, G., Hatzfeld, D. and King, G.C.P., 1983, An aftershock study of the El Asnam (Algeria) earthquake of 1980 October 10, Geophys. J.R. astr. Soc. 73, Perrey, A., 1847, Note sur les tremblements de terre en Algérie et dans l Afrique Septentrionale. Mémoire de l Acad. des Sci. Arts et Belles-Lettres de Dijon, année , Philip, H. and Meghraoui, M., 1983, Structural analysis and interpretation of the surface deformation of the El Asnam earthquake of October 10, 1980, Tectonics 2, Rothé, J.P., 1950, Les séismes de Kherrata et la sismicité de l Algérie. Bull. Ser. Carte géologique de l Algérie, 4 e série, Géophysique 3. Roussel, J., 1973a, L activité sismique en Algérie de inclus. Extrait du Bulletin de la Société d Histoire Naturelle de l Afrique du Nord, t. 64, 3 et 4, Roussel, J., 1973b, Les zones actives et la fréquence des séismes en Algérie ( ). Extrait du Bulletin de la Société d Histoire Naturelle de l Afrique du Nord, t. 64, 3 et 4, Yahia Ouahmed, A., 1997, Analyse sismologique des séismes récents du Sahel d Alger, Thèse de Magister, Université des Sciences et de la Technologie Houari Boumediene (USTHB), Algiers, 178 pp. Yielding, G., Ouyed, M., King, G.C.P. and Hatzfeld, D., 1989, Active tectonics of the Algerian Atlas Mountains. Evidence from aftershocks of the 1980 El-Asnam earthquake, Geophys. J. Int. 99,