Key words: active faulting, deterministic seismic hazard, Algeria, seismicity. Abstract. Seismotectonic zonation studies in the Tell Atlas of Algeria, a branch of the ...
Journal of Seismology 4: 79–98, 2000. © 2000 Kluwer Academic Publishers. Printed in the Netherlands.
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Seismogenic potential and earthquake hazard assessment in the Tell Atlas of Algeria A. Aoudia1,2 , F. Vaccari1,3 , P. Suhadolc1 & M. Meghraoui4 1 Dipartimento
di Scienze della Terra, University of Trieste, Italy; 2 Abdus Salam International Centre for Theoretical Physics, SAND Group, Trieste, Italy; 3 CNR, Gruppo Nazionale per la Difesa dai Terremoti, Rome, Italy; 4 EOST, Institut de Physique du Globe, Strasbourg, France Received 23 October 1998; accepted 22 June 1999
Key words: active faulting, deterministic seismic hazard, Algeria, seismicity
Abstract Seismotectonic zonation studies in the Tell Atlas of Algeria, a branch of the Africa-Eurasia plate boundary, provide a valuable input for deterministic seismic hazard calculations. We delineate a number of seismogenic zones from causal relationships established between geological structures and earthquakes and compile a working seismic catalogue mainly from readily available sources. To this catalogue, for a most rational and best-justified hazard analysis, we add estimates of earthquake size translated from active faulting characteristics. We assess the regional seismic hazard using a deterministic procedure based on the computation of complete synthetic seismograms (up to 1 Hz) by the modal summation technique. As a result, we generate seismic hazard maps of maximum velocity, maximum displacement, and design ground acceleration that blend information from geology, historical seismicity and observational seismology, leading to better estimates of the earthquake hazard throughout northern Algeria. Our analysis and the resulting maps illustrate how different the estimate of seismic hazard is based primarily on combined geologic and seismological data with respect to the one for which only information from earthquake catalogues has been used.
Introduction Regional earthquake hazard studies require a multidisciplinary approach in geology and geophysics along with innovative methods of investigations supported by powerful mapping tools. In several countries of the Euro-Mediterranean regions, national seismic hazard models are often based on incomplete seismicity data and fragmentary and poorly significant geological observations. This kind of practice is potentially misleading because seismicity records and tectonic analyses are seriously limited in space, time and quality. The identification and characterization of active faults as earthquake sources are essential parts of seismic hazard evaluation because they enable forecasts to be made of locations, recurrence intervals, and sizes of future large earthquakes. This is especially true when the average repeat time of large earthquakes
on individual seismogenic faults, is larger than the period covered by the seismicity record. Nowadays, in some places around the world, the combined use of seismological and geologic data is the key to refining seismic hazard estimates (i.e., Wesnousky et al., 1984; Wesnousky, 1986, Ward, 1994; Van Dissen and Berryman, 1996). In contrast, in many other places, even though compilations of active faults have been available for many years, their input into hazard models (Muir-Wood, 1993; Woo, 1994; Valensise, 1995) is either ignored or done in the framework of smaller-scale, site specific hazard analysis. In the Algerian Tell Atlas, the mapping and understanding of active structures has initiated following the 1980 El Asnam-Cheliff earthquake. This region is a branch of the Africa-Eurasia collision zone and is characterized by shallow seismicity, active faulting, and short historical record of earthquakes. Existing countrywide seismic hazard models (Mortgat
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Figure 1. A) Shaded relief (from Cornell University) of northern Algeria showing the different morphostructural units. Algiers, Oran, Annaba and Constantine are the principal megacities in Algeria. El Asnam refers to the location of the 1980 Ms 7.3 earthquake. B) Map showing active and Quaternary faults in the Tell Atlas (after Meghraoui, 1988), and directions and rates of convergence along the Africa-Eurasia plate boundary from Nuvel-1 model (Argus et al., 1989). Note that the main cities lie where significant Quaternary faults and large topographic basins are evident.
and Shah, 1978a,b; Hattori, 1988) did not provide reasonable maps of seismic hazard because of the incomplete and inhomogeneous data sets they used (Benouar, 1993). In this paper we assess the earthquake hazard in northern Algeria using a deterministic procedure with a multidisciplinary input based on a careful analysis of the active tectonics. After an introduction to the structural and seismological setting of the Tell Atlas, this paper consists of three parts: 1) We delineate twelve seismogenic zones on the basis of geological, active tectonics and seismological data, and on our state of knowledge and understanding of the seismotectonics of the area. 2)
We discuss the seismogenic potential in the Tell Atlas and patterns of seismic activity with emphasis on the earthquake hazard approach. On the basis of this seismotectonic regionalisation we will first assess the earthquake hazard as pictured only by the historical seismicity, and second we will use also active faulting data to predict the worst hazard scenario. 3) We assess the seismic hazard using a deterministic method based on the computation of complete synthetic seismograms up to 1Hz. This article brings several new and already existing data into hazard assessment and attempts to generate comprehensive seismic hazard maps.
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Figure 2. Historical and instrumental seismicity in Algeria. The time period 1365–1900 includes only largest events and is compiled from several sources mentioned in the text; the time period 1900–1990 includes foreshocks and aftershocks and is taken from Benouar (1993); the time period 1991–1995 is extracted from NEIC.
Structural and seismological setting of Algeria Northern Algeria is formed by 4 morphostructural domains, namely: the Tell Atlas, the High Plateaus, the Sahara Atlas and the Sahara Platform (Figure 1a). The Tell Atlas consists of a succession of mountain ranges and valleys parallel to the coastline, and showing diverse morphological aspects, with juxtaposed platforms (alluvial basins) and high topography reaching 1500 m of relief. Parallel ridges and valleys correspond to E-W to NE-SW trending alluvial basins and thrust and fold systems with a transport direction to the south and southeast (Figure 1b). The High Plateaus zone in-between the Tell Atlas and the Sahara Atlas, is an elevated region (1000 m) of relatively tabular topography. The Sahara Atlas domain is a mountain range with a folded Mesozoic-Cenozoic cover. The Sahara Platform limits to the South the whole region. Earthquakes in Algeria must have been felt since the ancient (Punic-Roman) time and the first detailed description began as early as the Arabic conquest (800 AD, Poirier and Taher, 1980). However, first adequate estimates of earthquake location and magnitude exist since the beginning of the century. As shown on the map in Figure 2, the seismicity is mainly distributed around the most marginal domain that is the Tell Atlas. Few earthquakes appear in the High Plateaus, and Montesus de Ballore (1906) had already observed
the seismic stability of this zone. But according to Benouar (1993) and in the absence of a reliable local seismic network, they may be miss-located by the ISC. Although the Sahara Atlas is characterized by a low seismic activity, Benouar (1993) studied several events that were strong enough to cause damage to property and loss of lives. The Tell Atlas, by its tectonic setting and high population density, is the place where earthquakes pose a serious natural risk. This geological domain lies within the active collision zone between the Eurasian and African plates in the western Mediterranean area. The tectonic regime in this part of the alpine chain is mostly compressional since the early Cenozoic, with a late Quaternary N-S to NW-SE convergence. Neogene and Quaternary post-napes basins exhibit E-W to NE-SW striking folds and related reverse faults. This complex tectonic setting within an actively deforming zone that absorbs 4 to 6 mm/year (Figure 1b, Argus et al., 1989) of crustal shortening and dextral shearing (Meghraoui, 1988; Meghraoui and Pondrelli, 1996), is responsible for the contemporary seismicity and late Quaternary active faulting. The causal relationships between earthquakes and geological structures was well established in the Tell Atlas some 18 years ago following the El Asnam 1980 earthquake (Ouyed et al., 1981; Philip and Meghraoui, 1983). This earth-
82 quake raised a totally new conception on crustal deformation and seismogenic potential of fault-related folding structures in compressional domains. The El Asnam coseismic surface faulting and related active folding was taken as reference example for the identification of other active structures in North Algeria and helped in characterizing their seismic potential (i.e., Meghraoui et al., 1986; Meghraoui, 1991; Aoudia and Meghraoui, 1995).
Seismotectonic source zones and active faulting characteristics in the Tell Atlas In this section, we define seismotectonic source zones for earthquake source characterization and seismic hazard assessment in the Tell Atlas of Algeria. Along with this section, we provide an Appendix where we review the available data concerning seismicity and active faulting for each seismotectonic source zone. As shown in Figure 3, we delineate 12 source zones that are named either after the corresponding geological basin they cover or the main town they contain. The rationale behind such a partition are: 1) distribution of well-identified Quaternary and active faults in Neogene basins, 2) our understanding of the thrustrelated crustal tectonics and kinematic puzzle at local and regional scales, 3) causal relationships established between geological structures and earthquakes, and 4) patterns of clustering of earthquake epicentres. In the Appendix, we describe typical examples of active faults and folds found within each zone, including estimates of their average slip rates, lengths, and recurrence interval of earthquakes whenever known. We assign a typical fault type for each zone either from focal mechanisms, when available, or from geology. We also assign a maximum expected magnitude for each zone either inferred from the seismic catalogue or from fault and fold characteristics (Table 1). To translate these fold and fault characteristics into estimates of earthquake size, we used the empirical relationships proposed by Wells and Coppersmith (1994) between subsurface rupture length and moment magnitude. These relationships are appropriate for estimating magnitudes for expected ruptures along multiple segments and occurring on subsurface seismic sources (blind faults, i.e., Stein and King, 1984; Stein and Yeats, 1989). They may help to overcome uncertainties associated with estimating rupture length from geomorphological and geological evidences. We note that the three external zones, z3, z9 and z10 (Fig-
Figure 3. Seismotectonic source zones in northern Algeria. The geometry of these zones is defined according to our present understanding of the active tectonics and spatial extent of active faults and earthquakes. Discussion of each zone and tabulation of the related active faulting data can be found in the Appendix and Table 1, respectively. Index ‘z1’ stands for ‘zone 1’. The number in bold character is the maximum magnitude estimated from active faulting data, whenever available. The number in plain character is the maximum recorded magnitude. The focal mechanisms associated to each zone are either taken from the literature (see text for references) or inferred from tectonic features. SBS-TAF path is a phase velocity profile computed by Marillier (1981).
ure 3) extend also into Tunisia and Morocco. However in this paper we restrict our discussion to the mainland of Algeria.
Seismogenic structures and seismicity evolution: bearing on a regional earthquake hazard approach Two principal requirements are crucial in order to understand the earthquake hazard in a region of interest: the first is to delineate active geological structures and determine how often great earthquakes have occurred along them; and the second is to look at the evolution of the present day seismicity. In the previous section and related Appendix, we have shown that the Algerian Tell Atlas is undergoing active compression across Neogene and Quaternary intermontane basins elongated parallel to the coastline (Figure 1a). We have also shown that fault-related folds appear to be the main type of active structures by which the seismic energy is released (Figure 1b). The most recent reverse faulting earthquakes, El Asnam 1980, SahelAlgiers 1989, and the Mascara 1994, reinforce the idea that fault-related folding and blind thrusting in the Tell Atlas constitute potential sources of future large
83 Table 1. Active faulting data Seismogenic zones
Active faults
Cheliff zone z1
El Asnam Tenes-Abou El Hassan Boukadir Dahra
Mitidja zone z2
Sahel
Oran-Beni Chougrane zone z3
Ghriss Habra Saline d’Arzew Murdjadjo
Typical fault type
Reverse
Reverse Reverse
Fault length (km)
Probable maximum earthquake magnitude Mw
36–40 30 30 28–30
7.5 7 7 7
70
7.6
30 – 40 60
7 – 7.3 7.4
Cherchell zone z4
Oued El Abiodh
Thrust +strike-slip
20
6.5
Constantine zone z5
Constantine
Strike-slip (senestral)
30
6.5
Hodna zone z11
Chott El Hammam
Reverse
30
7
Soummam zone z6
–
Reverse
–
–
Babor zone z7
–
Reverse
–
–
Kabylie zone z8
–
Reverse
–
–
Annaba zone z9
–
Strike-slip
–
–
Guelma zone z10
–
Normal
–
–
Sahara Atlas z12
–
Strike-slip
–
–
Figure 4. Spatial distribution of earthquakes larger than 5.5 Ms and VIII MSK in the Tell Atlas.
84 earthquakes. As pointed out in several examples of seismogenic folds (King and Vita Finzi, 1981; Philip and Meghraoui, 1983; Meghraoui, 1991; Aoudia and Meghraoui; 1995), fold growth is clearly related to the degree of activity of its underlying fault and illustrates its capability of producing future large earthquakes. So far, in the Algerian Tell Atlas the mapping and understanding of active structures has initiated following the 1980 El Asnam earthquake. In the Cheliff basin, numerous geological, seismological and geodetic studies (Ruegg et al., 1982; Meghraoui et al., 1986, Meghraoui, 1988; Dewey, 1991, Yielding et al., 1989, Aoudia and Meghraoui, 1995) pointed out the complexity of reverse fault zones and the evolution of the related seismicity. This basin has been the site of three destructive earthquakes during this century: the Cavaignac 1922, Ms = 5.9; the Orléansville 1954, Ms = 6.7 and the El Asnam 1980, Ms = 7.3 (Figure 4). These three events were all reverse faulting earthquakes generated by segmented faults and occurred in a 50 km wide zone. The two most recent and larger events had focal areas that overlap in map view (Dewey, 1991) and took place within a time period of less than three decades. Dewey (1991) clearly stated how unusual was the 1954–1980 earthquake sequence and discussed two different fault behavior models that would likely represent the mode by which the seismic strain was released i.e., the characteristic displacement model of Schwartz and Coppersmith (1984) and the time predictable model of Bufe et al. (1977) and Shimazaki and Nakata (1980). In addition to this short-time period irregular occurrence of earthquakes and related uneven clustered spatial distribution, wellresolved paleoseismological data on the El Asnam fault zone (Meghraoui and Doumaz, 1996) show that there are temporal variations in recurrence intervals of great earthquakes on a larger time scale and significant variations in late Quaternary deformation rates. Therefore, the seismicity evolution in the Cheliff basin cannot be considered as following a normal seismic cycle. The clustering of moderate sized and large earthquakes and the long-term irregular recurrence rate exhibited by the El Asnam fault data imply that earthquakes are hardly predictable. If we enlarge our view at the scale of the whole deforming zone in northern Algeria as shown in Figure 4, we see that the historical seismicity (from 1365 AD to 1995 AD) does cluster in space and time showing sometimes clear patterns of earthquake migration from one zone to another. Similar patterns have been described by Ambraseys
Figure 5. Variation of b-value within 95% confidence interval (b+,b-) for the seismicity recorded in the zones z1-z2-z3. Note that for magnitudes larger than 5.9, b-value is statistically meaningless.
(1975) in Iran. Another important feature shown in Figure 4 is that following the 1980 El Asnam 7.3 earthquake, several destructive moderate sized events occurred along the Algerian plate boundary probably self-accommodating the slip in the Cheliff basin. Bearing in mind the short time-window of the Algerian earthquake catalogue, which is less than the average recurrence interval of large earthquakes estimated from paleoseismic analysis, i.e., 500 to 700 years, caution should be exercised in the extrapolation of statistics deduced from short time scale data sets to long time scales. To highlight this issue we look at the catalogue of Benouar (1993) spanning from 1900 up to 1990, where the dominant magnitude is Ms, that we extend up to 1995 using NEIC data. In order to work with a seismotectonically and statistically meaningful sample of earthquakes, we limit our attention to the seismicity recorded in the following neighboring zones: Chelif, Mitidja and Oran-Beni Chougrane, respectively z1, z2 and z3 (see Appendix and Figure 3). We first analyze the completeness and then look at the variation of b-value in different magnitude ranges within 95% confidence interval using Molchan et al. (1997) algorithm. The catalogue is complete from magnitude 4.7 Ms, which is close to the completeness value, 4.8 Ms, calculated by Benouar (1993) for the full catalogue. The magnitude ranges are adopted by shifting of 0.2 the completeness magnitude up to the fixed maximum recorded one. As shown in Figure 5, from magnitude 4.7 up to 5.9 Ms, b-value varies from 0.48 up to 0.73 within a relatively wide confidence in-
85 terval, but still the b-value slope is rather stable within this magnitude range. In contrast, for magnitude larger than 5.9, b-value is statistically meaningless. Therefore a conventional form of seismic hazard analysis could be misleading if only based on the historical earthquake catalogue (i.e. Sieh, 1996; Scholz, 1997; Molchan et al., 1997). It is clear that for the Algerian Tell Atlas, further historical and paleoseismic studies are needed to constrain earthquake statistics to the degree necessary for reliable hazard assessment.
Earthquake hazard assessment In this section, we will compute the seismic hazard using a deterministic procedure with a multidisciplinary input. The deterministic procedure we employ is fully described by Costa et al. (1993) and is briefly summarized below. The multidisciplinary input consists of a database we are currently handling with GIS tools. To illustrate the contribution of active faulting data to earthquake hazard estimation, we will compute two variants of seismic hazard maps: one based only on historical records of seismicity, the second one based both on active faulting data and historical seismicity. The deterministic procedure allows for a first-order seismic hazard mapping. It is based on the computation of complete synthetic seismograms, whose parameters are extracted from a large geological and geophysical data set. The procedure uses regional polygons that limit the area of validity of the proposed structural model, and parameters such as focal mechanisms, active faults, seismogenic areas, earthquake catalogues to characterize the seismic sources. The flowchart of the procedure is shown in Figure 6. Seismic sources are grouped in homogeneous seismogenic zones, and for each group the representative focal mechanism is kept constant. The seismic moment associated with each source is either estimated from the analysis of the maximum magnitude observed in the epicentral area or from active fault characteristics using the empirical relationships of Wells and Coppersmith (1994). As a result, we generate seismic hazard maps of maximum velocity, displacement and design ground acceleration that blend information from geology, historical seismicity and observational seismology leading to better estimates of the earthquake hazard throughout the region of interest.
Input Estimates of seismic hazard depend on the current knowledge of potential earthquake sources, seismic wave paths and local site conditions. Here, we focus on the understanding of the first two input data. Local site conditions are relevant for more detailed modeling at a specific site of interest, and this goes beyond the scope of this paper where only the gross features of the seismic hazard are defined. Potential earthquake sources are represented within each seismogenic zone according to the recorded seismicity, and in terms of active faults capable of producing large earthquakes. Seismogenic zones The seismogenic zonation and active faulting are discussed in Appendix with 12 zones being delineated. This allows an efficient hazard computations. Working catalogues Reliable earthquake catalogues are fundamental to seismic hazard assessment, since they provide the essential information about distribution of earthquakes in both space and time. Probabilistic seismic hazard analyses are particularly sensitive to the catalogue completeness, while for deterministic approach completeness is requested only for strong events. In the absence of a homogenized catalogue of earthquakes for the Algerian Tell Atlas, based on original data treated in a uniform way, it has been necessary to compile a working catalogue from readily available sources. Our catalogue is a modified and updated version of that compiled by Benouar (1993) for the Algerian part spanning from 1900 to 1990. Modifications include the following: we excluded earthquakes smaller than magnitude 5, we added earthquakes since 1990 up to 1995 from the NEIC, and we added all available information on historical earthquakes since 1365 AD. We have carefully merged the information on historical seismicity as reported in already existing papers and studies (see Appendix), keeping in mind the restrictions this places on the quality of the resulting catalogue. As a result of preliminary statistical analyses, the catalogue is believed to be complete for earthquakes larger than magnitude 5 since 1800. For events prior to 1900, some catalogues give only maximum intensity and no magnitude values. In these cases a magnitude had to be estimated. We used the magnitude-intensity relationships computed by Benouar (1993) for the Algerian territory. Where intensities are given as ranges, the midpoint was chosen,
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Figure 6. Flow charts (after Costa et al., 1993) describing: A) the procedure for the first-order zoning B) the detail concerning the definition of seismic sources.
where an intensity given as >I is considered as of intensity I+1. Starting from this catalogue (WC1), we compiled a second working catalogue (WC2), where we added estimates of maximum expected earthquake magnitudes from fault and fold characteristics using the world-wide empirical relationships of Wells and Coppersmith (1994). These two catalogues WC1 and WC2 will be used for the computation of seismic hazard. Focal mechanisms For the definition of the source mechanisms, we adopt already published fault plane solutions or we assign a typical mechanism based on geological considerations (Figure 3). The fault plane solutions used are derived from McKenzie (1972), Girardin et al. (1977), Deschamps et al. (1982), Cisternas et al. (1982), Meghraoui et al. (1986), Meghraoui (1991) and CMT solutions. Structural model The velocity model we use is the one computed by Marillier (1981) who analyzed the dispersion of phase velocities of teleseismic Rayleigh waves along the
SBS-TAF profile running across northern Algeria and Tunisia, in conjunction with existing WWSSN stations. This profile is shown in Figure 3. It samples from east to west the whole study area. The dispersion data were measured with the ‘two-station’ method and then a non-linear inversion, ‘hedgehog’ (Valyus, 1972; Panza, 1981) was applied, which gives a set of compatible upper mantle models. Because of the relatively low maximum periods (not greater than 120 s) at which the phase velocity could be measured, Marillier (1981) could not sample the sub-channel layer. The structural model has been therefore extended to greater depths using the I-Data set computed recently by Du et al. (1998) for the Euro-Mediterranean region. For the most superficial levels, we analyzed a set of strong motion data following the El Asnam main shock of October 10, 1980 to constrain a velocity model in the uppermost three kilometers of the basin. We applied the Frequency-Time Analysis (FTAN) technique (Levshin et al., 1992) to measure the dispersion properties. The same non-linear inversion procedure ‘hedgehog’ is performed to retrieve the shear-wave velocity models from the dispersion data.
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Figure 7. Velocity model used for the computation of synthetic seismograms. The upper 3 km are derived from strong motion analysis (see text), the other lithosphere-asthenosphere data come from Marillier (1981) and Du et al. (1998).
We believe that the velocity model of the El Asnam basin can be extrapolated to other active basins in the Tell Atlas where the gross geological features are very comparable at that wavelength. The entire structural model is shown in Figure 7. The different layers are described by their thickness, density, P and S wave velocities and attenuation. Computations We compute earthquake seismic hazard for two models. The hazard models are as follows: (1) seismicity only, using historical data and maximum recorded magnitudes (WC1), (2) seismicity plus maximum fault hazard using historical data plus estimates of maximum earthquake magnitudes associated with each of the fault sources (WC2). The choice of these two models is constrained by the data already available and serves to illustrate how different would be the hazard maps based on seismicity from the ones where active
faulting data are taken into account. For both models the computation steps are the same. The seismicity is first discretized into 0.2◦ × 0.2◦ cells: to each cell is assigned the magnitude value of the most energetic event that occurs within it. A smoothing procedure is then applied to account for the spatial and magnitude uncertainties relevant to earthquake catalogues and for source dimensions. Therefore, the maximum magnitude to be associated with each cell is searched also in the cell surroundings, through the application of a centered smoothing window with an n = 3 radius, which correspond to considering 3 cells around the central cell (Costa et al., 1993). Only the cells located within a seismogenic area are retained for the definition of the seismic sources that are used to generate the synthetic seismograms. The maps shown in Figure 8 are the results of the application of this method to the two working catalogues WC1 and WC2. A double-couple point source is placed in the center of each cell. The orientation
88 of the double-couple associated with each source is obtained from the database of the fault plane solutions, as mentioned before. Receivers are then placed on a grid (0.2◦ × 0.2◦) covering the whole area of study. To reduce the number of computed seismograms, the source-receiver distance is kept below an upper threshold, which is taken to be a function of the magnitude associated with the source. The maximum source-receiver distance is set to 25, 50, and 90 km for M
