IC/98/173

United Nations Educational Scientific and Cultural Organizationand

International Atomic Energy Agency

THE ABDUS SALAM INTERNATIONAL CENTRE FOR THEORETICAL PHYSICS

SEISMOGENIC POTENTIALAND EARTHQUAKE HAZARD ASSESSMENT

IN THE TELL ATLAS OF ALGERIA

A. AoudiaDipartimento di Scienza delta Terra, University of Trieste, Trieste, Italy

andThe Abdus Salam International Centre for Theoretical Physics, SAND Group,

Trieste, Italy,

F. VaccariDipartimento di Scienza della Terra, University of Trieste, Trieste, Italy

andCNR, Gruppo Nazionale per la Difesa dai Terremoti, Rome, Italy,

P. SuhadolcDipartimento di Scienza della Terra, University of Trieste, Trieste, Italy

andThe Abdus Salam International Centre for Theoretical Physics, SAND Group,

Trieste, Italy

and

M. MeghraouiCNR, Istituto di Ricerca sulla Tettonica Recente, Rome, Italy.

MIRAMARE - TRIESTE

October 1998

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 is the estimate of seismic hazard based primarily on combined geologic and seismological

data with respect to the one for which only information from earthquake catalogues has been used.

1)- 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 and Shah, 1978a,b; Hattori,

1988) did not provide reasonable maps of seismic risk 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 lHz. This article brings several new and already existing data

into hazard assessment and attempts to generate comprehensive seismic hazard maps.

2)- 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 la). 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

3 500 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 lb). The

High Plateaus zone in-between the Tel! Atlas and the Sahara Atlas, is an elevated region (1000m) 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 greatest population density, is the place where

earthquakes pose a serious natural hazard. 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 mostiy compression at 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 lb, 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 Et Asnam 1980 earthquake (Ouyed et al., 1981; Philip and Meghraoui, 1983). This

earthquake 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).

3)- 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 thrust-related 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,

]989). 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 zlO

(Figure 3) extend also into Tunisia and Morocco. However in this paper we restrict our discussion

to the mainland of Algeria.

4)- 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 la). 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 lb). The most

recent reverse faulting earthquakes, El Asnam 1980, Sahel-Algiers 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 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, 1989, 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, Mw=6.0; the Orleansville 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 (Dcwey, 1989) and took place within a time period of less than three decades. Dewey

(1989) 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 ShimazakJ and Nakata (1980). hi addition to this

short-time period irregular occurrence of earthquakes and related uneven clustered spatial

distribution, well-resolved 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 more

unpredictable than often assumed in probabilistic hazard analysis. 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 (1975) in Iran. Another important feature shown in Figure 4 is that

following the ]98O 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. 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 zi, 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 a!. (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

interval, 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.

5)- 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 the Geographic Information System 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 epicentra! 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

9

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 compiie 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, where an intensity given

as >I is considered as of intensity 1+1. Starting from this catalogue (WC1), we compiled a second

10

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 Rayieigh 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. 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.

11

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° X 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 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° X 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<6, 6<M<7 and M>7,

respectively. The minimum source-receiver distance is set to =13 km and therefore no near fault

effects on seismograms are accounted for.

12

Once the source characteristics and the structural model are defined, the synthetic

seismograms are efficiently computed by the modal summation technique (Panza, 1985; Florsch et

al., 1991) for an upper frequency limit of IHz. This is justified by practical considerations related to

modern design technologies and approaches (e.g. seismic isolation) that tend to lower the free

oscillation frequencies of buildings (e.g. Panza et al., 1996), and by the source rupturing process that

cannot be neglected when going to higher frequencies. All seismograms of sources below

magnitude 7 are computed for a constant hypocentral depth of 10 km. Sources larger than

magnitude 7, frequent in the second hazard model, are assigned a hypocentral depth of 15 km.

P-SV (radial and vertical components) and SH (transverse component) synthetic

seismograms are computed for a seismic moment of 1x10"^ N.m. The amplitudes are then properly

scaled according to the smoothed magnitude associated with the cell of the source using the

moment-magnitude relation given by Kanamori (1977) and the spectral scaling law proposed by

Gusev (1983) as reported in Aki (1987). At each receiver, the horizontal components are first

rotated to a reference system common to the whoie territory (N-S and E-W directions) and then the

vector sum is computed.

Among the parameters representative of the strong ground motion we have focused our

attention on the horizontal ground velocity and displacement in the considered range of frequencies

(up to IHz), Since we compute the complete time series we are not limited to this choice, and it is

possible to consider other parameters or quantities that can be of interest in earthquake engineering

or engineering seismology. To overcome the limitation of 1 Hz, which is relevant for accelerations,

we computed the DGA (Design Ground Acceleration) by fitting (Panza et al., 1996) the response

spectra computed for the synthetic accelerograms (at 5% damping) to the long-period part of the

design spectra as given by Eurocode 8 (1993). The latter defines the normalized elastic acceleration

response spectrum of the ground motion, for 5% critical damping. Therefore, representative

response spectra (RSS) are obtained for the considered receiver locations and the DGA defined as

the T=0 intercept of the corresponding RSS can be compared with the observed peak ground

acceleration. Since recordings of many different sources are associated to each receiver, different

maps can be produced. If one single value is to be plotted on a map, then only the maximum value

13

of the analyzed parameter is considered. For the two hazard models, we generate maps of maximum

velocity (Vmax), maximum displacement (Dmax), and design ground acceleration (DGA).

Results

The resulting maps provide useful information regarding the distribution of seismic hazard

in northern Algeria. Discussion is limited here to a description of the major features that

characterize the maps. Specific geographic regions are discussed qualitatively in the Appendix and

quantitative estimates of the expected strong motion are presented in Figures 9 and 10.

The suite of maps in Figure 9 showing the maximal values of displacement, velocity and

DGA are based on the seismicity model derived from WCL The maximal values are recorded

around the Cheliff basin where the largest ever recorded event in Algeria, the 1980 El Asnam

earthquake, took place. The general shape and tendency of the DGA 0.4 g - 0.6 g area reflect the

extent of the Cheliff basin which experienced the highest seismic strain during this century. We see

that all the sites in central and western Algeria arc susceptible to high ground accelerations and that

DGA values exceed 0.1 g. The values of DGA obtained from our modeling correlate well with

available observed peak ground horizontal acceleration. The only main shock strong motion

available to us is the one of the 1989 Ms=6.1 Tipaza earthquake. The horizontal peak ground

acceleration recorded for this shallow earthquake (depth=6 km) at an epicentra! distance of 9 km is

0.28 g. Our computations done at a minimal epicentral distance of 13 km and a constant depth of 10

km provide a 0.2 g of DGA. These realistic values and their geographic distribution which

correspond to the extension of main active faults in the Quaternary basins validate our analysis and

computations. Beside this, the computed DGA are in very good agreement with the most recent

predicted values of peak horizontal accelerations as a function of distance for moderate sized and

large earthquakes computed by Ambraseys et al. (1996) in the Euro-Mediterranean region.

The second variant of seismic hazard maps shown in Figure 10 help provide a synoptic view

of our current understanding of seismic hazard in northern Algeria. A detailed assessment of the

maps will send us back to the comprehensive understanding of the geological and seismological

data already presented in a previous section and used in compiling Table 1. The level of seismic

14

hazard, as shown in Figure 9, is changed significantly when active faulting data are taken into

account (Figure 10). Figure 10 portrays five main high level hazard areas reflecting the number of

mapped faults shown in Figure 1, capable of producing ground shaking at levels of DGA > 0.1 g.

These five areas correspond to places showing evidence of significant active faults and where the

largest population density and megacities are reported.

6)- Conclusion

The seismic hazard maps presented in this study result from of a careful analysis of the active

tectonics in the Tell Atlas of Algeria along with a specific interpretation of the seismic behavior of

each seismotectonic zone. This analysis and the resulting maps illustrate how different is the

estimate of seismic hazard based primarily on combined geologic and seismological data with

respect to the one for which only information from earthquake catalogues has been used. It should be

further emphasized that the maps we are presenting in Figures 9 and 10 show the worst case for both

models and reflect our actual state of knowledge. Therefore, caution should be exercised in the

interpretation of these maps because a significant number of seismogenic faults remain

unrecognized. Hence, we should increase our knowledge of both distribution and rate of slip across

active faults. Slip rate data of faults are still sparse at the scale of the Tell Atlas and cannot be used

in the construction of probabilistic hazard maps yet. Therefore, the contribution that can be made

will be the further acquisition of geological and geophysical data. It is perhaps this ability to collect

data that represents the greatest advantage over conventional forms of seismic hazard analysis,

which are hampered by the existing and limited historical data. Even though the method we have

followed is deterministic, it is suitable to be used in new integrated procedures which combine

probabilistic and deterministic approaches and allow us to minimize the present drawbacks which

characterize them when they are considered separately. We believe that the data we present in this

paper may contribute to a better understanding of the seismic hazard in northern Algeria,

Furthermore, our multidisciplinary procedure may help those wishing to undertake comprehensive

and detailed studies of earthquake hazard in northern Algeria, and for those (e.g. Civil Protection)

who have to decide of measures needed to reduce the effects of future catastrophic earthquakes.

15

Acknowledgements

This work was initiated while AA was visiting DST Trieste, thanks to a grant from TRIL-ICTP

program headed by Prof. G. Furlan. This research has been financially supported by EU Copernicus

contract CIPA-CT94-0238, EU DG12 contract "Strong Ground Motion Estimates" (ENV4-CT96-

0296) and Italian funds MURST 60%. We thank G. R Panza for fruitful discussions. Thanks to N.

Ambraseys, B. Mohammadioun and J. P. Dervin for providing digital strong motion data. Thanks to

D. Benouar for providing his Algerian earthquake catalogue and Cornell University for the

topographic data they provided.

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Appendix

The seismic hazard maps developed in this study are the result of specific interpretation of our

knowledge and understanding of the active tectonics in the Tell Atlas of Algeria. In the appendix,

readers may find details about the active tectonics and main seismicity of the seismotectonic source

zones presented in Figure 3. Map of the active and Quaternary faults is shown in Figure lb.

The Cheliff zone (zl, Figure 3)

The Cheliff basin is one of the well-known and most studied areas in the Tell Atlas (Meghraoui et

al, 1986; Yielding et al., 1989). It was the site of destructive and large earthquakes such as the 1954

22

Orleansville Ms=6.7 and the El Asnam, Ms=7.3 earthquake. The 1980 earthquake is the best

instrumentally and geologically documented seismic event in the western Mediterranean area and is

very representative of the neotectonics in the Tell Atlas. The associated coseismic surface ruptures

exhibited 36 km of segmented NE-SW striking thrust faulting, showing 2m of average vertical

displacement (Philip and Meghraoui, 1983). Paleoseismic studies along the El Asnam fault-related

fold have produced a wealth of information on its late Quaternary activity (Meghraoui et al, 1988a;

Meghraoui et al, 1988b; Meyer et al, 1990). Significant progress has been made on the resolution of

the recurrence interval and slip rates through a detailed analysis of earthquake-induced flood

deposits and leveling profiles across the main thrust fault and secondary faulting by Meghraoui and

Doumaz (1996). An average recurrence interval is calculated to be 720 years during the Holocene

time. During periods of high seismic rate, this recurrence time may reduce to 300 to 500 years. The

minimum uplift rate is 0.6 mm/y and with coseismic slip variation, it may range from 0.25 mrn/y to

1.6mm/y from which Meghraoui and Doumaz (1996) estimated a shortening rate ranging between

0.17 mm/y to 1.2 mm/y. The El Asnam fault zone offers the necessary elements to understand the

thrust fault behavior and the related folding (King and Vita Finzi, 1981; Meghraoui, 1988) and is

taken as a reference example for the identification of other active geological structures in northern

Algeria.

The Tenes-Abou El Hassan active fold, comparable to the one of El Asnam and situated 50 km to

the North, displays on his southern flank a 23 km long reverse faulting with flexural slip (Aoudia

and Meghraoui, 1995). Three segments were identified along strike, indicating some evidence for

the occurrence of possible large earthquakes in the past. The 1922 moderate sized earthquake

(Mw=6.0) was probably the result of a rupture displacing about 13 km of the central segment.

Morphological evidences such as steep fold scarp, tilted young quaternary deposits, uplifted alluvial

terraces, southward progressive migration of meanders and the impressive graben-Hke structure on

the top of the anticline refer to fold growth process by successive coseismic movements and yield

0.5 mm/y of uplift rate.

The Boukadir active fold (30 km) located between the El Asnam structure and the one of Tenes-

Abou El Hassan, displays a NO65 trending thrust fault (Meghraoui, 1988). Another structure,

corresponding to the eastern edge of the Chelif basin, is the Dahra active broken fold. South of the

23

Dahra range Anderson (1963) describes several asymmetrical anticlinal ridges with a transport

direction to the southeast. South and southwest of the Tenes-Abou El Hassan region, flexures are

juxtaposed to flat-lying young deposits where the maximum thickness of Neogene and Quaternary

formations may reach 5000 m (Perrodon, 1957). Meghraoui (1982) and Thomas (1985), in their

neotectonic analysis of the oriental and occidental parts of the basin, respectively, determined N-S

to NNW-SSE shortening directions. In the western Cheliff basin, Thomas (1985) points-out the

existence of E-W trending folds with NE-SW striking axes controlled by inferred E-W striking

deep-seated dextral faults.

We believe that each of these structures can be associated with parallel thrust and reverse faults

(Figure lb) and each of them shows a similar behavior as that of the El Asnam fault. Then a

maximum earthquake magnitude of about 7.5 can be expected in the Cheliff basin (Table 1).

Mitidja zone (z2, Figure 3)

Like in the Cheliff zone, the Mitidja zone shows the same features of active deformation. The

Mitidja region, in terms of seismic history as reported by Rothe (1950), Ambraseys and Vogt

(1988), has experienced several destructive earthquakes in the past, like the earthquakes of January

2, 1365 (IX, MSK); February 3, 1716 (XI, MSK); December 3, 1735 (VII, MSK); March 17, 1756

(VIE, MSK); November 8, 1802 (VIE, MSK); June 18, 1847 (VHI, MSK) and November 5, 1924

(VIII, MSK). These events were followed by a long sequence of aftershocks suggesting a high

magnitude for the main shocks. The strongest seismic event recorded in the coastal region of

Algiers is the Chcnoua-Tipaza earthquake of October 1989 (Ms=6.1). Meghraoui (1991) associated

this event to a blind reverse faulting related to the Sahel anticline. The focal mechanism solution

yielded a reverse fault striking ENE-WSW that is in good agreement with the western segment of

the active Sahel folding structure delineated from geology. Two other segments can be observed

along strike, yielding in total 70 km of folded structure (Figure lb). The northern flanks of this

coastal structure show a steplike morphology of marine terraces reflecting recent uplift movements.

From the C*4 and U/Th dating of two samples of marine terraces (Steam and Thurber, 1965;

Roubet, 1967) Meghraoui (1991) proposed an uplift rate ranging between 0.14 mm/year and 0.25

24

mm/year. In this complex thrust sheets system and related flexured Neogene deposits already

described by Lepvrier (1967) and Glangeaud (1932), the 70 km Sahel anticline constitutes the most

striking feature from the seismic hazard point of view. Hence the probable maximum expected

magnitude in the Mitidja basin is 7.6 (Table 1). This basin clearly highlights the important issue of

blind faulting that remains one of the main problems to be faced from the seismic hazard point of

view.

Oran-Beni Chougrane zone (z-3, Figure 3)

To the western part of Algeria, the Oran-Beni Chougrane zone is represented by the Beni

Chougrane mountains to the south and Murdjadjo basin to the North. The Beni Chougrane

mountains are made of Cretaceous napes unconformably covered by folded Neogene and

Quaternary deposits. The south-eastern edge of these mountains is separated from the flat Ghriss

alluvial basin by a 30 km long NE-SW striking and NW dipping thrust fault showing highly

deformed young quaternary deposits and striations within vertical bedding planes (Meghraoui,

1988). To the northwest, the Beni Chougrane are also bounded by a Quaternary alluvial basin

(Habra basin), which is the western continuation of the Cheliff basin. South-east dipping thrust

faulting limits the Habra basin and Beni Chougrane fold and thrust belt, while in the middle of it,

another southeast dipping thrust affects the Neogene formations. A thrust sequence with folds

having a transport direction to the southeast, accompanied by antithetic thrusting, constitutes the

structural framework of the Beni-Chougrane mountains (Figure lb). Several damaging historical

earthquakes occurred in this zone (Rothe, 1950; Roussel, 1973) confirming its seismogenic

character. The largest are: March 1819 of intensity X, November 22, 1851 of intensity VIE and

November 29, 1887 that had an intensity of X MSK. For the recorded seismicity, the largest event is

the August 1994, Ml=5.7 earthquake (Benouar et a!. 1995). It occurred very close to the epicenter of

the shock of July 13, 1967 (Ms=5.1). The Harvard CMT solution shows almost a pure thrust with a

strike=252°, dip=43° and slip vector 112°, The faulting mechanism of the 18 August 1994 is very

similar to that of 13 July 1967 calculated by Meghraoui et al. (1986) and it can be correlated to the

NE-SW striking thrust and fold system of the Beni Chougrane mountains. This faulting system

25

accommodates one part of the vertical and horizontal movements associated with NNW-SSE

directed shortening of the crust in this zone. Other structures that are taking up one big part of the

deformation are the Saline d'Arzew and the Murdjadjo active folds. Thomas (1985) and Meghraoui

(1988) provided a detailed description of these two compressive structures. The Saline d'Arzew

fault-related fold has a length of 40 km and a topographic offset of 205±20m. It is an asymmetric

fold lying on the northern part of a young Quaternary basin. The Murdjadjo active fold (Meghraoui,

1988) is striking NE-SW, with a transport direction to the SE and a topographic offset of 490±20 m.

It has 60 km of length. The largest historical earthquake reported in its vicinity is the one of October

9, 1790 with an intensity of X (Rothe, 1950; Roussel, 1973). It has devastated the town of Oran, the

second largest city in Algeria. Grangeand (1954) provided valuable information on the November 4,

1949 earthquake where the general tendency of the isoseismals can be correlated with the Murdjadjo

fold and related basin. Using the empirical relationship of Wells and Coppersmith (1994), the

maximum expected magnitude in the case of the Murdjadjo fold is 7.4 (Table 1).

Cherchell zone (z4, Figure 3)

The Cherchell zone is between the Mitidja and Cheliff basins. In this area Glangeaud (1932) and

Perrodon (1957) reported the existence of thrusts with a strike slip component affecting Cretaceous

napes that are unconformably covered by folded Neogene and Quaternary deposits. However, in

contrast to the previous zones, the quaternary basins of this province are very squeezed making the

tectonics more complex. The peculiar distribution among pre-Noegene, Neogene and recent

formations shows that considerable strike-slip motion is required to explain stratigraphic

mismatches across individual basins. It is rather difficult to single out the seismogenic sources,

because of the intricate and puzzling tectonic fabric accompanied by very fast erosional processes

emphasized by regional landslides. Ambraseys and Vogt (1991) in their analysis of the 1891

Gouraya earthquake (I=X) provided valuable information on the type and direction of movements

that were predominant in the epicentral area. The movement was chiefly vertical. This is attested by

several localized upliftings, for instance, the uplift of the alga levels of the coastline by 30 cm, very

26

large sea waves travelling seawards from the South to the North. As a result of the earthquake, a

huge landslide clearly observable on aerial photos points out also the great hazard of earthquake

induced processes in this zone. Although no active fault has been identified in the highly shattered

bedrock in this region, the uplifted and tilted terrace risers present in the Oued El Abiodh basin

indicates that an active fault might exist at this location.

Constantine zone (z5, Figure 3)

In contrast to the Central and Western part of Algeria, the Constantine basin is localized at higher

altitudes and exhibits a rather different faulting mechanism from the four above-mentioned zones.

Its geomorphology shows very squeezed and deep valleys with steep slopes (Figure la). Filed

investigations and conventional geological maps when cleaned up from older structures, show the

existence of active faults. Many of these faults are trending NE-SW in the same direction as the

surface ruptures due to the most significant event that took place in this region, the 1985 Ms=6.0

(Bounif et al, 1987). The strike-slip focal mechanism solution is in good agreement with the nature

of surface ruptures and reflects well the squeezed morphology in this uplifted zone, where much of

the actual deformation is more likely to be accommodated by transcurrent motions than by

compressional ones. Rome (1950) and Grangeand (1954) provided a list of moderate historical

earthquakes that are close in location to the 1985 earthquake such as the 1908 and the 1947 events.

These three events provide a return period of 39 years although we have to be cautious due to the

short time period of observation. This zone of moderate seismicity requires more investigations and

a detailed mapping of active structures. From the spatial extent of the 1985 aftershock sequence a 30

km maximal fault length is inferred. Using Wells and Coopersmith (1994) empirical relationships

for a strike slip fault type, we estimate that the Constantine fault can generate an earthquake of

magnitude 6.5.

Soummam zone (z6, Figure 3)

27

The Soummam zone is represented by a Neogene elongated basin clearly observable on large-scale

geological maps (Figure la). As clearly shown by its geometry, this zone points out two different

structural strikes along its length, corresponding to two sub-basins, one striking almost E-W and the

second striking NE-SW. The first sub-basin was the site of the third largest shock that occurred in

Algeria after the Orleansville (1954) and El Asnam (1980) ones, that is the Aumale June 24, 1910

Ms=6.6 event (Benouar, 1993). The E-W general trend of the isoseimals provided by Sieberg (1932)

for this earthquake can be correlated with the main strike of the Soummam basin. The maximum

damage, as reported by Ambraseys and Vogt (1988) and Benouar (1993), is located along the east

west Soummam sub-basin. The second sub-basin is characterized by a high recent moderate

activity. The pure thrust source parameters of the November 25, 1973 earthquake are in good

agreement with the NE-SW strike of the sub-basin and the shortening vector in north central

Algeria. In terms of seismic history, the Soummam region as reported by Hee (1933 and 1950),

Rome (1950), Mezcua and Martinez (1983) and Ambraseys and Vogt (1988), appears to be the site

of many destructive earthquakes in the 200 years preceding the 1910 event. Further field

investigations are required in order to know whether a larger earthquake is to be expected in this

region.

Babor zone (z7, Figure 3)

The Babor ranges is located to the east of the Soummam Valley, and is a prolongation of the

Djurdjura Mesozoic chain. It is a fold belt characterized by deep valleys and high mountain ranges

(2004 m). The limestone massif of Kherrata within the Babor zone was the site of some destructive

earthquakes but a lot of smaller ones have been reported in the last 200 years by several authors

(Rothe, 1950; Grandjean, 1959; Hee, 1933 and 1950; Roussel, 1973; Benhallou, 1985 and Benouar,

1993). The seismic history of the region shows that the seismic activity is relatively moderate and

constant, about 5 felt events per year and a moderate-size earthquake every two years on the

average, with a macroseismic area equivalent to that of the January 15, 1949 earthquake (Benouar,

1993). As reported by Rothe (1950) this moderate sized earthquake generated surface ruptures

striking N070E that are likely associated with an active asymmetric fold (Figure lb) present in the

28

area (Meghraoui, 1988). This faulted-fold seems to be responsible of the constant activity in the

Babor zone. To the northeast within the same zone lies the town of Jijel. This coastal city was

completely inundated and heavily damaged by two large earthquakes in August 21 and 22, 1856 (IX

and X, MSK respectively; Rothe, 1950). Ambraseys (1982) in his paper on these two historical

events proposed an isoseismal map elongating in a NE-SW direction. This trend and the tidal waves

following the seismic events, confirm the existence of an off shore source, having roughly the same

main strike as other active folding structures observed in the same zone and elsewhere in the Tell

Atlas.

Kabylie zone (z8. Figure 3)

The Kabylie zone is characterized by high alpine mountains within a complex structural

environment. It is represented by thrusts underlined by nape fronts striking roughly east-west with a

vergence to the south involving basement rocks. There are no geological or morphological witnesses

of recent activity in this area. This zone is considered to have a low degree of seismic activity, when

considered independently from the neighboring zones.

Annaba zone (z9, Figure 3)

The Annaba zone is the easternmost zone in the Tell Atlas of Algeria and extends to Tunisia. The

outcrops in this area are mainly basement Palaeozoic rocks. The most recent faulting system is

striking NE-SW. Like in the Kabylie zone, there are no witnesses of recent tectonic activity. No

large or moderate earthquakes were reported in this zone.

Guelma zone (zlO, Figure 3)

In contrast to all the other zones, the Guelma zone presents a different faulting mechanism.

Meghraoui (1988) described in this region a pull-apart basin formed between two overlapping east-

west dextral strike slip faults. Crustal extension occurs in between. The size of the pull-apart is quite

29

important and this is linked to the amount of overlap (25km) and distance between overlapping

segments. At the ends of the pull-apart basin there are series of N-S to NNW-SSE bounding normal

fault systems that intersect the sub parallel shear faults. Further field investigations are needed to

decipher the seismogenic potential of these extensional features. Nevertheless, the Guelma pull-

apart basin, by its size and related cluster of earthquakes, is of concern from the seismic hazard

point of view. The largest seismic event that ever occurred in this area is the one of February 10,

1937 (Ms=5.2; Benouar, 1993), Two destructive earthquakes of intensity VIII (MSK) struck the

same region on June 17, 1908 and December 3, 1928.

Hodna zone (zl 1, Figure 3)

The Hodna basin lies southeast of Algeria. It is an integral part of the Tell Atlas. It is a Neogene

basin with relatively gentle topography, except where it is warped or broken into a series of ridges

and valleys by folds and reverse faults trending E-W to NE-SW. The dominant geomorphologic

features of the Hodna zone consist of peneplain surfaces that are flat with internal drainage

catchments, called chotts, composed of very soft materials. This region has experienced several

destructive earthquakes in the past (Chesnau, 1892; Rothe. 1950). The largest recorded earthquake

that occurred in the Hodna zone is the M'Sila earthquake of January 1, 1965, Ms=5.5 (McKenzie,

1972; Benouar, 1993). The isoseismal map provided by Benouar (1993) shows that isoseismals are

noticeably elongated along the main direction of the Hodna folding system. The focal mechanism of

the same event is also in good agreement with the geological structures described by Rothe (1950)

and Meghraoui (1988). Rothe (1950) associated the February 12, 1946 event (I=DC, MSK), with the

focus in the neighbourhood of the 1965 M'Sila event, to E-W asymmetrical folds with transport

direction to the South, indicating that the underlying faults dip to the North. According to

Meghraoui(1988) this event has been probably generated by the Chott El Hammam reverse fault

with an exposed fault length of approximately 30 km. Thus this fault is capable of generating an

earthquake of about Mw=7.0 (Table 1).

30

Sahara Atlas zone (zl2, Figure 3)

This is the only zone that does not belong to the Tell Atlas chain. The Sahara Atlas is considered as

another structural domain made of thrust and folded structures (Villa, 1980; Wildi, 1983), defined

as being active until at least the beginning of the middle Pleistocene (Outtani et al, 1995).

Unfortunately there is no knowledge of whether there are evidences of more recent activity or not.

This zone is separated from the active Tell Atlas Mountains by the stable High Plateaux zone.

Therefore it is not within the plate boundary zone, and consequently the seismic activity is low as

clearly pointed out in Figure 2. However, the few events that took place in the area were strong

enough to cause damage and loss of lives, mainly due to the low quality of building materials

(Benouar, 1993). We would not describe this zone as being stable or tectonically inactive, but it

simply behaves differently from marginal active orogens.

31

Figure Captions

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.

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.

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 "zl" 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).

Figure 4:

Spatial distribution of earthquakes larger than 5.5 Ms and VTfl MSK in the Tell Atlas.

Figure 5:

Variation of b-value within 95% confidence interval (b+,b-). Note that for magnitudes larger than

5.9, b-value is statistically meaningless.

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.

32

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).

Figure 8:

Magnitude distribution within the seismotectonic source zones after the application of discretisation

and smoothing to the seismicity data given in: a) working catalogue WC1 (only recorded seismicity)

b) working catalogue WC2 (recorded seismicity and active faulting data). See text for explanation.

Figure 9:

Seismic hazard maps based on historical seismicity only (catalogueWCl) showing the spatial

distribution of: a) Horizontal maximum displacement; b) Horizontal maximum velocity; c) Design

ground acceleration (see text).

Figure 10:

Seismic hazard maps based on historical seismicity and active faulting data (catalogue WC2)

showing the spatial distribution of: a) Horizontal maximum displacement; b) Horizontal maximum

velocity; c) Design ground acceleration (see text).

Table 1:

Active faulting data

33

Table 1

Seismogenic zones Active faults • • Typical fault; - Fault length .. Probable maximum' -;v.k-;' .:vrv ' ' • >.'-\i-'~'$t'-'::'j-j •....' • • V ' t y p i ^ j ' 1 ? ' . : . CB?nJ'̂ .'.'-":' .earthquake magnitude,

Cheliff zone zl

Mitidja zone z2Oran-BeniChougrane zone z3

Cherchell zone z4

Constanrine zonez5Hodna zone z l lSoummam zone z6Babor zone z7Kabylie zone z8Annaba zone z9Guelma zone zlOSahara Atlas zl2

El AsnamTenes-Abou El HassanBoukadirDahra

SahelGhrissHabraSaline d'ArzewMurdjadjoOued El Abiodh

Constantine

Chott El Hammam------

Reverse

Reverse

Reverse

Thrust+strike-slipStrike-slip(senestral)ReverseReverseReverseReverseStrike-slipNormalStrike-slip

36-40303028-30

7030

406020

30

30------

7.5777

7.67

7.37.46.5

6.5

7------

34

Figure 1

• • - • : • / , . . . : : - ' :1 ' :." ';-•• •»'!.• •"• - - V " ssiii'ir'Si.1..-?:-"-.-

£:$•*• Western Me4iterranMiiii;;.se

* C . * ^ j - . T f i. •:•.-. ;••• • • v V " . •. • ...

« r % -\ .̂••,;;-;̂ i;;v;:;;;:::-,- - j . ^

3 2 -

35

o 1900 -1995

1365-1900

Figure 2

36

30-3 -1 1 11

Figure 3

37

Figure 4

3 8 -Chenoua,Ms 5.7, 1989

Aumale,Ms 6.6, 1910

Bouira,Ms 5.6, 1946

Ain Tagrout,

Tenes,Ms 5.9, 1922

Hadjout,Ms 5.6, 1988

Orleansville,Ms 6.7, 1954

Ms > 6.5

• 5.5iUsi6.5O I>vm MSK, 1365- 1900

Figure 5

b-value2 -

1.75-

1.5-

1.25-

1.0-

0.75 -

0 . 5 -

0 . 2 5 -

"I 1I

A / ^

/VVl /Ms

38

Figure 6

REGIONAL

POLYGONS

STRUCTURAL

MODELS

FOCAL

MECHANISMS

SEISMOGENIC ZONES+

ACTIVE FAULTING DATA

EARTHQUAKECATALOGUE

SEISMIC SOURCES

SITES ASSOCIATEDWITH EACH SOURCE

TIME SERIES

PARAMETERS

P-SVSYNTHETIC

SEISMOGRAMS

SHSYNTHETIC

SEISMOGRAMS

VERTICAL

COMPONENT

HORIZONTAL

COMPONENTS

EXTRACTION OF

SIGNIFICANT

PARAMETERS

FOMECH/

SALWISMS

OBSERVEDEVENTS:

- location- orientation- magnitiide

SEISMOGENIC ZONES

ACTIVE FAULTING DATA

EXPECTEDEVENTS;

- location-magnitude {from fault

characteristics)- mechanism type

CHOICE OF THEREPRESENTATIVE

FOCALMECHANISM

FOR THE WHOLESEISMOGENIC

AREA

EARTHQUAKECATALOGUE

OBSERVEDEVENTS:

- location

DISCRETIZEDAND SMOOTHED

SEISMICITY

CHOICE OFMAXIMUM

MAGNITUDE

FOR EACH CELLAFTER

SMOOTHING

SEISMIC SOURCES

A B

o o © o ©CM co >* m to

o © o or*- oo o> o

§D)

CM

O)

00

<D

IO

* f

CO

CM

iTTTh—r

I-1

1--++-

4

' : i i ! I

UJJ

; i i i ! ; ! I

; I

44+44

MIT

H-j-

; • • } • • [ • • : • • ! • • [ •

f+

m l

OQ.

Q

ca

I

CO

- CO

- CM

CMOCO

oin o

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40

Figure 8

Magnitudes7.50-7.75

'•}• 7 .00-7 .50

' 6.75-7.00

* 6.50-6-75

* 6.00-6.50

* 5.50-6.00

5.00-5.50

B 0-500

Figure 9

Veloc (crh/s)

50-100

20-50

10-20

5-10

2-5

1 - 2

;* -1

DGA(g)

0.4-0.6

0.2 - 0.4

0.1-0.2

0.05-0.1

- 0.05 ,

Figure 10

• • • ' > * • • • • • » • •

* < * • . . . . , _ - . . ,

* * • = - . - - - - : - . - .

•••MM *•••••••••#••••»••••• * • • • • • • • • • * • • • • • • •

• • * • • • • • •

• • • * »

• • * *

•«••»*»»»••••

•••«•«••••••*••••••

Etfspl (cm)

* id

5 -

•2 -

• : ; , t :T :

; : » . : ' . : . : . ! : ; • ! :

,20

10

5

2.;- ' .

illlij

20

10

5-

2-

1 -

• •»

(cm/s)-too

-50

-20

10

5

2

1 • • • : : : : - : : : :

0.2 - 0.4

0.1-0.2

0.05

0.05

43