18Benouar.qxd 555 ANNALS OF GEOPHYSICS, VOL. 47, N. 2/3, April/June 2004 Key words Historical seismicity – Algeria 1. Introduction This work discusses some of the most im- portant reported earthquakes that occurred in Algeria since 1365, and the issues involved with their study. For the early times in Algeria, only one reli- able earthquake catalogue for the province of Algiers has been published (Ambraseys and Vogt, 1988), which extends back to the four- teenth century. With regard to regional studies, in Tunisia, the earthquake catalogue compiled by Rothé (1980) dates from 410 but does not contain enough information about the sources used. For Morocco, Ramdani et al. (1988) pub- lished a list of historical earthquakes. Mezcua and Martinez-Solares (1983) compiled an earthquake catalogue for the Ibero-Maghreb re- gion for the period 500 B.C.-1980. Alexis Perrey is without any doubt the pio- neer in this domain. Perrey published a note on the earthquakes in Algeria (Perrey, 1848) and he continued his publication by notes on earth- quakes in Algeria up to 1871 in his annual lists. Later M. Chesneau published in the Annales des Mines, a note on the earthquakes during the pe- riod 1716-1887 containing some interesting de- tails (Chesneau, 1892). At the same time, Montessus de Ballore (1892) published some comments on the number of earthquakes felt in Algeria under the title «La France et l’Algérie sismiques». The seismicity of North Africa is al- so addressed in the book «Les tremblements de terre, géographie sismologique» published by Montessus de Ballore (1906). This book in- cludes a map showing the earthquakes reported in Algeria. During the last century, several notes were published in the Comptes Rendus de l’A- cadémie des Sciences (CRAS) about seismic events in Algeria. A further listing of Algerian earthquakes was published in the Bulletin du Materials for the investigation of historical seismicity in Algeria from the records of past earthquakes Djillali Benouar Built Environment Research Laboratory, University of Bab Ezzouar (USTHB), Civil Engineering Department, Bab Ezzouar, Alger, Algeria Abstract This paper presents some materials relating to the investigation of historical seismicity in Algeria. The histori- cal seismicity in Algeria is imperfectly known: its coverage is discontinuous and its record is grossly deficient. The seismicity of the twentieth century is relatively well documented; however, the further back in time, the harder it becomes to collect data. Even so, important research work in studying the seismicity in Algeria has been made in the past by several studies, notably those by Alexis Perrey, M. Chesneau, F. de Montessus de Ballore, N.N. Ambraseys and J. Vogt. This paper discusses the problems of historical earthquake investigation in Algeria and presents a selection of notable historical earthquakes in the country. Mailing address: Dr. Djillali Benouar, Built Environment Research Laboratory, University of Bab Ezzouar (USTHB), Civil Engineering Department, BP 32 El-Alia, Bab Ezzouar, Alger 16111, Algeria; e-mail: dbenouar@yahoo.com 556 Djillali Benouar Service Météorologique d’Algérie during the pe- riod 1889 to 1908. Roussel (1973) listed Alger- ian earthquakes during the period 1716 to 1970, to analyse the frequency of the seismicity; he prepared also a map of observed intensities. More recently, Ambraseys and Vogt (1988) pub- lished a study on the historical seismicity in the province of Algiers during the period 1365 to 1902, and Guidoboni et al. (1994), in her «Cat- alogue of ancient earthquakes in the Mediter- ranean area up to the 10th century», mentions a seismic event, which occurred in Setif (Algeria), known at that time as Sitifis, in 419 A.D. The seismicity of Algeria throughout the 20th centu- ry is addressed in detail by Benouar (1994). 2. The historical period The period of interest for historical studies should be extended as back in time as far as the data allow. The period before 1900 (the pre-instrumen- tal period) has to be studied from macroseismic data. Events are typically characterised by a brief description of the macroseismic field, from the data available today. Events reported in the older national catalogues cannot be in- cluded in the parametric date set to be used in seismic hazard assessment where the data used in these events are not sufficient for a reliable analysis. These events were usually not studied in a systematic manner or following a defined and documented methodology; they were gen- erally collected from various authors from dif- ferent periods. These catalogues were compiled by their authors for the preoccupation of their times and not with the needs of present studies in mind. 3. General methodology This section presents the methodology used to re-evaluate the seismicity of Algeria and ad- jacent regions. This requires: (i) the retrieval and revision of macroseismic data; (ii) the de- velopment of a methodology for the preparation of earthquake catalogues. For historical earth- quakes (pre-instrumental events) and even for twentieth century earthquakes for which there are no instrumental data, but for which intensi- ties and radii are available, magnitudes are cal- culated from macroseismic data (using calibra- tion relationships). 3.1. Measurement of time, intensity and magnitudes In Algeria, as in other countries, the meas- urement of the exact time of earthquakes, which occurred in different historical periods, is very important for our preoccupations today. Not tak- ing this seriously will affect the catalogue through duplicating events and conflating sepa- rated events. Through history, different calen- dars have been used, which makes the measure- ment of time of seismic events a complex work. Destructive earthquakes in Algeria and else- where have always caused substantial damage and/or total collapse to adobe and other prevail- ing types of houses. These structures have shown throughout history a high vulnerability and a very low resistance to seismic loads, and even to heavy rain. As a result of their weakness, maximum in- tensity in any destructive earthquake in Algeria seems to saturate; that is, at intensity 9 or less on the MSK scale, all adobe houses are destroyed, and thus any settlement or hamlet would be equally, but no more, devastated at higher inten- sities of the scale. Because the building stock in Algeria has numerous variable characteristics such as age, building materials and structural sys- tems, an extensive investigation has to be carried out to reveal what type of constructions were ex- posed and what state they were in during the time of each event. Due to certain particularities in the construction materials, the assessment of intensi- ties poses some significant problems. Another serious problem that arises is the correct interpretation of written accounts accord- ing to the historical context of the period con- cerned. Certainly, exaggerations exist in the sources; usually made to attract more attention to documents, but these are not very difficult to dis- cover, particularly when there is more than one source. The contribution of illustrative photo- graphs to the damage survey, when they exist, is noteworthy. 557 Material for the investigation of historical seismicity in Algeria from the records of past earthquakes Magnitude scales allow the classifying of earthquakes objectively and independently of local ground conditions and environment. For a variety of reasons, many earthquakes in Al- geria remain without surface-wave magni- tudes or simply without any type of instru- mental magnitude. To solve this problem, Ms is estimated when possible from semi-empir- ical relationships between Ms and mb or Ms and ML (Benouar, 1993). Ms for historical earthquakes may also be estimated from the radius of perceptibility (r3), which is defined as the mean epicentral distance of an area within which the shaking was felt with an in- tensity equal to or greater than 3 MSK, and then using a relationship between Ms and r3, or by using a relationship between Ms and I0 (Benouar, 1993). The location of macroseismic epicentres is also of great value, in terms of understanding tectonics features, particularly during the peri- od before 1900 as well as for the first half of the 20th century. Macroseismic information pro- vided by the available source documentary ma- terials usually allows relatively accurate loca- tion of the epicentral zone. 4. Macroseismic information (literary sources) Documentary source materials are essen- tial for a retrospective reconstruction of the macroseismic field data of past earthquakes. Such materials are found to be available in li- braries and archive centres across Algeria and abroad. These archives do not always give full macroseismic information for the historical earthquakes during the period pre-1900. A particular case demonstrating the incomplete macroseismic data of Algeria is that of the city of Algiers. There are very few literary sources and documentary materials referring to de- structive events in Algiers. The earthquake damage reported in the few records known to have occurred in the city during the last seven centuries of its history is very infrequent, small and mainly due to relatively large earth- quakes originating either on land, away from the site of Algiers, or offshore at a consider- able epicentral distance from Algiers. The earliest known earthquake to have caused loss of lives and considerable damage in Algiers occurred in 1365 (Ambraseys and Vogt, 1988). Earlier authors writing on the subject include Shaw (1743) who discussed «De la fréquence des tremblements de terre en Algérie» in his book «Voyages dans plusieurs provinces de Barbarie». Many factors influence the content and the survival of macroseismic information. These include site accessibility, geographical position of the region, and density of the pop- ulation, socio-economic conditions, political and military situations, and building stock characteristics. For instance, during the French colonisation period of Algeria, the lack of macroseismic information for native settlements was obviously conditioned by the censorship imposed by the French adminis- tration. To understand better the importance of an earthquake and the macroseismic data contained in the contemporary source docu- ments, it is imperative that retrieved informa- tion be carefully analysed in their whole his- torical context, to avoid serious miscalcula- tions. For many parts of Algeria and other parts of the world with comparable literary production and density of inhabitants, it is well known that the cities, large towns or ma- jor settlements constitute the main sources of macroseismic information. In cases when the event occurs within the surroundings of such a town, damage information is more likely to be well reported, while that occurring in re- mote sites could be left unclear. The scarcity of available macroseismic information obvi- ously makes the catalogue very incomplete. 5. Notable historical earthquakes in Algeria Table I presents a sample of historical earth- quakes illustrating the seismic activity in Alge- ria during the period before 1900. Both these events and the other known historical earth- quakes in Algeria still need to be investigated fully in order to be able to include them in the catalogue and use them with an acceptable de- gree of reliability in seismic hazard analysis. The locations of the principal places affected are shown in fig. 1. 558 Djillali Benouar Table I. Selected historical earthquakes of note in Algeria (see fig. 1 for location of places). Date Place Notes Reference 419 Setif Mentioned by Augustine Guidoboni et al. (1994) 3 January 1365 Algiers Destructive Ambraseys and Vogt (1988) 10 March 1673 Algiers Similar to 1716 event Ambraseys and Vogt (1988) 3 February 1716 Algiers Destructive; many casualties reported Ambraseys and Vogt (1988) 29 November 1722 Algiers Several casualties Ambraseys and Vogt (1988) 9 October 1790 Oran Well documented; 2000 dead. Lopez Marinas and Salord (1990) March 1819 Mascara Extensive damage in Mascara; damage also in Oran Mokrane et al. (1994) 3 March 1825 Blida Damage at Blida and Algiers, 7000 dead Ambraseys and Vogt (1988) 22 November 1851 Mascara Damage at Mascara but no deaths Rothé (1950) 22 August 1856 Djidjelli Destructive; tsunami Ambraseys (1982), Aucapitaine (1856), Senarmont (1856), Glautier (1856) 9 March 1858 Kherba Damage in Kherba area Ambraseys et al. (1991) 2 January 1867 Mitidja Destructive Cochard (1867), Ambraseys and Vogt (1988) 16 November 1869 Biskra 200 houses damaged and 30 dead Rothé (1950), Roussel (1973), Ollivier (1870) 3 December 1885 M’sila One of the strongest in East Algeria: 33 dead Rothé (1950), Harbi (2000), contemporary press reports 29 November 1887 El Kalaâ At least 331 houses collapsed; 20 dead Chesneau (1892), contemporary press reports 15 January 1891 Gouraya Heavy damage; 36 dead Pomel (1891), Rothé (1950), Ambraseys and Vogt (1988) Fig. 1. Location of places mentioned in table I. 559 Material for the investigation of historical seismicity in Algeria from the records of past earthquakes 6. Discussion and conclusions The general conclusion is that long-term his- torical seismicity of Algeria for the period be- fore 1900 is very imperfectly known. The main reasons for this situation are the limited avail- ability and the quality of contemporary docu- mentary records, the prevailing circumstances during the time of the event, the geographical lo- cation and problems relating to the intensity. For the period before 1900, the local sources are re- ally the only means of describing the impact of the event for the region, and in the case of non- availability of such sources, the macroseismic data will remain unknown and thus incomplete. This may explain the apparent low seismic haz- ard of a number of regions shown on actual modern earthquake hazard maps in Algeria that may not be correct; it may illustrate the absence of long-term observations. The types of material and the construction techniques during the period before 1900 are different in many respects from today’s con- structions and thus attributing intensities to such destructive events without taking this into account could be misleading. It may be con- cluded that the historical seismicity in Algeria remains incomplete, and unknown events dur- ing the period before 1900 cannot be included in the earthquake catalogue to be used for seis- mic hazard analysis. In Algeria, as in other regions where the seismicity is medium to low, the importance of the role of historical data is most evident. 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