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Geophys. 1. Int. (1992) 110, 141-158 The Armenian earthquake of 1988 December 7: faulting and folding, neotectonics and palaeoseismicity H. Philip,' E. Rogozhin,2 A. C i ~ t e r n a s J. , ~ C. Bousquet,' B. Borisov2 and A. Karakhanian4 'Laboratoire de Tectonique, Universite' des Sciences et Techniques du Languedoc, Place E. Bataillon, 34095 Montpellier Cedex 05, France 'Inrtitute of Physics of the Earth, Soviet Academy of Sciences, Bolshaya Gruzinskaya 10, Moscow 123242, USSR 'Inrtitut de Physique du Globe, 5 Rue Rent! Descartes, 67084 Strasbourg, France 'Institute of Geological Sciences, Armenian Academy of Sciences, Prospect Marshala Bagramiana 24-0, Yerevan 375019, USSR Accepted 1992 January 21. Received 1992 January 21; in original form 1991 February 4 SUMMARY The Spitak earthquake of 1988 December 7 is the first well-documented event directly associated with surface breaks in the Transcaucasian regions of the USSR. The earthquake was located within the ESE-WNW oriented Pambak-Sevan thrust and fold zone corresponding to the southern front of the Lesser Caucasus. The mechanism of the earthquake is consistent with the nearly NS compressive tectonics due to the active continental collision between the Arabian block and the Russian Platform. The rupture is composed by several branches, two of which reach the surface. The first branch, oriented N140", begins near the village of Alavar, has a length of 11km and disappears at about 4 km SE of Spitak. It consists of a right lateral en kchelon system of strike-slip faults, with a maximum offset of 50cm. The second branch is the main one and breaks for about 8 k m between Spitak and Gekhasar, with a general orientation N120", showing reverse faulting dipping to the north with a right lateral component. Surface ruptures between Gekhasar and Spitak show either a narrow band of pressure ridges in alluvial deposits and soil, or a fault scarp in the bed rocks when soil is absent. Maximum displacements, observed between Spitak and Gekhasar, attain 160cm of vertical motion and 90cm of horizontal dextral offset. The displacement also varies within this branch, showing finer segmentation. Few secondary deformations are observed: some normal faults near Gekhasar correspond to the collapse of the uplifted block. An anticline fold, oriented parallel to the fault scarp and situated along its northwestern prolongation, emphasizes the regional compressive tectonics. Landslides were activated on its flanks. A 200m long reverse fault break observed along the hinge of a secondary fold suggests the occurrence of a blind thrust at depth and that the hidden branches, which continue the main central segment towards the NW, are associated to surface folding. Therefore, the total length of the fault and fold zone is about 40km in agreement with the rupture length and seismic moment obtained on the basis of surface wave modelling. Well-developed uplifted terraces in the'northern block and subsiding valleys in the southern block indicate past Quaternary activity in the fault region. Palaeoseismological evidence of ancient earthquakes has been recognized in trenches across the Spitak fault. One old event occurred between 17000 years BP and the beginning of the formation of present-day soil. Key words: Armenia, earthquake source, folding, rupture, tectonics. 141 142 H. Philip et al. 1 INTRODUCTION The destructive earthquake that struck northern Armenia on 1988 December 7 devastated the region around the cities of Leninakan, Spitak and Kirovakan. This earthquake was located within the Lesser Caucasus (latitude 40.88"N, longitude 44.26"E, depth 10 km according to Dorbat et at. (1992), and magntiude M, = 6.9 according to NEIC). This is a region of continental deformation subjected to compression and roughly characterized by a mountainous landscape with E-W folding and reverse faulting, N-S oriented volcanic alignments and large fN45" strike-slip faulting (Fig. 1). The source region was known to be a seismic area, but no historical record of such a large event was previously available (Shebalin & Borisov 1989; Ambraseys 8i Adams 1989; Reisner 1989). Thus, this was an unusual event that aroused a strong and generalized feeling of solidarity and also much scientific attention. A multidisciplinary programme of study of the earthquake source was immediately set up. A general account of the observations made by a French-Soviet team around the epicentral region has been published elsewhere (Cisternas et al. 1989; Borisov & Rogozhin 1989; Trifonov et al. 1989). The study of coseismic surface ruptures and deformations, palaeoseismicity, aftershock distribution and mechanism, world-wide broad-band seismic records and other observations (Haessler et al. 1992; Dorbath et al. 1992) permitted a comprehensive understanding of the rupture process and its relations to the tectonic regime. It has also been possible to obtain information concerning seismic hazard. The purpose of this paper is to give a detailed description of the surface tectonics, main ruptures, folding and secondary features, to establish their connection with the geophysical observations and to propose a mechanical model of the earthquake source. Neotectonic and palaeoseismic observations that give clues to the ancient activity of the Spitak fault are also described. 2 GEODYNAMICS The geodynamic context of the Great and Lesser Caucasus has been described by Gamkrelidze (1986), Khain & Milanovsky (1963), Reisner (1982), Sholpo (1982), Vardapetian (1979), Zonenshain & Le Pichon (1986), Philip et af. (1989) and Balyan et af. (1989) among others. Global plate tectonics modelling permits identification of the northern drift of the Arabian Plate at a rate of 3cm-'y as the motor of the tectonic processes. The Iranian and Anatolian plates are laterally ejected (Fig. l ) , but the Lesser Caucasus is pushed against the Mesozoic back-arc basins to the north. The closing of the back-arc basins, whose remains are the Black Sea and the southern Caspian Sea, initiated the process of continental collision. Thus, the relief of the Great Caucasus corresponds to a young collision mountain belt (Middle Pliocene). At the same time, the Lesser Caucasus has been subjected to intense deformation, folding Figure 1. Present-day seismotectonic features in the surrounding area of the Armenian earthquake after Philip el af. (1989). 1-Recent volcanos; 2-relative motion with respect to Eurasia; %major strike-slip faults; &major thrust faults; h c e a n i c or intermediate crust; kontinental crust; 7-main sedimentary basin; &recent folding at the border of the Arabian Plate; %picentre of the 1988 December 7 Spitak earthquake. G.C.--Great Caucasus; D-Daghestan; AR.-Arabian Plate; Z-Zagros; K.-Kasbeg volcano; E.-Elbrus L.C.-Lesser Caucasus; T-Talysh; El.-Elborz; Tur.-Turkish block; volcano; A.-Aragats volcano; P.S.F.-Pambak-Sevan fault zone. The Armenian earthquake 143 4 Foyre 2. Main seismotectonic features of the Armenian region. 1-Epicentre of the 1988 December 7 earthquake; 2-fault segments (blind fault, fold, reverse and strike-slip faults) activated during the main shock; %major strike-slip faults; &major reverse faults; 5-volcanos; &volcanic alignments and extension axes; 7-regional direction of shortening. (a) Volcanic axis of Akhalkalak; (b) volcanic axis of Guegam; (c) Alavar fault; and (d) Amasiya-Sarikamish fault. and reverse faulting, its southern front being overturned and thrusted to the south. The Pambak-Sevan fault zone corresponds to the main suture that limits the most deformed structures of the Lesser Caucasus to the south. Milanovsky (1968) described the Pambak-Sevan synclinorium as a thick folded and faulted package of Cretaceous to Palaeogene sediments, situated on the southern margin of the Lesser Caucasus and formed by an en kcchefon system of young basins filled with Plio-Quaternary sediments several hundred metres thick, the disposition of these depressions being consistent with a dextral lateral component of the slip along the PambakSevan fault. Furthermore, the rivers are shifted by several kilometres in a dextral sense across the fault (Figs 2 and 3). The kinematics of the fault system are consistent with a N-S compression and an E-W extension deduced from the roughtly N-S oriented extensional fissures and the normal faulting related to the recent volcanism (Philip et al. 1989). Two volcanic alignments are shifted by several tenths of a kilometre by the Pambak-Sevan and Alavar fault system, which acts as a right lateral transform fault (Fig. 2). A portion of the displacement along the Pambak-Sevan and Alavar fault system corresponds to the extension associated to the two N-S volcanic alignments. Satellite images taken before the earthquake d o not show direct morphological evidence of the Spitak fault. O n the other hand, the Amasiya-Sarikamishkii, the Pambak-Sevan and the Alavar faults may be neatly followed for lengths that vary from several tenths to several hundred kilometres (Fig. 2). 3 S U R F A C E R U P T U R E S A N D FOLDING The surface expression of the fault shows an important degree of complexity characterized by segmentation and transition from faulting t o folding associated with a blind thrust. Coseismic secondary features are also present: landslides and rock falls, gravity induced normal faulting associated to thrust, earthquake originated springs with gas emanations some of them intermittent, burnt bushes near cracks, natural dams on fault scarps and associated ponds, mole-hills indicating unusual animal activity near the fault during winter (the name of the mole is Ellobius talpinus according to Borisov & Rogozhin 1989). 3.1 Typical structures along the fault The nature of the surface breaks varies along the fault. Different types of structures can b e recognized depending 144 H . Philip et al. F i r e 3. Topographic map showing the surface breaks, active folds and uplifted terraces (dotted areas) in the epicentral zone of the 1988 December 7, Armenian earthquake. upon the properties of the near-surface materials, the geometry of the fault and the topography as observed in the case of other earthquakes on reverse faults (Everinghan el Qf. 1969; Kamb et af. 1971; Gordon & Lewis 1980; Philip & Meghraoui 1983). (1) In some cases (Fig. 4a) the fault scarp appears very clearly at the surface (simple thrust scarp) without any secondary features and slickensides may be observed. (2) A gravitational collapse of the hanging wall may happen when the fault mirror is too steep (hanging wall collapse scarp, Fig. 4b). (3) Sometimes, a pressure ridge is formed that may be broken at the hinge (simple pressure ridge, Fig. 4c). Figure 4. Typical fault scarps along the Spitak fault. The following names are used in the text: (a) simple thrust scarp; (b) hanging wall collapse scarp; (c) simple pressure ridge; (d) dextral pressure ridge; (e) back-thrust pressure ridge; (f, low angle pressure ridge; (g) en khelon pressure ridges. 1-bedrock; 2 s o f t Quaternary sediments; %turf. The Armenian earthquake 145 3 km Figure 5. Topographic map of the southeastern Alavar segment of the surface ruptures. Several subsegments are formed by en kchelon strike-slip faults. The overall motion is right lateral. The rectangle shows the detailed area of Fig. 7. (4) Gravitational sliding may produce normal faulting and tensile cracks behind the thrust front (Fig. 4d), this situation being observed at different scales (metric to hectometric). This configuration, related to the change in the dip of the fault as it approaches the surface and triggered by gravity, is similar to that observed during experiments of compressional fracturing under a confinement pressure (Friedman et al. 1976). En kchelon tensile cracks within the hanging wall indicate the lateral component of slip (dextral or sinistral pressure ridge). (5) In other cases, the thrust front generated a fan-like compression ridge (back thrust pressure ridge, Fig. 4e). (6) The thrust front may also produce a detachment of turf layer inducing a fold ahead of the fault (low angle pressure ridge, Fig. 4f). (7) In cases where strike-slip is dominant and vertical offset is weak, a system of en kchelon transverse faults connecting folds or pressure ridges or en kchelon cracks may be observed in the turf cover (en kchelon pressure ridge, Fig. 4g). 3.2 The southeastern segment This segment was discovered only during the spring of 1989 after the snow melted away. The ruptures form a right lateral en kchelon system, oriented N140", with four main subsegments, each one measuring about 2km. They are complemented by two conjugate en khelon left lateral subsegments oriented N010". They are, in general, situated at high altitude (between 1750 and 2200 m) along the Spitak mountain ridge (Fig. 5). The first subsegment begins near the village of Alavar 146 H . Philip et al. 81 Figure 7. Detailed map of the en echelon folds and cracks in turf of the area indicated in Fig. 5. Folds, or broken folds, are connected by right lateral transform faults. The overall system is right lateral horizontal shear as shown by the large arrows. The direction of the main compression axis u, is also shown. with a series of tensile cracks oriented N140". They are followed by an en echelon system of right lateral transform faults that connect a sequence of folds in the turf cover [Figs 4g, 6 and 8 (opposite page)] similar to those already described in Peru by Philip & MCgard (1976). The next subsegment (Fig. 5) is formed by two parallel branches. The northern one exhibits a reverse fault on a saddle (Fig. 8, opposite page) with 50cm of vertical offset and a right lateral offset of similar amplitude. The vertical offset diminishes towards the SE and there is a progressive transition to an en echelon dextral system. The southern branch is formed by two conjugate en echelon systems. The third and fourth subsegments are oriented N140" and they are separated by a N010" trending left lateral en 6chelon system. The third one is formed by two parallel branches. A detailed map of a part of the northern branch is shown in Fig. 7. Folds are oriented N080" to N095" and the transverse faults trend N-S to N170". A 40cm lateral displacement is measured in the central strike-slip transverse fault along lozengic openings. The fourth subsegment exhibits also a reverse portion across a saddle. The last subsegment, oriented NOlO", consists of a large system of left lateral en echelon cracks. The overall direction of shortening deduced from these series of structures is NNW-SSE to N-S. Nevertheless, local irregularities are observed, showing that the fault is complex at the surface. Ruptures are discontinuous and localized within a 1km wide band in the central part. Conjugate subsegments increase the complexity. Vertical slip is important only at the high saddles and disappears at the bottom of the river valleys. 3.3 The central segment This is the most important surface feature of the Spitak earthquake from the point of view of maximum slip. It corresponds to an 8 km long reverse fault with a right lateral offset formed by three subsegments. The first one begins near the road from Spitak to Yerevan, which is offset 15 cm in a right lateral sense (Fig. 9). The horizontal displacement increases rapidly to the west and reaches 100cm in a field, west of the road to Yerevan, on a right lateral strike-slip branch. A maximum vertical amplitude of 160cm is observed further away towards the northwest, at about 700 m from the road. The corresponding horizontal displacement at the same site is 40cm and the fault runs along the geological contact between Palaeogene volcanic rocks to the north and Cretaceous limestones to the south (Fig. 10). A gouge zone of about 2 m thick is present and the layers of limestone show strong deformation and cleavage. The aspect of the ruptures along the fault is given either by a simple thrust scarp, or a hanging wall collapse scarp or a dextral pressure ridge (Fig. 4) according to the thickness of the soft sediment cover or soil. The first subsegment ends at about 2 km west from the Yerevan road at the intersection with an irrigation channel. Before the crossing with the duct, the direction of the fault changes to N050" and the horizontal component becomes sinistral. On the other hand, the vertical offset decreases to a value of about 80 cm. The irrigation duct is shortened by about 45 cm by the fault. After an undisturbed interval of some 500m across a cultivated field, the fault splits into two branches surrounding a hill and converging towards the west of it (Fig. 9). The motion is reverse and the shape of the surface rupture corresponds to a hanging wall collapse scarp (Fig. 4b). The amplitude of the slip is very small (20-25cm and only vertical) on both branches near the field and increases rapidly uphill towards the northwest (130 cm vertical and 40cm horizontal slip). The shape of the fault is well described by a low angle pressure ridge [Fig. 4f and Fig. 11 (opposite page)], one kilometre west from the field. In fact, a bifurcation of the thrust front is evident in Fig. 12 where a double band of low angle pressure ridges (Fig. 4f) is connected by tensile cracks. A sequence of cross-sections illustrating the splitting of the thrust front and the relationship between the main fault trace in depth and the turf detachment and folding is given in Fig. 13. A vertical offset of about 130 cm may be obtained from these sections. The tensile cracks follow the orientation of the maximum compressional stress N170", similar to what was observed Figure 6. Photograph of a detail of the en echelon folds and cracks in the turf (Fig. 7) along the southeastern segment of the fault. Figure 11. Photograph of the fault scarp with a double band of pressure ridges near the pass at 4 km west of Spitak (Fig. 12). View towards the northwest. Figure 8. Photograph of a reverse fault scarp on the saddle showing 50cm of vertical offset. southeastern segment of the fault. Figure 15. Photograph of normal faulting associated with the collapse of the hanging block of the thrust fault near Gekhasar (location o n Figs 9 and 17). View towards the northeast. Figure 24. Photograph of the fault scarp 1 km west of Spitak. View towards the west showing the dip to the north. The uplifted northern block is formed by volcanics. The profile has been regularized by erosion and there is no evidence of an older scarp. Figure 14. Photograph of the fault scarp in the saddle at 4 km west from Spitak. Vertical offset: 1.6 m. Horizontal offset: 0.9 m. View towards the east. The Armenian earthquake 147 Figure 9. Topographic map of the central segment of the scarp between Spitak and Gekhasar. The square indicates the location of Fig. 12. 1-Thrust; 2-strike-slip; >normal fault; &uplifted terraces; %location of trenches; &railroad; 7-main Figure 10. Cross-section across the fault scarp 700 m west of Spitak. 14retaceous limestone; 2--gouge zone; and %breccia in palaeogenic volcanic rocks. along the southeastern Alavar segment. Nevertheless, the road; and &rivers. simple thrust scarp to a simple pressure ridge (Figs 4a and c). There, we observed a remarkable pressure ridge in the turf layer with a vertical offset of IlOcm. Further down, along the ravine, the northern slope becomes very steep and gravitational effects perturb the tectonic structures. Several lateral streams running N-S are cut by the fault, the V-shape of the valleys being offset accordingly, due to the uplifting and dextral motion of the northern block with respect to the southern one. The second time the fault cuts the main stream it forms a one metre high dam within the bedrock with a small pond behind. Farther west the fault disappears for 50 m and reappears in the cultivated fields as a simple pressure ridge. A rural track orthogonally intersected by the fault trace is offset and no other special feature is found on the field. angle between the main stress and the fault trace is about Next, the scarp runs uphill towards a second saddle before 60" in this case, while the corresponding value for the descending towards Gekhasar. The limit of a laboured patch located uphill is apparently displaced more than 2 m by the fault (Trifonov et af. 1989), but the offset evaluation can be overestimated because of the combined effect of topography, slip vector and gravity. This is the case of a reverse fault with a constant horizontal slip component that, cutting a hill, shows attenuation of the displacement on one slope and exaggeration on the opposite one. Along the second saddle, the fault looks like either a simple thrust scarp or a dextral pressure ridge, the vertical offset being 1 m (Figs 4a and d). The hanging wall shows en ichelon tensile cracks in agreement with the right lateral component of slip. Down the slope, towards Gekhasar, the fault runs across volcanic rocks without any indication of previous geomorphological markers. Nevertheless, a fault gauge confirms recent activity. Vertical offset is almost southeastern segment was about 30" only. The highest point on this central segment, with an elevation of 1930 cm, is located at about 200 m NW from the bifurcation discussed above. It corresponds to a saddle where the fault scarp is very well exposed (Fig. 14, opposite page). The character of the fault along the saddle is represented either by a simple thrust scarp or by a hanging wall collapse scarp (Figs 4a and b). Striations on the fault plane show a vertical offset of 160cm and 90 cm of dextral horizontal component. En ichelon tensile cracks on the hanging wall are compatible with the dextral horizontal slip. West from the saddle, in the direction of Gekhasar, the trace of the fault is linear for about 1 km and runs almost parallel to a stream, cutting it in two places. At about 100 m west from the saddle the scarp undergoes a transition from a 148 H . Philip et al. q$................;. ; ."? ....... h \ I Fmre 12. Detailed map of double pressure ridge formed by (location in Fig. 9). . . . lorn, . . en kchelon folds and transform faults east sw 2 m , I of the high pass in the central segment purely reverse and attains 50cm within a portion oriented N085". The main branch cuts an irrigation duct close to a grove tree, next to Gekhasar with a shortening of 75cm. Then it bifurcates on the slope next to Gekhasar as a dextral pressure ridge (Fig. 4d), the scale being hectometric (Fig. 9). A branch in normal faulting on the hanging block follows the crest of the hill northeast of Gekhasar [Figs 15 (opposite p. 147) and 161, but some minor branches are also located along the northern slope of the hill. Westaway (1990) interpreted both branches as reverse faults, but the arc-like shape of the normal branch and the tension cracks are clearly against his model. Vertical and horizontal offsets diminish very rapidly between the railroad and the highway Spitak-Leninakan (20-30 cm vertical and 14 cm horizontal offset) and finally the breaks disappear north of the road. Different amounts of thrust across the fault produced extension along the Spitak-Leninakan railroad. The rails were thrown out of their sleepers some 800m east of the intersection with the fault, rail-clips and the rails themselves were broken and pulled 28cm apart. The rails were also broken at the intersection but as a result of compressional and strike-slip motion (about 20cm in each direction, see Fig. 17). 3.4 Fold and ruptures in the northwestern segments 3.4.1 Third hidden segment Figure W. Cross-sections across the double band of pressure ridges corresponding to detailed map of the reverse fault shown in Fig. 12 and vicinity. Northwest of Nalband (Fig. 3) there is a large anticline formed by Palaeogene volcanic and sedimentary layers that creates a ridge, 8 km long, between the Pambak river valley and that of its confluent, the Chichkhan river. Secondary folding within the main one are observed in the hills next to and north of Nalband (Fig. 18). No major coseismic rupture has been observed in this region; nevertheless, some direct or indirect structures related to the earthquake are localized on the anticline or its immediate vicinity. These structures are of three types: (1) Landslides that are present on the periphery of the The Armenian earthquake 149 Photo ftg.15 Figure 16. Section across the end of the central segment near Gekhasar, showing normal faulting in the hanging block (Fig. 15, opposite p. 147) and several terraces next to the Pambak river in the uplifted compartment of the thrust fault. The small inset in the lower left comer shows the situation just before the gravitational collapse. fold along the flanks or on the pericline end. The most spectacular one is the Sarapat landslide, 9 km NW of Nalband, on the end of the anticline and next to an abandoned dam project. It was triggered by the gliding of limestone layers on top of soft layers of marl and it cannot be interpreted as being part of the tectonic rupture. Several fossil landslides on the northern flank of the fold were not activated by the earthquake of 1988 December 7. O n the other hand, two landslides were activated on the southern flank. (2) Tension gashes. Rectilinear tension cracks with an opening of 10-15 cm and several metres long, are located on the crest of the ridge that corresponds more or less to the top of the anticline. They are well exposed near the Nalband television antenna. (3) Reverse fault. A short broken segment, 200111 long and 8 cm of reverse relative displacement dipping north, was observed some 3 km to the northwest of Nalband along the core of one of the secondary folds (Fig. 18). The collection of these observations suggests that the fold was active and deformed during the earthquake, the broken surface being hidden under the fold. This statement is also in agreement with the distribution of aftershocks as shown below (Dorbath et al. 1992). 3.4.2 Fourth hidden segment Moreover, the aftershock distribution and focal mechanisms indicate that another fold, northeast of Leninakan, was also deformed during the Spitak earthquake. This fold, limited to the south by a fault scarp not active during the earthquake, but clearly recognizable in satellite images, is parallel to and of the same length as the previous one. Therefore, a fourth segment, parallel to and shifted to the WSW with respect to the third one, should be considered, the fault being again hidden under the fold. Two landslides along the southern flank of this fold were weakly activated. 3.5 Interpretation of the field observations The above observations, combined with the results of seismological studies (Dorbath et al. 1992; Haessler et al. 1992), indicate the presence of five major segments from SE to NW (Fig. 3): (a) The southeastern branch of the fault is oriented N140" approximately, with a dip of about 70" to the NE, and shows a dominant right lateral strike-slip offset of about 40-50 cm. The breaks begin near the village of Alavar and extend for a length of 11 km, disappearing at about 4 km SE of Spitak. (b) The central branch of the fault is oriented N120", with a dip of about 50" to the north. It has a dominant thrust component that may reach 160cm and an observed maximum right lateral offset of 90cm. This rupture extends from Spitak to the village of Gekhasar for about 8 km. (c) The third segment to the northwest of Nalband shows very minor surface breaks (reverse faulting and tensile cracks). Most of this deformation is located along an 8 km long anticline oriented N120". The only great landslides activated furing the earthquake are located on the flanks of this fold. (d) The distribution and focal mechanisms of aftershocks (Dorbath et al. 1992) and the presence of another anticline to the SW of the previous one suggest that a fourth segment has been active as a reverse fault at depth, but we have not been able to find any surface expression of its activity up to now. (e) A fifth segment (Dorbath et al. 1992), with a right lateral slip on a vertical plane oriented N140", is evident only from a linear cluster located at the northwestern extremity of the aftershock cloud (see Fig. 2). The second, third and fourth branches are disposed en kchelon with a separation that varies between 2 and 5 km. The slip vector on each segment of the fault is in agreement with the regional compressional stress that was described in 150 H. Philip et al. the previous section and with the modelling of broad-band teleseismic waveforms (Haessler et al. 1992). 3.6 Quantitative distribution of the slip along the fault Figure 17. Damage on the railroad near Gekhasar at the intersection with the surface ruptures. The rails were compressed (a) at this point, but farther away they were pulled apart (b). Vertical and horizontal offsets are of the order of 20 cm in (a). The eastern (right hand) limb is upthrusted. Rectangles show the sites of the cross sections in Fig. 21. . . . . . v Figure 18. Active folding NW of Nalband. Small reverse fault, landslide and extensional cracks on top of the anticline. Slip along the fault has been measured at different sites, in particular along the central segment between Spitak and Gekhasar (Fig. 19). Vertical displacement has been measured directly on the fault scarp and horizontal displacements have been measured from the lateral offset of linear features (tracks, furrows, field boundaries, etc.) across the fault. The presence of slickensides on the fault mirror at several sites permits the joint determination of both horizontal and vertical slip. Several important features may be observed in Fig. 19, where the values collected along the central segment are shown: (1) The absolute value of the vertical and horizontal components of the slip, and their ratio, vary along the central segment. The latter one depends upon the orientation of the fault trace, which varies from an EW to a NS direction. This is particularly true at the eastern end of the central segment, near Spitak, where a small branch of the fault with a direction close to N-S, has an almost pure dextral strike-slip motion (Fig. 9). Another segment oriented ENE-WSW corresponds to a reverse fault with a sinistral lateral component while the E-W oriented branches have a pure reverse motion. Only the segment oriented ESE-WNW shows a dextral lateral component. The whole set of observations (Fig. 20) is thus compatible with a u1 main stress direction close to N-S, whether obtained from the inversion of the data from striations produced by the main shock (by using the method of Etchecopar et al. 1981) or from the inversion of the focal mechanisms of aftershocks (by the method of Rivera & Cisternas 1990). (2) The magnitude of the displacement vector also shows a variation along the central segment. This variation determines a finer structure constituted by three subsegments each about 2 km in length. It is quite remarkable to observe that the places where the dislocation is zero corresponds to the passage of valleys at the ends of the central segment (Pambak river valley to the NW and Kachkara valley to the SE) and to two transverse valleys separating the three subsegments (Fig. 9). A topographic profile along the fault shows a very good correlation with the variations of the amplitude of the displacement (Figs 19a and b). This correlation alows the identification of an even finer structure within the southeastern subsegment next to Spitak. It suggests that there exists a relationship between the amount of slip on the fault and the topography. In fact, the slip decreases from its maximum values at the high passes to a zero value before entering into the soft sediments of the valleys. This is a most important problem since, as stated above, we cannot say whether the absence of slip is due to the presence of soft sediments on the valleys or, on the contrary, if the valleys are situated on the regions of absence of slip. The fact that the displacement disappears above the surface of the basins favours the second hypothesis. This means that each earthquake in the past probably had the same distribution of slip within a scale factor, and that the fine segmentation of the central branch has therefore been maintained for a long time. . . 1800. 1600. b o-~ slip vector .-.vertical displacement *-* horizontal displacement Figure 19. Diagram showing the relationship between surface displacements (a), and topography (b) along the central segment of the fault. Figure 20. Stress tensor determinations. (a) Histogram showing the angular differences between calculated and measured slip directions. (b) Mohr diagram showing normal and tangential stresses for observed striations. (c) Lower hemisphere Schmidt equal area projection showing: (1) fault planes; (2) slip vector directions on fault mirrors, squares being calculated striae and arrows the observed ones; and (3) main stress axes calculated either from fault mirror observations (filled symbols) or from first motions of aftershock recordings (open symbols). 152 H . Philip et al. Fewer quantitative measures were possible on the SE branch of the fault between Spitak and Alavar. Nevertheless, a similar correlation between topography and slip may also be established. Slip is absent at the bottom of the principal valley traversed by the fault trace (Fig. 5 ) , and becomes important on the slopes and on the high elevations. More precisely, we may say that the vertical displacement is maximum on the high points and that it diminishes and vanishes rapidly from the top. One possible explanation of these observations is given by McTigue & Mei (1981) after modelling the effect of gravitational loading due to topographic ridge upon a compressional tectonic stress. Their model shows that the effect of gravity due to the presence of a ridge is of the same order of magnitude as the tectonic stress, the transverse component of stress is reduced and may become extensional on the top of the ridge and that it is increased and is always compressional on the foothill. This effect could favour the rupture in places of high elevation while inhibiting it at the base of the ridges. 4 DISCUSSION A B O U T THE SEISMOTECTONIC F E AT URE S In this section, we compare the surface ruptures and deformations with the results obtained from the temporary seismic network (Dorbath et al. 1992), in particular the distribution and focal mechanism of the aftershocks. The aftershocks are mainly located to the north of the surface ruptures, their depth increasing from southeast (6 km) to northwest (14 km). The cross-sections across the different fault segments delineate the rupture surfaces dipping to the north rather well, but it is difficult to determine the transition from one segment to the next. The focal mechanisms within each segment are coherent and well-determined due to the good coverage and density of the seismic network. (1) Southeastern segment. In this 11 km long segment the aftershocks are very shallow, most of them above 6 km, and the surface dislocation does not exceed 40-50cm. Hence, the corresponding seismic moment is small, of the order of M, = loz5dyne-cm, about 1/6 of the total moment, contrary to the estimation proposed by Pacheco et al. (1989) on the basis of body wave modelling. The distribution of aftershocks shows an almost vertical rupture, oriented N140" and dipping 70" to the east. It is remarkable to observe that the aftershocks are very shallow near the edges of the segment and that they become deeper towards its centre. Thus, it appears that the ruptured surface is surrounded by a concentration of aftershocks. The focal mechanisms show an almost pure right-lateral strike-slip in good agreement with the surface observations. (2) Central segment. The hypocentre of the main shock is close to the lowest point of the line of intersection between the fault surface of the central segment and the almost vertical rupture surface of the southeastern segment. The depth distribution of aftershocks indicates a fault surface, oriented N120" and dipping 50" to the north. The seismic moment associated to this segment is about M , = 4.3 x dyne-cm (Haessler et al. 1992). Most of the aftershocks exhibit focal mechanisms corresponding to a thrust of the northern block over the southern one with a right lateral offset, in agreement with the surface measurements. (3) Northwestern segments. These are segments with deep seismicity not connected directly with surface breaks. The seismicity located below the fold that exists to the northwest of Nalband delineates a fault surface oriented N120" and dipping some 50" to the north. Hence, we may assume that plastic deformation took place within the fold and that the rupture did not reach the surface. The situation becomes more complex to the west of the western edge of the Nalband fold. The aftershock data show that the rupture bifurcates, one of the branches being shifted south and hidden under another fold, while keeping the character of a thrust with a right-lateral strike-slip component. The other branch, further north, has a N140" direction and a pure dextral strike-slip on a vertical fault plane. The transition between the third segment and the fourth is not evident, but in several sections of the aftershock distribution we observe a concentration of aftershocks between 3 and 5 km of depth. This is particularly clear at the northwestern edge of the Nalband fold, where shallow aftershocks are concentrated within a narrow linear band roughly oriented NE-SW and show a left-lateral strike-slip focal mechanism. The shallow depth of these aftershocks suggests a detachment surface at the contact between the basement and the sedimentary cover, and the focal mechanisms explain the offset of the folding to the SW. The correlation of the aftershock distribution and the surface observations shows that the length of the fault is longer than what appears at the surface, showing that the Spitak earthquake was produced by a half-hidden rupture. One of the most striking features of the Spitak fault is that it has a very complex appearance. Nevertheless, the complete set of focal mechanisms of the aftershocks on its different branches and the slip observations on the fault mirrors at the surface are all consistent with a singe stress tensor (Fig. 20). The focal mechanism of the main shock, either its average value (Cisternal et al. 1989) or a more detailed model (Haessler et al. 1992), is also consistent with that same stress tensor. 5 NEOTECTONICS A N D PALEOSEISMICITY Several neotectonic and geomorphologic observations along the fault indicate Quaternary activity. Thus, we have begun a programme of trenching across the fault in order to obtain fundamental data for the evaluation of past activity and recurrence times of earthquakes similar to that of 1988 December 7. Nevertheless, we have only a few accurate C14 datings at this moment, hence we cannot establish precise slip rates across the fault, but can only advance preliminary results. 5.1 Neotectonic observations Direct observations that confirm the Quaternary motion of this fault have been performed in two sites of the central segment, between Spitak and Gekhasar. (a) At the entrance of the gorge of the Pambak River, north of Gekhasar (northwestern end of the central segment The Armenian earthquake \ 1 ' NE NE of the fault). Along both sides of the walls of the road from Leninakan to Spitak, Quaternary sediments (volcanic tuffs, alluvium and lacustrine deposits) are tilted (up to 25") and offset near the trace of the fault (Figs 21a and b). These deformations had been observed previously by Milanovsky (1968), who thought the motion on the fault was normal. The more recent alluvial terrace overlays in unconformity the tuffs and is itself deformed. In fact, a levelling survey conducted on both sides of the Chichkhan River shows that the surface is bent as a syncline south of the fault and as an anticline to the north of it (Fig. 22). This terrace that lays some 10m over the river-bed seems to be of Wurmian age (Milanovsky 1968). The trench corresponding to the railroad between Leninakan and Spitak, in the same sector, cuts the same formations. Several reverse faults are present (Figs 21c and d). We observed that the fault corresponding to the rupture of the railway line shows a vertical offset of 2.5 m. This dislocation is larger than the one corresponding to the earthquake of 1988 December 7 (10-15cm). This offset, which is visible at the bottom of the trench, is rapidly attenuated in the vertical direction and does not reach the free surface. (b) At the main saddle, 4 km west of Spitak. An older scarp of the fault cuts some colluvium formations deposited along the slopes between Spitak and Gekhasar. The contact between the colluvium and the fault is clear some hundred metres northwest from the highest pass. In fact, it corresponds to a branch that was not activated during the last earthquake. This scarp, covered by the recent soil, is separated from perioglacial deposits by a breccia blended with palaeosoil (Fig. 23). 5.2 (4 Figure 21. Cross-sections (see Fig. 17) along the road (a) and (b) and along the railway (c and d) between Spitak and Leninakan near Gekhasar. (1) Alluvial deposits and lacustrine sediments; (2) Quaternary ignimbritic tuff and (3) alluvial terraces. i'" sw 5m . i A I .----- loom -- ---- , l I I 'a ##/ The general disposition of the relief on both sides of the fault zone does not present significant geomorphological features, indicating past tectonic activity. In fact, at some sites, the subsiding compartment looks higher than the uplifted one, though this may be due to differential erosion I aI ! I 1 I I I I I I I Geomorphological observations _-_-/-- I I 153 I I Chichka? riv ..,'t - NE . I 1 I , I Figure 22. Section along the Pambak River north of Gekhasar. (A) Topographic profile of the Wurmian terraces on the northern bank of the Pambak River. (B) Geological cross-section along the same profile (vertical exaggeration: 1.3). (1) Palaeogenic volcanic rocks; (2) alluvial and lacustrine sediments; (3) Quaternary ignimbritic tuff (4) Wurmian terraces; and ( 5 ) fault of the 1988 December 7 earthquake. 154 H . Philip et al. F p r e 23. Neotectonic observations along the central segment of the fault west of the saddle at about 4 km west of Spitak. (1) Palaeogenic volcanic breccia; (2) fault breccia blended with palaeosoil; (3) Periglacial colluvium formations; (4) modern soil; and ( 5 ) fault plane of the 1988 December 7 earthquake. Arrows indicate slickensides. since volcanic formations are in contact with limestones across the fault. However, a more careful study of the disposition of the alluvial formations on both sides of the fault, at a regional scale, reveals important accumulated deformations. For instance, the northeastern compartment is crossed by the Pambak river gorge at present and we find a system of imbricated terraces along its northern bank, between Spitak and the intersection with the Chichkhan River (Fig. 3). On the contrary, the disposition is completely different to the southwest of the fault, since the Pambak River runs over a basin smoothly filled with sediments southwest of Nalband. The same is true for the Kachkara River before joining the Pambak River south of Spitak. Moreover, the Milanovsky (1968) cross-section along the Pambak River cuts twice the Spitak fault and clearly shows, thanks to borehole loggings, a thickening of Quaternary sediments in the subsiding compartment. We have examined the morphology of the relief along the fault zone. The southeastern segment, between Alavar and Spitak, exhibits surface ruptures near the top of a zone of elevated topography ranging from 1600m to 2100m. At some places, the fault runs across saddles located over the slope in the interfluves. The surface ruptures of the last earthquake indicate mostly horizontal displacements along this segment, and the regularization of the fault scarps dominates over the tectonics. The central segment of the fault (between Spitak and Gekhasar) is situated on the southern Bank of a more or less continuous ridge, twice interrupted by the valleys of tributaries of the Pambak River. This ridge, formed by volcanics, corresponds to a fault scarp most often regularized over several tenths of a metre in front of the fault (Fig. 24, opposite p. 147). Steeper topography, accompanied by breaks in the slope along some transverse profiles, seems to correspond to places not affected by regularization rather than to the traces of successive uplifts. Weak scarps (one to two metres at most) may be observed within the Quaternary formations and soils at the level of the transverse valleys (trench excavated for the railroad, Fig. 21c, for example) several metres behind the 1988 ruptures. These fault scarps may be interpreted in two ways: either they correspond to a branch of the fault that was not activated during the 1988 event, or to a scarp that has receded by erosion from the original one that was located over the fault. The first hypothesis has been put forward by Westaway (1990) for a scarp situated more than 10m from the 1988 ruptures. Unfortunately, the trench excavated across the 1988 rupture has not been extended yet towards the old scarp. However, from the railroad trench north of Gekhasar, we may conclude that the scarp on the alluvial terrace does not correspond to a different fault and has therefore receded (Fig. 21c). On the other hand, the analysis of the different trenches indicates an accumulation of Quaternary deposits on the subsiding side of the fault and a correlative erosion of the uplifted block, a mechanism which favours the second hypothesis. The recurrence times for major earthquakes from trench data (seven to ten thousand years, as discussed below) is long enough for the erosion to move the scarp backwards. If that were not the case, and the old scarp belonged to a different fault, we would be forced to admit that erosion had not acted for at least some 5000 years according to the estimation of the recurrence proposed by Westaway . 5.3 Palaeoseismic observations In this section, we will describe preliminary results obtained in some trenches excavated across the fault. The conditions for palaeoseismology are quite different from those at El Asnam, where a fault controlled lake permitted exploration of the palaeoactivity for 6000 years (Meghraoui et al. 1988). Nevertheless, several outcrops in the central segment of the fault where the Quaternary is overthrusted by the volcanic formations gave evidence of ancient displacements corresponding to earlier earthquakes. Thus, we selected sites where recent sediments might be disturbed by the fault during past events. Four trenches were excavated across the fault between Spitak and Gekhasar (see Fig. 9 for locations). Trench I, dug in the middle of the central segment, shows the main fault scarp offsetting soil formations, a series of extension cracks filled with soil on the upper block and an antithetic reverse fault at the northern end (Fig. 25a). The thickness of Quaternary sediments to the south of the fault trace (Fig. 25b) suggests that three to four events, similar to the last one, did occur in the past. Trench 11, at about 2 km west from Spitak, shows an old gouge parallel to the 1988 mirror of the fault, offsetting colluvium deposits (Fig. 26). This can be interpreted as a clear sign of past activity. Trench I11 (Fig. 27), 100m to the east of Trench 11, contains a layer of palaeoturf that is covered by Quaternary deposits and offset by the fault, and it is the most interesting one since it gives quantitative palaeoseismic information. Two certain events (including the last) and possibly a third one may be observed in this trench where Quaternary deposits are overthrusted by Palaeogene volcanic rocks. The sites where CI4 dating was made gave the following ages: site A, old colluvium in the uplifted block: 24 765 f 770 yr The Armenian earthquake 155 N (b) TRENCH 1 bore hole . . . . Figure 25. Trench I across the fault scarp located 800 m to the east of the high pass in tnc: central segment (Fig. 9). (a) Western wall of the trench showing the main fault to the left. 1-Palaeogenic volcanic rocks; 2-yellow colluvium; %white colluvium, silt gravel and laminated gravelly sand; and &modern soil and turf. (b) Two boreholes drilled by the Armenian Academy of Sciences confirm the accumulation of several metres of colluvium in the lower compartment of the fault. s Figure 26. Trench 11 located 2 km west of Spitak (Fig. 9). 1-Palaeogenic and laminated gravelly sand; and +modern soil and turf. BP; site B, buried palaeoturf: 17 565 f 170 yr BP and site C, buried palaeoturf: 19 960 f 225 yr BP. These data indicate that the event which cut the palaeoturf is younger than 17565 years, and that two events occurred between that date and the present time. Other events possibly offset the colluvium dated from 24 765 years, after its deposit. The reconstitution of past events is based on the successive elimination of the offsets by recovering the continuity of the different levels across the fault (Fig. 28). Undoing the effect of the 1988 earthquake (about l m vertical offset at this site), it becomes clear that the palaeoturf (3) was deformed by an older event before the deposition of the recent colluvium (4) (Figs 28 I and 11). The vertical displacement corresponding to this event must be N volcanic rocks; 2-yellow colluvium; >white colluvium, silt gravel greater than the one that is observed in the cross-section because the older deposits, in particular the paleoturf, have been eroded and are truncated by the base of the recent colluvium. Moreover, the palaeoturf also disappeared from the uplifted compartment due to erosion after the older event. The above data clearly indicate that two similar events (including 1988) occurred within the last 17 OOO years (samples B and C, Fig. 28 111). The identification of events before the formation of the palaeoturf (Fig. 28 IV) becomes more difficult because of the lack of levels which are clearly identified and dated. Sample A at 25000yr BP does not give the age of a precise level but indicates only that a part of the breccia (2b) is older than 25000 years. One or more events, though we cannot specify them, might be necessary 156 H . Philip et al. - N l m TRENCH ravine Figure 27. Palaeodeformations and CI4 datings on Trench 111. (a) Trench site situated 100 m to the east of the trench of Fig. 26. Black squares are the locations of the C14 samples. 1-Palaeogenic volcanic rocks; 2a-yellow colluvium with distorted elements of palaeosoil; 2 b S l o p e breccia and yellow colluvium with distorted elements of palaeosoil; 3-palaeoturf; and &white colluvium. (b) Geological cross-section in a ravine located 40 m south of the trench, allows us to estimate a thickness of about 6 m for the colluvium deposits. in order to explain the remaining accumulated vertical offset. The presence of a ravine 40m south of the trench (Fig. 27b), where the brook cuts down to the Palaeogene volcanics, permits one to measure a total thickness of 6 m for the old Quaternary deposits laying on top of the Palaeogene, and to formulate the hypothesis that at least four important earthquakes occurred in the past, if we consider a characteristic vertical offset of 1 . 5 m for each event. Neither the palaeoturf nor the avalanche breccia were observed on the section at the ravine. 6 CONCLUSIONS The seismic source of the Spitak earthquake shows a number of new features studied through a multidisciplinary approach in which tectonics and seismology were integrated. It complements previous detailed studies of other major earthquakes in areas of continental deformation like El Asnam, 1980 (Philip & Meghraoui 1983); Tabas E-Golshan, 1978 (Berberian 1979); San Fernando, 1971 (US Geological Survey Staff 1971); Meckering, 1968 and Calingiri, 1970 (Gordon & Lewis 1980). The mechanism of the Spitak earthquake, in its average form and in its details at different scales, is consistent with the regional geodynamic scheme that had previously been obtained from neotectonic studies in the Caucasus (Philip et al. 1989). High quality data permitted a detailed and accurate picture of the rupture and of the mechanics involved. The source is complex and consists of five main branches working in different ways according to their disposition with respect to the regional stress orientation and shape (Fig. 29). Three segments, the main surface rupture between Spitak and Gekhasar and two en kchelon blind faults, are oriented N120" and work as thrusts dipping 50"NE with a right lateral component. The Alavar southeastern segment and the hidden northwestern segment are oriented N140' and they correspond mainly to right-lateral strike-slip faults along almost vertical planes. The average length is about 8 km for each segment. The central segment, between Spitak and Gekhasar, shows a finer segmentation correlated to topography with a characteristic length of about 2 km. The epicentre of the main shock is located in depth at the intersection of the Alavar and the Spitak-Gekhasar segments. Hence, the rupture was bilateral (Shebalin & Borisov 1989). Moreover, the aftershock distribution (Dorbath et al. 1992) suggests that the propagating rupture lost energy by dividing itself into several branches before stopping, similar to the El Asnam earthquake. - The Armenian earthquake . . . . . . . . . . . . , . 1 event . Present day , . 157 December 7,1988 earthquake . .. . . . . , _ . ' .. Figure 28. Tectonic reconstruction in Trench 111 (symbols as in Fig. 27). Black squares indicate sites where CL4dating was made. (I) Last event offsets the formations in (11) and leads to the. actual configuration. (11) Turf has been cut by an older event and eroded completely on the uplifted block. New deposits cover the scarp and a new turf soil layer is formed. (111) Erosion and formation of the palaeoturf layer. (IV) Older colluvium was probably already cut by one event. Figure 29. Block diagram of the entire fault zone showing the five branches and the relative motion of the southern block with respect to the northern one. Shaded area corresponds to the ruptured surface as defined by the aftershocks. Arrows indicate slip vectors. Thick lines show surface breaks. Palaeoseismic information obtained from trenches across the fault trace indicates the existence of at least one more comparable event between 17000yr BP and the 1988 December 7 earthquake. One or more events might have occurred between 25 000 and 17 000 yr BP. The Spitak fault, being a half-hidden fault with a long recurrence period with respect to the period in which historical seismicity is known, presents an interesting problem for the evaluation of seismic hazard in the Caucasus and, in general, in the whole Euro-Mediterranean region. The fact that it is half hidden warns us against the direct evaluation of maximum possible magnitude from the surface ruptures alone, without examining the activity of associated folds (Stein & King 1984) and without careful geomorphological analysis. The long recurrence time indicates that historical seismicity is not sufficient to give 158 H . Philip et al. adequate estimation of the seismic hazard in regions of moderate seismicity and that palaeoseismic studies should be strongly recommended. ACKNOWLEDGMENTS This work received financial support from the French CNRS and INSU, and from the Soviet and Armenian Academies of Sciences. Maurice Mattauer, Michel Cara, Louis and Catherine Dorbath, Henri Haessler, Luis Rivera and Humberto Fuenzalida contributed with their comments. We are specially indebted to Daniela Pantosti, who made a remarkable work in reviewing the paper and provided numerous thoughtful comments and suggestions. REFERENCES Ambraseys, N. N. & Adams, R. D., 1989. Long-term seismicity of North Armenia, EOS, 70, 145, 152-154. Balyan, S. P., Lilienberg, D. A. & Milanovsky, E. E., 1989. Noveishaya i sovremennaya tektonika seismoaktivnikh orogenov armenii i raiona spitakskogo zemletryaseniya, Geomorfologiya Akad. Nauk SSSR, 4, 3-16. Berberian, M., 1979. Earthquake faulting and bedding thrust associated with the Tabas e Goldshan (Iran) earthquake of December 16, 1978, Bull. seism. SOC. A m . , 69, 1861-1887. Borisov, B. A,, Reisner, G. I. & Sholpo, V. N., 1975. Vydeleniye seismoopasnykh ion v alpiyskoy skladcharoi oblasti ( P O geologicheskim dannym), Nauka, Moscow. Borisov, B. A. & Rogozhin, E. A., 1989. 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