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NEOTECTONICS OF THE SEISMICALLYACTIVE ARMENIAN OROGEN AND THESPITAK EARTHQUAKE REGIONS. P. Bal'yan a , D. A. Lillyenberg b & Ye. Ye. Milanovskiy ca Yerevan Universityb Institute of Geography , USSR Academy of Sciencesc Moscow UniversityPublished online: 06 Jul 2010.
To cite this article: S. P. Bal'yan , D. A. Lillyenberg & Ye. Ye. Milanovskiy (1990) NEOTECTONICS OFTHE SEISMICALLY ACTIVE ARMENIAN OROGEN AND THE SPITAK EARTHQUAKE REGION, InternationalGeology Review, 32:1, 1-12
To link to this article: http://dx.doi.org/10.1080/00206819009465749
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NEOTECTONICS OF THE SEISMICALLY ACTIVE ARMENIAN OROGEN AND THE SPITAK EARTHQUAKE REGION
S.P. Bal'yan (Yerevan University)
D. A. Lillyenberg (Institute of Geography, USSR Academy of Sciences)
Ye. Ye. Milanovskiy (Moscow University)
From: "Noveyshaya i sovremennaya tektonika seysmoaktivnykh orogenov Armenii i rayona Spitakskogo zemletryaseniy," Geomorfologiya, 1989, No. 4, pp. 3-16.
The authors offer an overview of the geodynamic setting and the morphostructural development of the Lesser Caucasus and the Armenian Highland during the late Tertiary and Quaternary periods. Focussing on the region of the destructive Spitak earthquake, they find mat the complicated pattern of stress buildup and active deformation indicated by repeated levelling over the last half century (including one line measured a month before the quake and reoccupied since) continues the pattern of the geologic past.
The Transcaucasus, particularly Armenia, is the most seismically active region in the Soviet Union. Destructive and catastrophic earthquakes have occurred in historical times. However, the Spitak earthquake on December 7, 1988 was unexpected. This was no accident, because seismologists have had difficult relationships with the State Construction Commission. Under constant pressure to reduce building costs, they systematically underestimated earthquake magnitudes and intensities. Consequently, on the seismic zoning map of the USSR (OSR-78), earthquakes of intensity 9 or more are not predicted for the Transcaucasus, although such quakes have occurred there in historical times, such as the catastrophic Dvina earthquake of 823 in Armenia. However, the intensity and location of the Spitak earthquake was not unexpected, as we show below, for leading geologists and tectonicists who have studied the neotectonics of the region or for geo-morphologists who have examined the Quaternary and Recent movements.
In the number of deaths and the extent of damage, the Spitak earthquake has no parallel in the
Caucasus in this century. This was the largest catastrophe in the USSR after the Ashkhabad earthquake of 1948. The number of deaths exceeds 25,000, and preliminary estimates of the losses are 8-10 billion rubles, a figure which probably should be doubled, given that a similar and more likely larger sum would be required to make good the damage.
Neotectonics and Orogenic Morphostructure
The orogens in the Lesser Caucasus and in the Armenian Highland have morphostructural features governed by the general geodynamic setting in the Caucasus sector of the Mediterranean mobile belt, where the Eurasian, African, and Arabian plates interact [1-5]. Throughout the Mesozoic-Cenozoic period, the projection on the Afro-Arabian plate has been moving gradually to the northeast, and the obstacle represented by the Eurasian plate has led to transverse compression in protoorogenic structures in the mobile belt. After the Red Sea rift formed, separating the African and Arabian blocks, the northern wedge-shaped projection began to play the main
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part in the Caucasus sector. In particular, there was increased horizontal pressure to the northeast in the Pliocene-Quaternary period, which gave rise to the main longitudinal orogenic structures. That interaction caused the orogens to acquire folded-overthrust and dome-block structures, with characteristic asymmetry in the relief: steep short southern flanks and more gently sloping and longer northern ones [5]. The constant pressure from the wedge-shaped projection on the Arabian plate resulted in the curvature of the en echelon east-west mountain systems to form trapezoid forms with their convex sides to the northeast. This is seen in the arcuate system in the Armenian and Kurdistan Taurus mountains along the northern edge of the Arabian plate. The next echelon in the trapezoidal bending can be seen in the Aladag and Agridag system within the Armenian Highland. The trapezoid is particularly clear to the northeast of the North Caucasus in the section from the Dzha-vakhet uplands to the Araks lowlands. The Middle Kura intermontane depression here separating the Lesser and Greater Caucasus is highly compressed, uplifted, and underthrust under the southern flank of the High Caucasus. Finally, the northernmost echelon in this transregional transverse geodynamic system is formed by the Dagestan terrace in the East Caucasus.
Along the flanks of these trapezoidal steps are diagonal and transverse transregional fault and mor-pholineaments, which intersect the east-west mountain systems and cut through the entire crust and are frequently accompanied by shearing. These "anti-Caucasus" zones at the western margins include the morpholineament seen within the Lesser Caucasus and the Armenian Highland as the Idzhevan-Razdan transverse depression system, which governs arcuate curvature in the mountain system. On the continuation in the East Caucasus along the Avar-Sulak morpholineament, there is a sharp bend in the Dagestan ranges. The eastern surroundings of the flank coincide with the Gyandzha-Terter transregional morpholineament (in the Lesser Caucasus and the Armenian Highland) and the Samur morpholineament (in the East Caucasus) [5].
A system of transverse or diagonal faults is involved in the vertical and horizontal mountain displacements, which indicates extensive transverse stretching as the orogens rose. This process was accompanied by volcanism in the Caucasus segment, which during the Cenozoic accounted for almost half
of the volume of lava eruption throughout the Mediterranean belt [3].
This stress pattern predetermined the compression-tension mechanism. The morphostructural result was the development of en echelon under-thrusts-overthrusts, which form the basis of the orogens and the separating intermontane depressions. The mechanism has also given rise to large shear strains, the most prominent of which is the North Anatolian sinistral shear, of which the Sevan-Pambak zone is sometimes considered to be the eastern branch [3, 5].
Morphostructural evolution. In this geodynamic system, the Lesser Caucasus mountains developed from an extensive and deep geosyncline trough that arose in the Early Jurassic in the inner part of the Mediterranean mobile belt (Tethys). The growth of the orogen was accompanied by all stages by extensive igneous activity. In the Early Alpine cycle (Jurassic and first half of the Cretaceous), when the Mediterranean mobile belt and the Lesser Caucasus eugeosyncline in particular were subject to considerable horizontal stretching, the effects were most extensive in the internal Sevan zone, where the previous continental crust was not merely broken up and thinned but also completely disrupted and replaced by newly formed thin oceanic crust. This was a belt of extensive spreading, the width of which has been estimated by various investigators as from tens to hundreds (or by some advocates of extreme mobilism even thousands) of kilometers, which separated not only the flanks of the Lesser Caucasus geosyncline and the Mediterranean mobile belt as a whole but also essentially separated the enormous lithospheric blocks of the Eurasian and Afro-Asian plates bordering it to the north and south.
In the middle of the Cretaceous, the Lesser Caucasus attained a more mature geosynclinal stage; spreading on the whole weakened and was repeatedly replaced by horizontal compression, which was most extensive at the end of the Paleogene, while the structures in the Lesser Caucasus gradually became more complicated. The first compressional pulses occurred in the Sevan-Pambak zone at the boundary between the Early and Late Cretaceous (Austrian phase), and repeated pulses in the middle Late Cretaceous led to considerable width reduction in the belt containing oceanic crust, with general crushing and extensive dislocation because of the
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approach between adjacent tectonic zones. The oceanic or melanocratic basement was transformed into a highly deformed ophiolite complex, consisting of a series of tectonic nappes, serpentinite melange zones, and their erosion products.
General subsidence was renewed at the end of the Late Cretaceous, followed at the start of the Paleo-gene by crustal fracturing and stretching in the Lesser Caucasus, accompanied by large submarine eruptions. This transformed the area into a system of narrow deep trenches bounded by faults and separated by zones of new or residual anticlinal uplifts. The deepest and most magmatically active of these secondary rift geosynclines was the Sevan trench in the middle of the Lesser Caucasus.
At the end of the Eocene and in the Oligocene, the Sevan trough like others of the same age in the Lesser Caucasus, again underwent extensive compression, was dissected by upfaults and underthrusts, and was also intruded by granitoids; it ceased to exist as a trough and was converted into the Sevan synclinorium. Blocks and nappes derived from the pre-Late Senonian basement, including the ophiolite complex, were again brought to the surface. In the final orogenic stage of the Alpine cycle, Paleogene downwarping died out, and folding and overthrust structures were produced everywhere, which converted the Alpine Lesser Caucasus geosyncline into a complicated folded structure. In the overall orogenic stage, horizontal compression predominated throughout the Caucasus segment, and there was accentuated uplift in the Lesser Caucasus, which acquired a dome and block structure over a width of -200 km.
The uplift rates in the Lesser and Greater Caucasus increased particularly in the Late Miocene. The late orogenic or neotectonic stage had begun, with repeated episodes of subaerial volcanism, especially extensive in the Lesser Caucasus, and this completed the formation of the present mountain relief. The largest neotectonic movements occurred in the axial zone of the Lesser Caucasus, which corresponds to the Sevan zone.
Neotectonic morphostructures. In the Lesser Caucasus, these were produced as sequences of elongated horst-overthrust ranges 50-100 km long and 10-20 km wide separated by depressions or grabens (Fig. 1). In the last 10 million years these
ranges have been uplifted 2.5-3 km (in places 3.5 km), whereas uplift is the adjacent intramontane depressions has been no more than 1.5-2 km. The latest intramontane depressions at the axis of the Lesser Caucasus consist of oval or linearly extended or occasionally triangular depressions, grabens, and half-grabens 10 to 30 km (occasionally up to 50-60 km) long and from 2-3 to 10-20 or occasionally 30 km wide. They are filled by Neogene and Quaternary or entirely Quaternary continental sediments and subaerial volcanics with total thicknesses from 0.1 to 0.5-1 km [1, 2, 6].
The Armenian segment on the whole has an over-thrust-block structure and asymmetric relief; the northern flank is heterogeneous, as it contains elements with various ages, types, and ranks that were integrated in the neotectonic stage into a single entity. As a neotectonic structure, they constitute a monocline tilted to the northeast with a total height difference of up to 2.5-3.5 km. The relief in the monocline was finally formed in the Pliocene-Quaternary period. This segment is separated from the Middle Kura intermontane graben by the Lesser Caucasus fault, which is a hinge zone delimiting regions differing considerably in displacement. During the various neotectonic stages, the slope of the monocline has changed, which reflects the varying transverse compressional and tensional forces, which in part can be seen in river terrace deformations [6]. Some successively uplifted longitudinal terraces are separated by regional faults. The monocline is also divided into several variously uplifted blocks, which are separated by disjunctive faulting. In our area, particular importance attaches to the superimposed transverse Idzhevan graben, which is part of the transregional or morpholinea-ment system.
The axial part of the Lesser Caucasus orogen shows particularly complicated structure and geo-dynamics [1, 2, 6]; it consists of tree east-west mobile subzones showing markedly contrasting movements. The northern one is the Shirak-Shakhdag subzone, which consists of unsymmetrical uplifts: the Shirak, Bazum, Khalab, Areguni, Shakhdag, and other horst-overthrust ridges, separated either by trenches or by small and shallow grabens. To the south, this adjoins the Leninakan-Sevan subzone, which consists of a chain of depressions or grabens separated by transverse barriers. From the border with Turkey, these subzones are
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FIGURE 1. Neotectonic and seismotectonic sketch of the Lower Caucasus and adjacent parts of the Armenian Highlands (compiled by Ye. Ye. Milanovskiy): 1) boundaries of the Lesser Caucasus dome-block mountain structure; 2) Sevan structural zone; 3) regions showing Late Miocene, Pliocene, and Quaternary subaerial volcanism; 4) major polygenic volcanic structures; 5) Quaternary volcanoes; 6) Pliocene folded structures; 7) partial morphostructural uplifts; 8) active intermontane and intermontane depressions; 9) depressions: a) filled with Pliocene-Quaternary beds; b) with existing lakes; 10-12) reverse faults active in the Pliocene-Holocene (11—reverse, 12—reverse-shear, 13—normal, including buried ones); 13) supposed normal faults, tears, and deep fissures controlling contemporary eruptions; 14) active grabens injected by parallel dike series; 15) directions of general horizontal crustal compression in the Lesser Caucasus; 16) earthquake epicenters; 17) epicentral zones and earthquake years; 18) individual symbols: a) letters representing populated points; b) numbers representing neotectonic structures: A) Ani, L) Leninakan, N) Nalband, S) Spitak, K) Kirovakan, St) Stepanavan, Ts) Tsovagyukh, Ka) Kamo, Sh) Shordzha. 1-7) Uplifts: 1) Shirak, 2) Bazum; 3) Khalab; 4) Areguni; 5) Shagdag; 6-7) Pambak; 8-14) transverse uplifts between depressions (saddle blocks and saddle points); 8) Dzhadzhur, 9) Saraart; 10) Archut, 11) Tanzut; 12) Semenov; 13) Noraduz; 14) East Sevan; 15-22) depressions: 15) Leninakan; 16) Upper Pambak; 17) Middle Pambak; 18) Lower Pambak; 19) Gamzachiman; 20) Little Sevan; 21) Great Sevan; 22) Upper Razdan; 23) Upper Terter (Kel'badzhar) graben; 24-30) faults: 24) Pambak-Sevan; 25) Leninakan-Ani; 26) Dzhadzhur-Mokrogora; 27) Marmarik; 28) Sarykay; 29) Kyuzadzhik; 30) Gegam.
separated by the major Pambak-Sevan fault, on which the northern ridges are relatively uplifted by up to 1-2 km. In places, this is seen as a single fault, but in the main, it consists of a zone of adjacent stepped up to 2-3 km wide. In the west, Late Cretaceous and Paleogene beds are in direct contact along these faults with upfaulted Neogene or Quaternary beds, dipping 50-60° [7]. In places, this almost vertical rectilinear step in the Cretaceous or
Paleogene beds is 0.5 km or more wide. It is the geomorphological expression of the uplifted flank of the Pliocene-Quaternary or Quaternary steep fault, as yet comparatively little altered by erosion. In the adjacent parts of the downfaulted southern flank, the Neogene volcanic and sediments frequently dip to the south 30-60°, and the Pleistocene beds at up to 10°, which reflects the flexure over the fault. The predominant type of the latest displacements on the
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Pambak-Sevan fault is upfaulting, clearly indicating ongoing horizontal compression in the crust, which is confirmed by the linear folding in the Pliocene and Eopleistocene formations in the south (the Great Sevan depression) and to the north (in the Tsalka depression), as well as in various other features.
The western part of the Sevan zone is occupied by the extensive Leninakan graben, which is filled with Neogene and Pleistocene lacustrine-alluvial volcano-sediments with a total thickness over 0.5 km. A deep northeast-southwest fault in the Pre-Neogene basement beneath the middle of the depression has given rise to repeated destructive earthquakes during the last thousand years. To the north, this is probably screened by the Pambak-Sevan fault. In the east, the Dzhadzhur horst uplift bounds the depression.
East of the Dzhadzhur block is a system of three narrow sublatitudinal grabens with a total extent of 50 km, in which the Pambak River runs. These grabens are filled by up to 150 m of lacustrine-alluvial beds which, unlike the deposits in the Leninakan depression, formed mainly in the Quaternary period [2, 6]. Surface deposits in the westernmost depression (Upper Pambak) belong almost everywhere to the broad flood plain of the Pambak River, whose bed lies near the southern flank. Only in the easternmost part, at Nalband, is there a low Late Pleistocene terrace. A narrow transverse wedge is represented by the Saraart horst, which separates the Upper and Middle Pambak grabens. On the west, the uplift is bounded by several closely adjacent steep stepped faults with the downthrown block to the southwest, and in the east it is bounded by a major reverse fault, with the southeast block depressed more than 200 m. The middle part of the uplift is formed by a ridge in the Cretaceous-Paleogene basement, covered by thin Pliocene and Pleistocene beds wedging out eastward, into which the mouth of the Pambak has cut to a depth of 100 m, and where at the flanks one can trace a Late Pleistocene erosion terrace 25-30 m high.
Farther east are the Middle Pambak (Spitak) and Lower Pambak (Kirovakan) grabens, separated by the narrow Archut transverse horst. These are filled by Lower and Middle Pleistocene beds, which contain several tuff horizons, for a total thickness of up to 150 m. In the Archut uplift, these beds have an anticlinal flexure, with the crest rising to 70-80 m
above the bed of the Pambak River. A basement terrace 25-30 m high (Late Pleistocene) may be traced through the two grabens and the Archut horst. This has a fixed level within the Middle and Lower Pambak depressions and in the Archut and Saraart uplifts, which indicates that uplift in the latter has been retarded since that terrace was formed. On the other hand, the downwarp of the Upper Pambak depression relative to the Gbllidzha uplift and the deformation in the junction zone between them has continued to the present. At Kirovakan, the Pambak River turns sharply to the north and intersects the Pambak-Sevan fault zone, where it passes through a deep and narrow gorge in the growing Bazum-Khalab uplift. There are virtually no terraces here, but the gradient of the Pambak River increases substantially.
The Tanzut and Semenov transverse uplifts lie between Kirovakan and the Sevan depression, in which the Neogene basement rises 1.7-1.8 and 2-2.5 km, respectively; between them lies the small Gam-zachiman depression, where the Quaternary sediments are up to 140 m thick. Nevertheless, at the axis of the Lesser Caucasus, the transverse uplift for an extent of about 40 km is represented by a narrow graben separated by steep longitudinal fault from the Khalab and Pambak horst-overthrust ridges to the north and south, which show vertical throws of up to 2.5-2.7 km. The graben is of the ramp type, and its morphology reflects to development under transverse horizontal compression.
To the southeast, near Tsovagyukh, the bottom of the graben descends rapidly by several hundred meters on the Idzhevan-Razdan transverse morpho-lineament, and it passes into the most depressed northeast part of the Little Sevan depression. The Pambak uplift descends even farther in the morpho-lineament zone, and passes into the southwest part of the Little Sevan depression. The depression is separated from the adjacent Areguni horst uplift by part of the Pambak-Sevan fault, which is seen in the relief as a very steep step (up to 50-70°), slightly dissected and as much as 0.5 km in amplitude. The greatest depths in the floor at Sevan (about 99 m before entering the lake) closely adjoin the foot of the step. The narrow and unsymmetrical transverse Noraduz uplift is truncated by a northeast-southwest fault and separates the Little Sevan depression from the longer (60 km) and wider (30 km) Great Sevan depression. In the north, this is separated from the
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Shakhdag horst uplift by the Pambak-Sevan fault. There is a series of oblique north-south stepped faults of Late Quaternary age cutting Late Miocene and Pliocene beds in the southwest flank of the depression; the result being that the Great Sevan depression has a triangular configuration, and there are several extended one-sided horsts and grabens at the western end. To the southeast, the depression is closed in a centriclinal fashion and separated by a transverse uplift from the narrow stepped Kel'bad-zhar (Upper Terter) graben, whose axis is heavily penetrated by a series of Neogene and Quaternary parallel dikes, which clearly indicate that the neotec-tonic deformations have been tensional [3].
South of the Sevan zone lies the Central Armenian volcanic region, which consists of a series of oval domal uplifts covered by Late Cretaceous, Pliocene, and Quaternary subaerial volcanic and a series of large young central-type polygenic volcanoes together with a set of monogenic volcanic structures, many of which are associated with north-south faults and deep tension fissures (the Gegam chain of volcanoes and others). The similar Akhalkalak volcanic region, with several elongated chains of volcanoes and domal uplifts, bounds the western part of the Sevan zone on the north.
All of these morphostructures developed in the Pliocene and Holocene under conditions of overall horizontal crustal compression. The Lesser Caucasus was compressed in the SSW-NNE and stretched in the WNW-ENE direction with uplift in the late orogenic stage and expansion of the Transcaucasian transverse uplift, which continues the African-Arabian rift belt in a northerly direction.
Active Geodynamics
The style of the active neotectonic deformation and geodynamic mechanisms, which includes the seismicity and vertical movements, of the Lesser Caucasus and the Armenian Highland tend to remain the same as they have been in the past.
Seismic activity. In northern Armenia, this is associated with several major fault zones, which have high mobility and have shown displacements in various directions in recent times (Fig. 1). There is regional seismic activity on the longitudinal Pambak-Sevan suture zone, which essentially separates the Lesser Caucasus and Armenian Highland.
Associated with this zone, for example, are the epicenters of the intensity 6-7 earthquakes in 1916 and 1967, where the regions are ellipses between Kirovakan and Leninkan, almost coinciding with the epicentral region of the 1988 Spitak earthquake. The latter thus corresponds to the current site of seismo-tectonic activity. The West Sevan suture zone is also active, and it gave rise to the intensity 6 and 7 earthquakes of 1863 and 1945, which had their epicenters in the region of Tsovagyukh. In the neotectonic stage along that zone, there have been compressional-tensional stresses and reverse faulting, with the northern ridge rising.
A similar pattern occurs in the activity on the western part of the Ankavan-Zangezur fault adjoining the Sevan depression and the Pambak ridge to the south; this accounted for the intensity 6 earthquakes in 1905 and 1909 in the region of the Kamo horst, as well as the intensity 7 earthquake in 1827 in the Taskhkadzor region, and the intensity 6 earthquakes in 1869, 1928, and 1935 at the foot of the Pambak horst-overthrust ridge [7]. The entire east-west zone of junction between the Lesser Caucasus and the Armenian Highland thus has a high mobility and activity, with shifts along the zone over time.
Around the western face of the trapezoidal Lesser Caucasus step, there are active faults trending roughly north-south. Along the Akhuryan (Lenin-akan) fault, one finds the elongated epicenter regions for the earthquakes of 1046 (intensity 8), 1043, 1065, and 1319 (intensity 7), 1924 and 1928 (intensity 6), and 1926 (intensity 8-9). The obliquely linked Orom (Dzhavakhet) fault is associated with the earthquakes of 1935, 1950, and 1971 [7]. A similar situation occurs for the north-south trending Aragats-Spitak fault to the east. Particular activity occurs around Leninakan, where the western part of the Pambak-Sevan suture zone intersects the Akhuryan fault
The above two general geodynamic mechanisms thus apply to the North Armenian foci; there are other regional forms, but they generally generate weaker earthquakes.
Active vertical movements. These are detected by geodetic instruments and reflect not the geodynamics of instantaneous stress relief but long-time processes lasting tens or hundreds of years. In that
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respect, the region around the Spitak earthquake and the adjacent mountains have a unique position. This is the first seismic region in the USSR to be covered by repeated precision leveling at various intervals, which includes the triple series of measurements made along the Tbilisi-Kirovakan-Leninakan and Tbilisi-Akstafa-Dilizhan-Yerevan lines, and double series along the Sevan-Martuni-Yekhegnadzor and Akhalkalaki-Leninakan-Yerevan lines, both lines that cut across the main orogenic structures [4, 8, 9, 10, 11]. These measurements identify epochs of uplift in the current activation cycle, which indicates that the transverse compression mechanism is active, together with epochs of depression, which involve the transverse stretching mechanism (Fig. 2).
The 1911/29-1940/53 epoch in the Lesser Caucasus can be taken as an interseismic one, with a predominant tendency for overall depression under tensile stresses [4, 9, 10]. The homoclinal block structure at the northern flank of the orogen in that epoch began the reverse tilt from north to south with respect to the axis of the Lesser Caucasus by ~90-100 mm per 100 km (Figs. 2 and 3), with rale gradients of 0.4-0.7 mm/yr per km (0.01 s/yr) along the valley of the Agstev River and up to 0.8 mm/yr per km (0.016 s/yr) along the Debed River. The northern longitudinal zone showed the lowest amplitudes of depression (1-2 mm) and inflexions on the rate curve for the movement on the bounding Idzhevan fault. In the southern part of this homoclinal block, there was more a marked increase in the relative depression: from -4 to -5 mm/yr, with the tilt to the south increasing to 0.7-0.8 mm/yr per km. This sinking involved the axial Lesser Caucasus ridges (Khalab and Areguni), whereas the Sevan suture zone shows sharp bends on the rate curves.
The Sevan-Pambak graben showed the greatest depression. In the Little Sevan graben, rates reached -5 to -6 mm/yr. The block structure in the Pambak graben shows relative sinking rates increasing in steps from east to west from 5-6 to 7-7.5 mm/yr, i.e., this clearly reflects an inversion, against which background one can identify relative uplifting of up to 1 mm/yr in the Saraart horst in the transverse Aragats-Spitak fault zone. To the west, the rate of depression falls to 4-5 mm/yr at the intersection with the Dzhadzhur horst, but rises again in the Shirak depression (near Leninakan).
A different picture is seen in the Razdan plateau in the Gegam volcanic uplands, where although there has been some very slight sinking (1-1.5 mm/yr), one can clearly distinguish a region with uplift relative to the Lesser Caucasus [4, 9] and to the Sevan-Pambak, Shirak, and Yerevan grabens (Figs. 2 and 3). Rates of movement on the Ankavan-Zangezur, Ashtarak, and Yerevan faults are 0.7-0.9 mm/yr per km.
The epoch 1940/53-1972/74 (Fig. 2) shows a sharp shift in geodynamic conditions, with rapid uplift and increasing transverse compression in the regional flexure in the Lesser Caucasus [1, 11, 12]. This may be considered as a preseismic event, when seismic strain began to accumulate over a fairly large area. In contest to the area discussed above, here the morphostructures show a clear relation to contemporary geodynamics. For example, the northern flank in the Caucasus in general changed tilt from south to north by up to 140-160 mm over 100 km, with rates of movement ranging from 0.5-0.8 mm/yr on the valley of the Debed River up to 0.9-1.5 on the valley of the Agstev River, i.e., the tilt not only changed sign and sense but also became steeper, with rate differences of up to +6 or +7 mm/yr (Fig. 3). Against that background, one can trace differences between northern (1-2 mm/yr), the middle (3-4 mm/yr), and the southern (5-7 mm/yr) parts of the homocline, which are separated by longitudinal faults, and which is also reflected in steps on the rate curves.
The Sevan suture zone at that stage separates two essentially distinct geodynamic regions differing in structure and active vertical movement pattern (Fig. 2). The Little Sevan graben shows a general uplift in the surrounding ridges and in the plateau of over 7 mm/yr, but it itself rises at 6 mm/yr or less, i.e., relatively it is sinking. In the Pambak depression, the geodynamic conditions changed radically; the transverse system of "piano key" blocks in the basement showed relative stepped uplift, increasing to the west: Korovakan graben 4.5-5.5 mm/yr, Archut horst 5.5-6, Spitak graben 6-6.5, Saraart horst 6.5-7.5, Nalband graben 7-8, Kaltakhcha block 8-10, and Dzhadzhur horst 10-10.5. There are two characteristic features. First, all the block displacements are parallel to the transverse transregional Aragats-Spitak fault, which indicate activation on it and greater uplift on the western flank. A similar
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FIGURE 2. Vertical movements in the epochs 1911/29-1940/53 and 1940/53-1952/74 in terms of the relative creep rates in relation to Shulaveri and Akstafa at the margin of the Middle Kura depression (compiled by D. A. Liliyenberg): 1) repeated level lines; 2) movement isolines in mm/yn 3-5) areas of downwarp: 3) rapid; 4) moderate; 5) slight; 6-8) areas of uplift: 6) slight; 7) moderate; 8) rapid.
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FIGURE 3. Variations over space and time of ongoing vertical movements, after Liliyenberg and Yashchenko [12]: 1) Shula-veri-Kirovakan-Spitak-Leninakan profile; 2) Akstafa-Dilizhan-Abovyan-Mkhchyan profile; 3) projections of the Spitak earthquake isoseismal lines.
transverse differentiation appears to be present in the adjacent horst ridges (Bazum and Pambak). Second, the differences in creep rate on the faults have increased sharply on the Chichkhan and Saraart faults 2 mm/yr per km, and on the Spitak fault up to 2.5. Such gradients have often been observed when strain is accumulating.
Rapid increases in uplift rate have also been indicated by repeated leveling in the southern part of the Dzhavakhet volcanic uplands, where in the region of Gukasyan [11, 12] they have reached 12-13 mm/yr, the highest levels known in the Transcaucasus. A different pattern occurs in the Shirak depression, where the movement rates on the steps fall from 10 to 2-3 mm/yr: Leninakan 8-10, Aga 5, Bagravyan 4, and Ani 3-3.5. The gradients on the separating faults are 1-2 mm/yr per km, or 3 on the Akhuryan trans-regional fault near Leninakan. Further steps in the vertical displacement rates occur in the Yerevan depression.
There is a second region of vigorous uplift (up to 8-10 mm/yr), in the Razdan plateau and Gegam
volcanic uplands to the south of Sevan; the Spitak earthquake region thus lies between two rapidly rising regions (where the rates are 10 or more mm/yr), which has governed the geodynamic setting during the epoch 1940/53-1972/74: tectonic activation with a potential for thrust faulting.
The rate variations of vertical movement show clearly that the epoch of general sinking has been followed by one of general uplift over a very large area, 100-150 km across. Figure 3 shows that the epicentral region for the Spitak earthquake lies in the area with the greatest range of vertical movements, both positive and negative, and this clearly delineates the area where the highest stresses have accumulated (the 1967 earthquake relieved them only partly). The Sevan suture and the Aragats-Spitak fault have also shown increasing activity, as these are zones for seismic energy release; there has also been activation and displacement in the small blocks at the bottom of the Pambak graben [12].
Spitak earthquake. The earthquake on December 7, 1988 was of intensity 10 (M = 7) at the epicenter
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(area 80-100 km ); the intensity 9 isoseismal zone (area -800 km ) covers the Pambak graben and parts of the flanks in the adjacent ridges, whereas the intensity 8 isoseismal includes the complicated system in the Pambak graben, the Leninakan depression, and the Pambak and Bazum ridges. The focal zone was precisely associated with the Sevan-Pambak suture and is elongated along it with a dip of 50-60° to the north-northeast [13]. The earthquake was violent because the suture moved in three phases starting almost from the surface (0-5 km) and extended down to 30-40 km (Fig. 4), while encompassing a length of ~35 km and extending up to 40 km down the dip [13]. The Aragats-Spitak transverse fault also played an important part. The earthquake displacement was of reverse fault type, in which the northern block in the Bazum ridge and the bottom of the Pambak graben rose (to the east of the fault). At the surface, a diagonal northeast-southwest trending seismogenic break occurred to the west of Spitak, which was over 13 km long and coincided with a fault trace expressed in the relief along the lower reaches of the Chichkhan River. This shear-reverse fault with its dip 50° to the north-northeast had a vertical component of 1.3 m, and a dextral shear of 0.5-1.0 m. This shows that the quake was generated by transverse compression, which could have activated longitudinal, transverse, and diagonal tear faults of the reverse fault-dirust type arising from the Sevan suture. However, to the west of the Aragats-Spitak fault, the transverse tear faults did not show high activity, and the railroad tunnel through the Dzhadzhur transverse uplift was unaffected, aldiough to the west of Lusakhpyur, there were fissure-normal fault patterns with displacements of up to 0.2-0.3 m.
The morphostructural, seismic, and geodetic data (Fig. 4) show a striking agreement. The conseismic instantaneous phase of course was not recorded by repeated leveling, but leveling measurements were made in October-November 1988 on the Tumanyan-Spitak-Leninakan line, i.e., about a month before the earthquake [12], which indicated a general background of uplift but rearrangement in the contemporary vertical movements: slight relative sinking in the Lower and Middle Pambak grabens, more extensive sinking in the Upper Pambak graben, and hinging in the Aragats-Spitak fault zone between them, with local sinking extending to a zone up to 20 km wide.
The next leveling was done at the end of December 1988, several weeks after the earthquake and a month after the previous; it indicated residual deformations. The overall range of the displacements on the Chichkhan (Aragats-Spitak fault) was 100-120 cm (Fig. 4). The maximum uplift occurred in the Saraart wedge horst (110 cm) and in part of the intensity 10 isoseismal zone, as well as in the Archut block (65 cm). In the Spitak-Dzhrashen part, the displacement was 86-95 cm (whereas it did not exceed 1 cm in the Pambak ridge), and the faulting gradients attained 10 cm/km. The Lower Pambak graben showed relative uplift of up to 50 cm (within the intensity 9 isoseismal), whereas the southern block on the northern flank of the Low Caucasus gave up to 40-45 cm (within the intensity 8 isoseismal). At the same time, the western flank of the deep fault and the Upper Pambak graben showed relative sinking of up to 10 cm, with gradual displacement compensation toward Leninakan. Therefore, the highest stresses persisted after the earthquake in the Spitak-Tumanyan area, where later there were the strongest aftershocks, up to intensity V-VI. The postseismic state of stress weakened slowly. Further leveling at the end of January 1989 showed that the range in the residual deformations was -100 cm, so there was still considerable potential energy. A third leveling in March 1989 showed that the residual-deformation compensation was continuing slowly. Against that background, there was some increase in the relative sinking in the Upper Pambak graben, but with a marked decrease in the movements on the fault at Tumanyan: by 15 cm, gradient 5 cm/km (Fig. 4).
This geodetic monitoring indicates the scale, direction, and relationship to the block structure of the active vertical movements as well as the relationship to particular fault zones and the course of time. The main active structure is the Sevan-Pambak suture with its diagonal tear faults. The increasing transverse compression (followed by transverse tension) displaced the Bazum horst-overthrust ridge on the hinge zone, which controls the shift in the tilts in the north flank of the Lesser Caucasus. Along that zone, the aftershocks migrated over the entire range from Kirovakan to the Karakhag pass to the north of Leninakan. The amplitudes of block displacement largely coincide with the isoseismals. The geodetic measurements show that the increasing crustal stresses leading to the Spitak earrtiquake must be considered as accumulating over some decades in
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FIGURE 4. Relation between vertical movements from geodetic data [12], focal zone in the Spitak earthquake from seismic data [13], and structural differentiation in the Pambak graben valley [2]: 1) vertical displacement amplitudes (in cm) for various time intervals; 2) displacements in the preseismic phase; 3) displacements in the conseismic and postseismic phases; 4) earthquake epicenters; 5) coupling phases in the Sevan-Pambak suture (1,2, and 3); 6) ages of units; 7) faults; 8) top of the Late Cretaceous to Eocene folded basement.
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an area ~100-150 km across [12]. Once more this shows that it is wrong to limit earthquake prediction regions to narrowly local epicentral zones. Geo-dynamic monitoring should cover fairly extensive regions, where the seismic strains accumulate and sharp changes in the rates of active displacement occur.
References
1. Bal'yan, S. P., 1969, Strukturnaya geomor-fologiya Armyanskogo nagor'ya i okaymylya-yushchikh oblastey (Structural Geomorphology of the Armenian Highland and Surrounding Regions): Yerevan Univ. Press, Yerevan.
2. Milanovskiy, Ye. Ye., 1968, Noveyshaya tek-tonika Kavkaza (Neotectonics of the Caucasus): Nedra Press, Moscow.
3. Milanovskiy, Ye. Ye., 1987, Riftogenez v pod-vizhnykh poyasakh (Rifting in Mobile Belts): Nedra Press, Moscow.
4. Liliyenberg, D. A., 1980, General and regional patterns in morphostructural geodynamics of the Caucasus. In Sovremen. dvizheniya zemnoy kory (Active Movements in the Crust) (pp. 204-217): Naukova dumka Press, Kiev.
5. Gerasimov, I. P. and Liliyenberg, D. A., 1984, A geomorphological model for the Caucasus and the origin of its relief. In Bolshoy Kavkaz-Stara Planina (The Great Caucasus-Stara Planina) (pp. 8-25): Nauka Press, Moscow.
6. Geomorphology of Armenia. In Geologiya Ar-myanskoy SSR (Geology of the Armenian SSR) (Vol.1): 1962.
7. Gabrielyan, A. A., Sarkisyan, O. A., and Simonyan, G. P., 1981, Seysmotektonika Armyanskoy SSR (Seismotectonics of the Armenian SSR): Yerevan Univ. Press, Yerevan.
8. Liliyenberg, D. A., Mattskova, V. A., et al., 1969, A map of active vertical movements and morphostructures in the Caucasus. In Problemy sovremen. dvizheniy zemnoy kory (Problems of Active Crustal Movements) (pp. 142-156): Nauka Press, Moscow.
9. Liliyenberg, D. A. and Mattskova, V. A., 1970, Patterns of active vertical movements in the Lesser Caucasus and Armenian Highland: Do-klady AN SSSR, seriya geologicheskaya, Vol. 194, No. 1, pp. 171-174.
10. Karta sovremennykh vertikal'nykh dvizheniy zemnoy kory Vostochnoy Yevropy (Map of Active Vertical Crustal Movements in Eastern Europe), 1971: GUGK, Moscow.
11. Karta sovremennykh vertikal'nykh dvizheniy zemnoy kory na territorii Bulgarii, Vengrii, GDR, Pol'shi, Rumynii, SSSR (Yevropeyskaya chast), Chekhoslovakii (A Map of Active Vertical Crustal Movements in Bulgaria, Hungary, the GDR, Poland, Romania, the USSR (European Part), and Czechoslovakia), 1986: GUGK, Moscow.
12. Liliyenberg, D. A. and Yashchenko, V. R., 1989, Geodetic and morphostructural data for the region of the catastrophic earthquake in Armenia: Geodeziya i kartografiya, No. 10, pp. 18-27.
13. Shebalin, N. V., 1989, The Spitak earthquake of 7 December 1988: Nauka i zhizn', No. 4, pp. 16-20.
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