Download The Armenian earthquake of 1988 December 7: faulting and folding

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.
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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.
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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
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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.
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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
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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
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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
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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. Seismogennyi Razriv,
Priroda, 12, 26-31.
Cisternas, A., Philip, H., Bousquet, J. C., Cara, M., Deschamps,
A., Dorbath, L., Dorbath, C., Haessler, H., Jimenez, E.,
Nercessian, A., Rivera, L., Romanowicz, B., Gvishiani, A,,
Shebalin, N. V., Aptekrnan, I., Arefiev, S., Borisov, B. A , ,
Gorshkov, A., Graizer, V., Lander, A., Pletnev, K.,
Rogozhin, A. I. & Tatevossian, R., 1989. The Spitak
(Armenia) earthquake of 7 December 1988: field observations,
seismology and tectonics, Nature, 339, 675-679.
Dorbath, L., Dorbath, C., Rivera, L., Fuenzalida, A,, Cisternas,
A., Tatevossian, R., Aptekman, J. & Arefiev, S . , 1992.
Geometry, segmentation and stress regime of the Spitak
(Armenia) earthquake from the analysis of the aftershock
sequence, Geophys. J . lnt., 108, 309-328.
Etchecopar, A., Vasseur, G. & DaignPres, M., 1981. An inverse
problem in microtectonics for the determination of stress
tensors from fault striation analysis, J . Strucf. geol., 3, (l),
51-65.
Everingham, I. B., Frezson, D. J. & Doyle, H. A., 1969. Thrust
fault scarp in the western Australian Shield, Nature, 223,
701-703.
Friedman, M., Handin, J., Logan, J. M., Min, K. D. & Strearns,
D. W., 1976. Experimental folding of rocks under confining
pressure: part 111, faulted draps folds and multilithologic
layered specimens, Geol. SOC. A m . Bull., 87, 1049-1066.
Gamkrelidze, I. P., 1986. Geodynamic evolution of the Caucasus
and adjacent areas in Alpine time, Tectonophysics, l27,
261-277.
Gordon, F. R. & Lewis, J. D., 1980. The Meckering and Calingiri
earthquakes, October 1968 and March 1970, Geol. Surv. W.
Australia, I%, 229 pp.
Haessler, H., Deschamps, A., Dufumier, H., Fuenzalida, H. &
Cisternas, A,, 1992. The rupture process of the Armenian
earthquake from broad-band teleseismic body wave records,
Geophys. J. lnt., 109, 151-161.
Khain, V. E. & Milanovsky, E. E., 1963. Structure Tectonique du
Caucase d’aprPs les donntes modernes, in Volume a la
mkmoire du Prof. Paul Fallot; Mhnoires SOC. Giol. France,
Tome 2,663-703.
Kamb, B., Silver, L. T., Abrams, H. J., Carter, B. A., Jordan, T.
H. & Minster, J. B., 1971. The San Fernando, California,
earthquake of February 9, 1971: pattern of faulting and nature
of fault movement in the San Fernando earthquake, US
Geological Survey, Professional paper No 733, 41-54.
McCue, K., Barlow, B. C.. Demtram, D., Jones,T.iGibson, G. &
Michel-Leiba, M., 1987. Another chip off the old Anatolian
block, EOS Trans. Am. geophys. Un., 68, 609-612.
McTigue, D. F. & Mei, C. C., 1981. Gravity-induced stresses near
topography of small slope, J . geophys. Res., 86, 9268-9278.
Meghraoui, M., Philip, H., AlbarPde, F. & Cisternas, A , , 1988.
Trench investigations through the trace of the 1980 El Asnam
thrust fault: evidence for paleoseismicity, Bull. seism. Soc.
Am., 78, 979-999.
Milanovsky, E. E., 1968. Noveishaya tektonika Kavkaza, M.
Nedra, 483.
Pacheco, J. F., Estabrook, C. H., Simpson, D. W. & Nabelek, J.
L., 1989. Teleseismic body wave analysis of the 1988 Armenian
earthquake, Geophys. Res. Lett.. 16, 1425-1428.
Philip, H. & Mkgard, F., 1976. Structural analysis and
interpretation of surface deformations of the Pariahuanca
earthquakes (Central Peru), Tectonophysics, 38, 259-278.
Philip, H. & Meghraoui, M., 1983. Structural analysis and
interpretation of the surface deformations of the El Asnam
earthquake of October 10, 1980, Tectonics, 2, 17-49.
Philip, H., Cisternas, A., Gvishiani, A. & Gorshkov, A., 1989. The
Caucasus: an actual example of the initial stages of continental
collision, Tectonophysics, 161, 1-21.
Reisner, G. I., 1982. Osabennosti Chetbertichnoi tektoniki
yuzhnovo sklona vostochnovo Kavkaza, Problemi Geodinamiki
Kavkaza, Nauka, Moscow (in Russian).
Reisner, G. I., 1989. Why was the map erroneous? Priroda, l2,
19-23.
Rivera, L. A. & Cisternas, A , , 1990. Stress tensor and fault plane
solutions for a population of earthquakes, Bull. seism. SOC.
Am., 80, (3) 600-614.
Shebalin, N. & Borisov, B., 1989. Spitakskoye zemletryasenie,
Priroda, 4, 69-72.
Sholpo, V. N., 1982. Rol yablenii unasledovannostii novoobrazobaniya v razvitii Bolshovo Kavkaza, Problemi Geodinamiki
Kavkaza, Nauka, Moscow (in Russian).
Stein, R. & King, G. C., 1984. Seismic potential revealed by
surface folding: 1983 Coalinga, California earthquake, Science,
224, 869-872.
Trifonov, V. G., Karakhanian, A. C. & Kozhurin, A. I., 1989.
Activnie razlomi i seismichnost, Priroda, l2,32-38.
US Geological Survey Staff, 1971. The San Fernando, California,
earthquake of February 9, 1971: Surface Faulting, US
Geological Survey, Professional paper No. 733, 55-76.
Vardapetian, A. N., 1979. Pozdnekainozoiskaya tektonika plit
Chernomorsko-K aspiiskovo regiona, Okeanologiya, XIX(6),
1066-1074.
Westaway, R., 1990. Seismicity and tectonic deformation rate in
Soviet Armenia: implication for local earthquake hazard and
evolution of adjacent regions, Tectonics, 9, No. 3 , 477-503.
Zonenshain, L. P. & Le Pichon, X., 1986. Deep basins of the Black
Sea and Caspian Sea as remnants of Mesozoic back-arc basins,
Tectonophysics, U3, 181-211.