E. A. Rogozhin, A. N. Ovsyuchenko, A. R. Geodakov
Institute of Physics of the Earth, Russian Academy of Sciences
S. G. Platonova
Altai State University
A large earthquake with a magnitude of 7.3, as recorded by the Geophysical Survey of Russian Academy of Sciences (GS RAS), occurred in the south of the Altai Republic on September 27, 2003, at 11 hours 33 minutes, Greenwich time. The focus of this earthquake was located in the area of the North Chuya Range, the Chuya and Kurai intermountain basins, and the Chagan-Uzun upthrown block between them. The earthquake was felt over an extensive territory of Russia (in the Altai, Khakassiya, and Tuva republics, in the Krasnoyarsk Region, and in the Novosibirsk and Kemerovo districts), North-west Mongolia, North-west China and in East Kazakhstan. According to the records of the GS RAS, it had a intensity of 8 in the Beltir area, 6-7 in Aktash, 6 in Tashtagol, 5-6 in Prokopievsk, 4 in Novosibirsk, Ust-Kamenogorsk, and Semipalatinsk, 3-4 in Abakan, 3 in Krasnoyarsk, Zaisan, and Kemerovo, and 2-3 in Barnaul, Alma-Ata, Taldy-Kurgan, and Astana. The effects recorded in the epicentral zone, comprising Bel'tir, Kurai, and Aktash settlements, included the destructions of stoves, the falls of chimneys, and the formation of typical fissures in the walls between the windows, and the occasional tumblings of the walls and corners of slag concrete buildings. Wood houses were more resistant and their damage consisted mainly in log displacements and partial roof destructions.
A network of five temporal seismic stations was installed by the Geophysical Survey of the Siberian Division of Russian Academy of Science to record any aftershocks. In addition, twelve digital seismic stations of this organization operate in the Altai region. The data recorded by the local seismological network located in the near zone of the earthquake and in the Altai region (Geophysical Survey of the Siberian Division of the Russian Academy of Sciences), and by the earthquake recording network of Russia and by the World Seismological Observation Network are processed in the mode close to the real time one.
The seismic event concerned was accompanied by numerous aftershocks, two of which were especially high. The aftershock with M = 6.4, recorded at 18 h 52 min in September 27, 2003, was felt in Abakan and Biisk as an event of intensity 2-3 and in Novosibirsk as an event of intensity 2. The aftershock of October 1, 2003, was recorded at 1 h 03 min 28 sec Greenwich time as an event of magnitude 7.0 by the Geophysical Survey of the Russian Academy of Sciences. The aftershock of October 1 happened in the area of the Aktash settlement, where the damage of the buildings was estimated as equal to intensity of 7 and 8. The workers of the Geophysical Survey of the Siberian Division of the Academy recorded more than a hundred of repeated shocks with magnitudes of 2.2 to 7.0 on October 15, 2003. The epicenters of the aftershocks produced an elongated cloud, the long axis of which was oriented in the NW direction. The length of this oval was found to be about 90 km with a width of 20 km.
No earthquakes of this magnitude were observed during the entire period of observations performed in the south of Gornyi (Mountainous) Altai. However, the paleoseismological studies, carried out in 1996-1999, revealed that large earthquakes with magnitudes of 7.0-7.5 and recurrence interval of 1000 to 2000 years occurred in the Chuya and Kurai intermountain basins during the last 9 thousand years [Rogozhin et al., 1998a, 1998b]. The southern areas of the Gornyi Altai region are shown in the map of the general seismic zoning of the Russian Federation (GSZ-87) as belonging to a intensity-9 zone.
The earthquake of September 27, 2003, was accompanied by the formation of numerous earthquake-related ground-surface ruptures, including primary seismic fault systems and secondary seismic gravitational and seismic vibration ruptures. The subsurface shocks resulted in the violation of the natural topography mainly in the mountainous part of the pleistoseist area of an earthquake. The source region was exposed on the surface as a long (about 20 km) S-shaped system seismic faults.
In the course of the crust breaking the earthquake source was exposed on the surface in the form of the system of primary faults, which has been traced over a distance of 20 km in the Chagan-Uzun River basin in the area of the eastward lowering of the North Chuya Range.The nongravitational, that is, primary origin of these faults is indicated by their morphology and interrelations, and also by their arrangement into linear zones cutting across various topographic forms. In map view, this zone of the seismic faults has an S-shape form. Generally, this zone is an en-echelon system of compression and extension fissures arranged en-echelon into one NW-WNW striking line. This pattern of a seismotectonic fault supposes that the movement in the source was almost a pure strike-slip fault along the vertical plane.
At the bottom of the Taldura R. valley the en-echelon system of seismic breaks has an orientation of 290o-300o. Here, the alluvial deposits are dissected by opened fissures, up to 2 m wide and up to 50 m long, and by closed fissures of the same strike (320o-330o) with a dextral offset of 0.1-0.2 m. The amplitude of the offset was estimated using the displacement of the road located there. The extension fissures are accompanied by linear zones of compression showing uplifting of the soil to a height of 0.5 m. The orientation of compressive structures is of NE 40o-60o.
The most significant seismotectonic faults deformed the ground surface in a broad saddle at the Taldura R.-Kuskunnur R. watershed (Uzyuk area). The saddle is about 4 km wide and is composed of Middle Pleistocene moraine deposits, whereas it is surrounded in the west and east by gentle hills composed of Devonian schist. This area is distinguished by dense meadow vegetation with scattered dwarf birch trees.
The same area (the western slope) includes an old pressure ridge with a height of about 2 m, orienting perpendicular to the scarps, which was also rejuvenated during the main shock of the last earthquake (Figure 14). The walls of the open fissures often show the traces of the crushing and ferrugination of the loose sediments, and also the wedges and lenses of ancient soil and peat, buried under the coarse clastic material (Figure 16). These facts prove the previous seismic events there, which might have been comparable with or, potentially, greater than the earthquake of September 27, 2003.
According to new results of radiocarbon dating of the paleosoil and peat samples these high magnitude seismic events could occur in 3340 30, 1780 30 and 1280 30 years b.p. (samples No. 2817, 2823 and 2818 IGAN). Thus, the observed recurrence interval might be of 500-1500 years.
Apart from the direct manifestation of the seismic source on the ground surface as a seismotectonic fault, there are ruptures of secondary origin in the pleistoseismic area, produced by the shaking of the day light surface due to the arrival of seismic waves to the latter. The secondary breaches of the topography, mapped during this study, are situated inside of the oval area, 70 km long and about 15 km wide. The long axis of this oval is oriented along the NW-SE strike of the fault and correlates well with the long axis of the oval cloud of the aftershock epicenters. The concentration of the secondary breaches varies in a normal way, attenuating away from the linearly elongated region where the source manifested on the ground surface.
As follows from the observations of our colleagues from the Tomsk University, the earthquake shocks had caused substantial movements in the ice cover of the South Chuya Range. The previously existed cracks in the glacier bodies had grown larger. The nunataks had been exposed, and the glacier on the top of the Three Lake Dome had been detached from its source area.
Detachment cracks are abundant on the turf-covered, gentle slopes of the river terraces. Some of them had initiated microlandslides, where the rock material had moved over less than a few meters. Similar sodded loose landslides have been observed in the upper parts of the slopes from the upper-stage mountains. They were expressed by fissures in the sod cover produced by typical strike-slip and overthrust faults.
The areas of the man-made road beds had been broken by settling fissures and by the fractures that cross the road from one edge to the other. Some segments of the road bed had been displaced along these fractures.
It should be noted that the seismogravitational ruptures had been expressed less comparing to the seismotectonic ones. The largest avalanches have the form of cones up to 100 m wide and up to 300 m high, usually restricted to the steep, rocky slopes of the river valleys without covering them or reaching the opposite side. The large rock avalanches that dammed the rivers had been formed, for example, in Kirghizia during the 1992 Susamyr earthquake. This earthquake had a magnitude of 7.3 and the intensity of 9 in the epicenter. The movement in its source area was a strike-slip and thrust fault. The motion had a significant vertical component [Bogachkin et al., 1997]. Similar mass phenomena accompanied the Racha earthquake of 1991 in Georgia. This earthquake had a somewhat lower magnitude (M = 7.0-7.2) with its intensity of 7-8 in the epicenter. The focal mechanism solution was interpreted as an almost pure overthrust [Rogozhin and Bogachkin, 1995]. The geomorphological conditions in the Gornyi Altai area, where an earthquake took place in September, 2003, do not preclude the formation of large stone avalanches. Phenomena of this kind are known to have happened in the past and are usually associated with the already occurred seismic events [Rogozhin et al., 1998a, 1998b]. The absence of similar topographic disturbances during the earthquake of September 27, 2003, seems to have been associated with some peculiar type of oscillations. According to the eye-witnesses, the shaking during the main shock propagated in the horizontal plane. At the same time, it is known that high seismogravitational displacements arise during intensive vertical shaking and when seismic waves attain their critical acceleration, which diminish with the greater steepness of the slope and with the lower cohesion of the rock material [Shteinberg et al., 1993]. Judging by the presence of old stone avalanches and rock falls, no conditions of this type had been manifested during the previous earthquakes. On the contrary, the earthquake of September 27, 2003, appears to be a weak event in terms of its effect on the natural and man-made objects. This is proved by the absence of any serious destructions in the settlements located in the pleistoseist region.
The results of the preliminary study of the high-magnitude earthquake, which occurred on September 27, 2003, in the south of the Gornyi Altai region, allow us to derive a few conclusions on the specifics and results of seismic activity in this mountainous region. First, the Gornyi Altai region can be confidently interpreted as a direct northern continuation of the Mongolia and Gobi Altai, the regions where numerous earthquakes with magnitudes of > 7.0 had occurred repeatedly, and combine all of the Greater Altai mountainous systems into one seismotectonic province with high magnitudes of expected earthquakes. Secondly, the sluggish character of the earthquake, responsible for the comparatively moderate macroseismic effect at the ground surface, in spite of its high magnitude and of its source located in upper crust, suggests the complex structure of the earthquake source, possibly, associated with the curvilinear form of the fault plane.
Bogachkin, B. M., The Cenozoic Tectonic History of the Rudnyi Altai Region (in Russian), Nauka, Moscow, 1981.
Bogachkin, B. M., A. N. Korzhenkov, E. Mamyrov, et al., The structure of the source of the 1992 Susamyr earthquake, derived from the analysis of its geological and seismological manifestations, Physics of the Earth, (11), 3-18, 1997.
Devyatkin, E. V., Inner Asia (in Russian), in Recent Tectonics, Geodynamics, and Seismicity of North Eurasia, pp. 92-100, Moscow, 2000.
The Earthquakes and Seismic Zonation of Mongolia (in Russian), 220 pp., Nauka, Moscow, 1985.
Explanatory Notes to the Maps of the General Seismic Zones of the Russian Territory (GSZ) (in Russian), 57 pp., Scale 1:8 000 000, Institute of Physics of the Earth, 1999.
Molnar, P., R. A. Kurushin, V. M. Kochetkov, et al., Deformation and rupture formation during large earthquakes in the Mongolia-Siberia region, in Crustal Structure and Geodynamics of the Mongolia-Siberia Region (in Russian), pp. 5-55, Nauka, Novosibirsk, 1995.
Rogozhin, E. A., and B. M. Bogachkin, Alpine and recent tectonics in the area of the Racha earthquake, Physics of the Earth, (3), 3-11, 1995.
Rogozhin, E. A., B. M. Bogachkin, L. I. Ioganson, et al., Experience of detecting and tracing earthquake generating zones by the methods of geotectonic analysis in the territories of West Mongolia and the Zaisan-Altai fold area (in Russian), in Seismicity and Seismic Zoning of North Eurasia, pp. 132-152, Publ. Iss. 2-3, Institute of the Earth Physics, 1995.
Rogozhin, E. A., B. M. Bogachkin, Yu. V. Nechaev, et al., Paleoseismological investigations on the territory of Russian (Gornyi) Altai, Journal of Earthquake Prediction Research, 7, (4), 391-413, 1998a.
Rogozhin, E. A., B. M. Bogachkin, Yu. V. Nechaev, et al., New evidence of strong earthquakes in the Mountainous Altai Region, Izvestia, Physics of the Solid Earth, 34, (3), 244-250, 1998b.
Shteinberg, V. V., M. V. Saks, F. F. Aptikaev, et al., Methods of estimating seismic effects (in Russian), in Problems of Engineering Seismology, Iss. 34, pp. 5-94, Nauka, Moscow, 1993.