E. A. Rogozhin1, S. L. Yunga1, A. V. Marakhanov2, E. A. Ushanova1, A. N. Ovsyuchenko3, and N. A. Dvoretskaya2
1United Institute of Physics of the Earth (UIPE), Russian Academy of
2Institute of Physics of the Earth (IPE), Russian Academy of Sciences, Moscow, Russia
3Geological Department, Rostov State University
In recent years, geologic-geomorphologic studies have been conducted in the Northwest Caucasus, aimed at revealing traces of unknown large past earthquakes in active fault zones. The studies were focused on paleoseismologic issues and involved trenching and test-pitting across those landforms that could be classed as seismic ruptures caused by past earthquakes. Earlier, similar studies were conducted on the Greater Caucasus [Paleoseismogeology..., 1979] and on the Lesser Caucasus [Rogozhin and Filip, 1991], and they demonstrated the applicability of such techniques to the Caucasus region as a whole.
Data on fold and fault structures in the Northwest Caucasus were collected and systematized in the form of three structural-geologic transects across the strike of the orogen. The transects were drawn through all the principal zones of the meganticlinorium, (i) along the Dzhubga-Goryachy Klyuch motor road, (ii) along the line Bezeps-Inal Cove, and (iii) along the Novorossiisk-Rostov motor road. The cross sections were constructed at 1:25,000 scale. During fieldwork, shapes of folds of various orders, lithologies, and all the major and numerous minor faults were recorded on the cross sections.
In order to elucidate the deep structure, features observed in the nearsurface portion of the sections were continued downward to basement surface. This exercise was performed with due account for thickness of geologic horizons, as assessed from stratigraphic columnar sections for each tectonic zone, and for the style of major- and medium-size folds (dip of limbs and axial surfaces, angle between limbs, etc.).
Tectonic lineaments differ in terms of dominant strikes in different lithotectonic zones of the northern and southern slope. In the more northerly Semigorsky zone (Figures 1, 3), encompassing part of the south slope, and in tectonic zones composing the axial part and northern slope of the Main Caucasus Range (Goitkhsky, Papaisky, and Tkhabsky), the lineaments can be attributed chiefly to the two orthogonal structural directions, Caucasian (WNW or EW) and "anti-Caucasian" (roughly N-S). In the Anapa-Agoisky flysch zone of the south slope, principal faults belong to the diagonal NE and NW structural trends.
The largest faults bound the graben on the northeast and southwest. They are accompanied by wedges that reach 4-5 m deep and are filled in with rock debris and soil material (Figure 6). Apparently, displacements on these faults are extremely young (in all likelihood, a few hundred to a few thousand years in age), are of pulse character, and have been repeatedly reactivated. One can discern four pulses of activity, expressed in the formation of debris wedges and horizons and talus aprons and in three phases of graben quiescence, evidenced by abnormally thick paleosoil horizons.
Besides the largest faults, the body of the "stabilized" landslide and its detachment zone are affected by numerous secondary faults, fractures, and jointing zones connected with repeated displacements of the landslide body. A number of such displacements are accompanied by zones of wall-rock reworking--mylonitization, limonitization, and carbonates crushed to powder. Some of the faults are devoid of such alterations.
Virtually all the faults, with rare exceptions, are extensional structures. Total extension, in as far as it can be assessed from the system of excavations in the landslide body, is likely to exceed 5 or 6 m.
Importantly, the young graben-like extensional structure is confined to the southwestern limb of an ancient NW-trending fault zone. The fault is expressed in two high-angle, SW-dipping, 30- to 40-cm-wide zones of jointing, brecciation, and limonitization. In the subsoil layer, these zones are continued upward by a thick eluvium pocket. These faults are situated in the central part of the trench (Figure 6). A scarp over 4 m in height, crossed by this trench (Figure 7), stretches for 550 m in a northwesterly direction. Further NW, the fault runs along a narrow cleft. To the southeast, the scarp ends 50-80 m short of the trench. Therefore, the length of the well-marked scarp is some 600 m. As one moves downslope, as many as three gentle benches with steeper slopes in-between are observed, which is characteristic of landslide topography.
Quaternary strata within the small graben and in the upper part of the steep slope are abnormally thick (3-5 m), and are distinguished by an appreciable variety of facies and by the presence of several thick horizons of buried paleosol, whereas normally, eluvial and talus deposits are no thicker than 1 m in this area. The modern soil layer in the study region is also thin, 20-30 cm, paleosoils being encountered only in active fault zones. It is worth noting that in the vicinity of the small graben and step-like steep slope, the forest is stable. Tree trunks are almost vertical, and there is no evidence of "drunk forest." The age of the oldest trees on this slope, which is of likely landslide character, is 230-260 yr (based on year rings). It thus can be ascertained that active movements along the scarp or on the slope have not resumed at least during that time. Radiocarbon datings on horizons of paleosoils and bones, collected from the walls of the trench crossing the graben, may be helpful in determining active displacement periods and stable development phases. The origin of this extensional near-fault structure can be tentatively attributed to strong surface shocks during large past earthquakes. To use the classification of the Irkutsk seismogeologic school, such seismic ruptures are termed gravitational-seismotectonic dislocations [Paleoseismogeology..., 1979].
Therefore, in the Beregovoi fault zone, the small seismogenic graben accompanying this NW-trending rupture is manifest in bed rocks and in Quaternary deposits. Here, the age of the modern soil is determined as 130 40 yr (IGAN-2418). A buried paleosoil horizon in the most sagged part of the near-fault graben yielded, from a depth of ca. 1.2 m, human bones, whose age was determined as 2980 90 yr (GIN-11728), and samples collected from still older paleosol horizons, from 1.3-1.4, 2.0, and 2.2-2.3 m depths, yielded respective radiometric ages of 5210 200, 6840 230, and 8600 190 yr (IGAN-2429, IGAN-2417, and IGAN-2427).
The graben-like structure just described has, in all likelihood, a resonance-seismic origin. Albeit clearly confined to a specific rupture, it is not itself a surface expression of seismic fault. Hence, it cannot be classed as a primary seismotectonic rupture, as is the case with the Kuznetsovsky or Verkhnekhazarovsky faults (Figures 4, 5). The singular gravitationally unstable state of the southwestern limb of the Beregovoi fault, which is a tectonic block downthrown due to joint action of geologic and gravity-related, slope phenomena, rendered this block extraordinarily sensitive to seismic shocks. Apparently, any seismic shock that occurred nearby excited resonance oscillations in this unstable structure, each time causing a relatively small-magnitude (0.5-0.7 m), instantaneous gravitational displacement of the downthrown block in the southwest limb of the fault, extension, and sagging of the near-fault graben. Such sharp pulse displacements are recorded in the Holocene section filling in the graben as a sequence of alternating young loose sediments and paleosoils.
Periods of accumulation of soil horizons reflect phases of quiescence of the past gravitational-seismotectonic seismodislocation under study, whereas the overlying and intercalated talus/debris sequences record periods of renewed movements in the graben-like structure. This means that, based on these datings, seismic displacements occurred four times. The first occurred roughly between 9 and 7 ka; the second, between 7 and 5 ka; the third, between 5 and 3 ka; and, lastly, the fourth, between 3 ka and 100-170 (the modern soil age, IGAN-2418) or 250 (forest age) years ago. According to these results, the seismic shock that occurred in the year 1341 1 in the Malobzhidsky fault zone not far from the small graben is coeval with the last paleo-earthquake, just as the September 16, 1799 shock, related to the Verkhnekhazarovsky fault zone. Possibly, prior to these four seismic events, a yet another shock--the oldest of those recorded stratigraphically--took place in the late Pleistocene or early Holocene. Temblor of the graben-like structure at this shock gave rise to a minor debris wedge in the downfaulted, northeastern limb of the buried ancient scarp in the central part of the graben, in the base of the oldest paleosol horizon (Figure 6). Probably, two of the four ancient seismic events can be correlated to pulse displacements that left traces in the stratigraphy of the trenched Kuznetsovsky fault zone (Figure 5).
The recurrence period of major earthquakes in seismogenic zones of the south slope of the Northwest Caucasus, as deduced from these paleoseismologic data, is ca. 2000 years.
Hence, in all three cases, the rate of creep displacements on faults is between 1.5 and 2.0 mm/yr. These values are appreciably lower than those given by Trifonov  for highly active Eurasian faults (5 mm/yr or more) or than the maximum rate of modern vertical crustal movements estimated from direct geodetic measurements (up to 6 mm/yr for the Caucasus region) [Kuznetsov et al., 1997; Lilienberg and Yashchenko, 1989, 1991], but they are roughly equal to the values given for the Northwest Caucasus in the Map of Modern Vertical Movements [Map..., 1986].
Paleoseismologic studies in the Greater Caucasus, conducted in recent years and previously [Paleoseismogeology..., 1979; Rogozhin and Ovsyuchenko, 2001; Rogozhin et al., 2001], show large earthquakes that left well-marked surface seismic ruptures to have occurred in the past in some geologically active fault zones. The study of past seismodislocations in trenches enabled us to constrain the age of ancient earthquakes. Recurrence interval of such seismic events in the Northwest Caucasus is on the order of 2000 years. The rate of geologic displacements (creep) on faults, based on radiocarbon dating of modern soils, is assessed at 1.5 to 2.0 mm/yr over the past 100 years.
A New Catalog of Major Earthquakes in the USSR Territory from Ancient Times to 1975, 535 pp., Nauka, Moscow, 1977.
Kuznetsov, Yu., V. Kaftan, V. Bebutova, et al., Modern vertical movements of the earth's surface in the Caspian region (in Russian), Geodeziya i Kartografiya, (9), 29-33, 1997.
Lilienberg, D., and V. Yashchenko, Analysis of geodetic and morphostructural data for the area of the catastrophic Armenian earthquake (in Russian), Geodeziya i Kartografiya, (10), 23-29, 1989.
Lilienberg, D., and V. Yashchenko, Principal developments in geodynamics of mountain morphostructures of the Greater Caucasus based on new geodetic data (in Russian), Geodeziya i Kartografiya, (2), 21-28, 1991.
Map of modern vertical movements of the earth's crust in the territory of Bulgaria, Hungary, the German Democratic Republic, Poland, Romania, the USSR (European part), and Czechoslovakia (in Russian), scale 1:10,000,000, GUGK SSSR, 1986.
Nesmeyanov, S., Geomorphologic Aspects of the Paleoecology of the Mountain Paleolith: A Case Study in the West Caucasus, 391 pp., Nauchny Mir, Moscow, 1999.
Nesmeyanov, S., Neotectonic Zonation of the Northwest Caucasus, 254 pp., Nedra, Moscow, 1992.
Paleoseismogeology of Great Caucasus, 188 p., Nauka, Moscow, 1979.
Reisner, G., and L. Ioganson, Seismic potential of western Russia and other CIS and Baltic countries, in Seismicity and Seismic Zonation of Northern Eurasia, pp. 186-195, United Institute of Physics of the Earth, Russian Academy of Sciences, Moscow, 1993.
Reisner, G. I., E. A. Rogozhin, and L. I. Ioganson, Extraregional method for revealing the sources of expected earthquakes of different M, J. Earthquake Prediction Research, 8, (2), 186-202, 2000.
Rogozhin, E., and E. Filip, Geologic and tectonic study of the focal zone of the Spitak earthquake (in Russian), Izv. Akad. Nauk SSSR, Fiz. Zemli, (11), 3-17, 1991.
Rogozhin, E. A., and N. I. Ovsyuchenko, Relationship of geological and seismological components of tectonic motions on the Northern Caucasus, Tectonics of Neogey: Basic and Regional Aspects, Proceedings of Conference, v. 2, pp. 145-148, Moscow "GEOS" Publishing House, 2001.
Rogozhin, E., B. Bogachkin, and Yu. Nechaev, Seismotectonic significance of transverse zonation of the northwestern Greater Caucasus, in Constructing Models for the Development of the Seismic Process, State scientific/technologic program of Russia Global Changes of the Natural Environment and Climate, pp. 139-148, United Institute of Physics of the Earth, 1993.
Rogozhin, E., G. Reisner, and L. Ioganson, Assessing seismic potential for the Greater Caucasus and Apennines by independent methods, in Geophysics and Mathematics XX1, Modern Mathematical and Geophysical Models in Applied Geophysical Problems, pp. 279-299, United Institute of Physics of the Earth, Moscow, 2001.
Rogozhin, E., O. Ostach, R. Gibson, et al., Extensive landslide formation in Stavropol Territory as an example of a "quiet" natural disaster, in Federal System of Seismologic Monitoring and Earthquake Prediction, Informational and Analytical Bulletin, 1, (3), 15-19, 1994.
Shempelev, A., N. Prutsky, I. Feldman, and S. Kukhmazov, A geologic-geophysical model along the Tuapse-Armavir transect (in Russian), Proc. XXXIV Tecton. Conf. Tectonics of the Neogean: General and Regional Aspects, Moscow, Jan. 30 to Feb. 3, 2001, vol. 2, pp. 316-319, GEOS, Moscow, 2001.
Shteinberg, V., M. Saks, F. Aptikaev, et al., Methods for the assessment of seismic impacts, in Preassigning Seismic Impacts, Vopr. Inzhenern. Seismol., 34, pp. 5-94, Nauka, Moscow, 1993.
Trifonov, V., Neotectonics of Eurasia, 252 pp., Nauchny Mir, Moscow, 1999.