INTRODUCTION

Assessment of the modern rates of tectonic movements are based on analysis of the time series for GPS points and stations; the time series are calculated from the initial data represented by the sets of phase and code measurements three days long with the sampling frequency of 30 s (GMT) at two frequencies. For the majority of observation points, periodicity of measurements is multiple to one year.

To assess the modern horizontal movements in the Ossetian region of the Greater Caucasus, we used data from 25 stations of the satellite geodetic network created by the joint efforts of the Vladikavkaz Science Center of the Russian Academy of Sciences (VSC RAS) and the Sternberg Astronomical Institute, Moscow State University (AI MSU) for the periodic repeated GPS measurements and from one station for continuous GPS observations (Milyukov et al., 2015, 2018). In addition, we took into account records from two seismic stations that are part of the network of the United Geophysical Survey of the RAS located in this region (Table 1).

Table 1.   Longitudinal and transverse projections of horizontal movement rates for the points and stations of the Ossetian geodetic network relative to the stable Eurasian continent to the profile running at azimuth 15.88°
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The geodetic profile is located in the zone of maximum narrowing of all tectonic zones of the Greater Caucasus. The main moving tectonic boundaries are clearly expressed in the modern topography and deformations of the Late Pliocene–Eopleistocene geomorphic level (Fig. 1).

Fig. 1.
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Tectonic scheme of the area where the stations of the Ossetian geodetic network were installed, after (Milyukov et al., 2018). Arbitrary notes: (1) main reverse and thrust faults; (2) highs of crystalline basement; (3) Pliocene–Quaternary intrusions; (4) Pliocene–Quaternary volcanic rocks; (5) Oligocene–Pliocene deposits of the Ossetian depression, North Caucasian foredeep, Racha–Lechkhum trough, and Transcaucasian intermontane basin; (6) points along the microseismic sounding (MSS) profile, after (Rogozhin et al., 2014; Gorbatikov et al., 2015)); (7) points and stations of the Ossetian geodetic network; (8) clusters of epicenters of the Racha (М = 7.1) and Dzhava (М = 6.2) earthquakes’ aftershocks. Circles denote epicenters of earthquakes occurred in 2010–2016, based on the seismological network of the North Ossetian Branch of the United Geophysical Survey, Russian Academy of Sciences (http://www.ceme.gsras.ru). Numerals denote major faults: 1, Kakheti–Lechkhum (Orkhevi, Utsera); 1a, southern branch of the Kakheti–Lechkhum Fault (Potskhrevi); 2, Gebi–Lagodekhi; 3, Main Caucasian (Tib); 4, Adaikom-Kazbek; 5, Pui; 6, Balta; 7, Vladikavkaz.

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The projections of horizontal velocities of movements were extrapolated to the combined structural-geomorphic and geophysical profile running along the valleys of the Greater Liakhva and Ardon rivers (Fig. 2). Geophysical studies were carried out using the microseismic sounding (MSS) method (Gorbatikov et al., 2015) which enables us to adequately reveal configurations of both subvertical and subhorizontal velocity boundaries down to 60 kilometers depth. The geologic objects characterized by relatively high seismic velocities are seen in the microseismic field as zones with lower amplitudes, and vice versa. Combined with data on manifestations of tectonic processes in the Quaternary, the MSS deep profiles provide an insight into the modern geodynamic sources (Gorbatikov et al., 2015).

Fig. 2.
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Structural-geomorphic and geophysical profiles combined with the longitudinal and transverse projections of rates of horizontal movements fro the points and stations of the Ossetian geodetic network relative to the static Eurasian continent to the profile running along the azimuth 15.88°. Arbitrary notes: (1) main faults (K-L means Kakheti–Lechkhum; G-L, Gebi–Lagodekhi; MC(T), Main Caucasian (Tib); A-K, Adaikom-Kazbek; P, Pui; Vl(S), Vladikavkaz, southern branch; Vl(N), Vladikavkaz, northern branch); (2) Late Pliocene–Eopleistocene geomorphic level; (3) projection of the clusters for hypocenters of the Racha and Dzhava earthquakes’ aftershocks to the geological-geophysical profile; (4) modern movement rates. Letters in the upper part are abbreviations for points and stations of the Ossetian geodetic network (Table 1).

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The area illustrated in the profile included the epicentral zones of two remarkable earthquakes, giving us an opportunity to establish the character of horizontal movements and deep structure of the respective source zones.

DESCRIPTION OF SEISMIC EVENTS

The epicentral zone of the Khataldon earthquake (May 11, 2008, М = 4.5) is hosted by the Vladikavkaz deep fault, which represents the regional boundary between the Alpine folded-and-block structure of the Greater Caucasus and the Northern Caucasus foredeep (Ossetian depression). In the upper layers of the sedimentary cover, the fault zone is represented by the largest regional flexure-fracture system in rocks of the Paleogene, Miocene, and Pliocene–Quaternary (Gorbatikov et al., 2011). In the field of horizontal gravity gradients, the boundary between the Greater Caucasus and Ossetian depression is characterized by anomalously high values, i.e., there is a regional gravity step. According to the calculations, the near-surface part of the step corresponds to the laminated vertical flexure with the extent of the linking limb of up to 10 kilometers and dip angle DP = 65°–70°, traced down to 5–6 kilometers depth. It is supposed that the basement surface in the area of the Vladikavkaz Fault is shifted by approximately 5–6 kilometers (Gorbatikov et al., 2011).

Deformations of the Late Pleistocene–Holocene terraces, related to three branches of the Vladikavkaz Fault Zone, were described in detail for the first time by E.E. Milanovsky (1968). The fault later appeared on the active faults of the Crimean–Caucasian region (Trifonov et al., 2002), with all three branches being indicated, as well as reverse dip character and slip rates of at least 1 mm/yr. Note that the hanging walls of all three branches are northern ones. In the lineament–domain–focus model developed during the preparation of General Seismic Zoning normative maps OSR-97, the zone of the Vladikavkaz Fault is shown as a seismolineament with Мmax = 6.5 (Ulomov and Shumilina, 1999). According to the application of extraregional seismotectonic method, which is based on the cluster analysis of the set of geological-geophysical data, the maximum seismic potential of this zone ranges from 6.5 to 7.1 depending on the segment (Rogozhin et al., 2001).

As a result of these and other studies, it was found that the zone of the Vladikavkaz Fault was a potential seismic hazard. During the field works, the near-surface structure of the fault zone was investigated by geological-geomorphic methods. The deep structure was studied by low-frequency MSS (Fig. 2).

All three branches of the fault zone are expressed in displacements of young terraces and the respective deposit comprising them. Additionally, the southern and middle branches concentrate multiple block slides, as well as traces of ancient dammed lakes and river capture. The northern branch of the Vladikavkaz Fault Zone is expressed on the surface as a ledge cutting the deposits of young piedmont complex of the Greater Caucasus. The maximum height (up to 8 meters) of the ledge is observed on the surfaces of the 5th and 6th Late Pleistocene terraces. The arguments for activation (or more precisely, seismic activation) of the fault zone in the past are fractured displacements of young deposits, as well as colluvial fans and buried paleosoils at footwalls of faults, studied in special mine workings. The amplitude of likely singular seismotectonic vertical slip on the northern branch of the Vladikavkaz Fault Zone is up to 40 centimeters (Fig. 3, point 7).

Fig. 3.
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Positions of the branches of the Vladikavkaz Fault Zone in the topography map of the study area (isolines are drawn for every 30 m), modified after (Ovsyuchenko et al., 2011a, 2011b). Arbitrary notes: (1) active reverse faults (bergstrichs are directed toward the hanging wall); (2) supposedly active faults (strike-slips); (3) points of geological-geomorphic observations; (4) points of MSS performed along the VL1 (Vladikavkaz) and VL2 (Alagir) profiles; (5) isoseismal of the highest intensity for the 2008 Khataldon earthquake, after (Starovoit et al., 2014). AB means the line, along which the detailed geomorphic study of terrace complexes of the Gizeldon River was carried out.

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In the middle and southern branches, vertical amplitudes of seismotectonic slips also do not exceed 40 centimeters (Fig. 3, points 3, 5, and 6). Using the statistical relationships linking earthquake magnitudes with extents of zones of observed coeval seismodislocations, and amplitudes and kinematics of seismotectonic slips (Wells and Coppersmith, 1994), we may suppose that magnitude of the seismic events generated these paleoseismodislocations was 6.5–6.7.

Thus, the described deformations give us grounds to think that all branches of the Vladikavkaz Fault Zone were renewed (reactivated by earthquakes) in the past, causing the formation of both seismic ruptures and large block slides.

The field instrumental investigations of the deep structure of the Vladikavkaz Fault using MSS (Gorbatikov et al., 2010, 2011) included the construction of deep sections in terms of relative velocities of S-waves along two profiles (Fig. 2). The sections in terms of relative velocities of seismic waves (or seismic contrasts), obtained through MSS, are presented in Fig. 4. The intensity of microseismic signal is given relative to the base station. Absolute values of velocities are not determined.

Fig. 4.
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(a) Topography section across the profile direction (1) and along the river valley (2). Results of low-frequency MSS application along the (b) VL2 (Alagir) and (c) VL1 (Vladikavkaz) profiles, modified after (Gorbatikov et al., 2010, 2011). Dashed lines indicate active faults inferred from the geological studies. The star denotes the hypocenter of the 2008 Khataldon earthquake. Locations of the VL1 and VL2 profiles are shown in Fig. 3.

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The most significant inhomogeneities in both MSS profiles are thick subvertical zones of high contrasts of shear wave velocities. Interestingly, these subvertical zones are traced down to the base of the Earth’s crust and even below, into the upper mantle (Fig. 2), while the subhorizontal inhomogeneities of microseismic sections abruptly disappear within their limits and are not traced in the opposite crustal blocks. In the upper parts of the sections (Fig. 4), at 7–8 kilometers depth and less, several contrasting zones are distinguished: two and three zones are seen in the profiles VL2 (Alagir) and VL1 (Vladikavkaz), respectively. In the VL1 profile, at 15 kilometers deep, these zones unite into one. In the VL2 profile they are traced separately down to the base of the Earth’s crust, and here only the central branch sinks into the mantle (Fig. 2).

In the near-surface parts of the sections, the distinguished inhomogeneities clearly correspond to geologically studied fractures in young deposits. If we consider the zones with high contrasts of seismic noise values, revealed from MSS data, as those of lower rigidity, then fractures in young deposits can be associated with the zones of released deep stresses in the upper layers of the sedimentary cover. Such a zone can be seen in the clearest manner in the area of the town of Alagir. This enables us to perform a general segmentation of the Vladikavkaz Fault Zone in terms of its modern activity: the most active is its western flank, where the Khataldon earthquake occurred.

The epicentral zone of the Khataldon earthquake of May 11, 2008, is clearly confined in the study area, being hosted on the Vladikavkaz Fault (Fig. 3). The isoseismal corresponding to I = IV, oval in plan view, is located between the northern and central branches of this zone. Given the hypocentral depth (from 7 to 17 kilometers, according to different estimates), the source could be located in the lower part of the southern branch, dipping northwards as a listric structure (Fig 3a). Then the nodal plane in the source can be gently dipping (24°) one directed to the northwest (Starovoit et al., 2014), and the slip was a thrust.

The Oni-II earthquake of September 7, 2009 with Мw = 6.0 occurred in the area near the Georgian–South Ossetian boundary (Gabsatarova, 2015) (Fig. 5). The available seismological and macroseismic data allow us to construct a clear model of the source (Figs. 5, 6). According to the data and focal mechanism solutions, the nodal plane in the source was a gentle one dipping in NNE direction. The slip type was a thrust with an insignificant strike-slip component (Fig. 6). Major axes of ovals, corresponding to the isoseismals of I = III and IV, are stretched along the direction of the Main Caucasus Range (Fig. 7).

Fig. 5.
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Map of seismic activity A3.3 for the North Caucasus region of Russia and adjacent areas, after (Rogozhin et al., 2013). The star denotes the epicenter of the Oni-II earthquake of September 7, 2009.

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Fig. 6.
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Beach ball diagrams of the Oni-II earthquake of September 7, 2009, after the data from different seismological centers. Arbitrary notes: (1) nodal lines; (2) and (3) axes of principal stresses, tension and compression, respectively. The black part corresponds to compression waves, after (Gabsatarova, 2015).

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Fig. 7.
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Shaking intensity map with proposed locations of isoseismals and reported intensities for the Oni-II earthquake of September 7, 2009, after (Gabsatarova, 2015). The star denotes the epicenter location.

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The source of the 2009 earthquake is similar in this aspect to those of the 1991 Racha (Aref’ev et al., 1993), Dzhava, and Oni-I earthquakes (Gabsatarova, 2012). The tomographic analysis of lateral and deep distribution of heterogeneities in the field of longitudinal wave velocities allowed researchers to obtain some new data on the structure and time evolution of the source zone of the Racha earthquake (Arefiev et al., 2006).

In the field of longitudinal wave velocities, the ancient crystalline basement and Mesozoic–Cenozoic sedimentary-volcanogenic cover can be distinguished in the deep sections. The basement high of the Dzirula massif is outlines in the field of seismic velocities on west and east. The low-velocity linear inhomogeneities revealed in the basement domain correspond to the zones of the Kakheti–Lechkhum active fault and two Transcaucasian lineament-fault zones separating the Dzirula high from the Rioni intermontane basin on west and from the Kartli one on east, respectively. In the western and eastern parts of the high, these are the Rioni–Kazbek and Tskhinval–Kazbek faults, respectively (Rogozhin et al., 2014). It is these tectonic faults that confined the source zones of the 1991 Racha earthquake and the 2009 Oni-II earthquakes on west and east.

The notable feature is general correlation of the depth position for the hypocentral cloud of aftershocks and the velocity divide corresponding to the cover–basement boundary. It is also interesting to note the structure of the area of the Kakheti–Lechkhum suture zone at the base of the southern slope of the Greater Caucasus. Here, a narrow vertical low-velocity pocket, dividing the high-velocity crust of the southern Chiauri flysch zone to the north and the Okriba–Sachkhere margin of Georgian block to the south, can be seen in the geophysical profile down to about 15 kilometers depth (Gorbatikov et al., 2015) (Fig. 2). Along the surface of the Paleozoic crystalline basement, beneath the Alpine complex in the southern margin of the Chiauri flysch zone, a narrow low-velocity body about 3 kilometers thick, gently dipping northwards at about 15° is revealed. This gently sloped detached fragment of the sedimentary cover from the basement of the Georgian block was identified in the seismotomographic model for the zone of the 1991 Racha earthquake (Arefiev et al., 2006).

The slips in sources of the Racha and Oni-II earthquakes can be considered manifestations of the main tendency of horizontal movements in the central sector of the Greater Caucasus, namely, underthrusting of the basement of the Georgian block referring to the Transcaucasian slab beneath the anticlinorium of the Main Caucasus Range and flysch synclinorium. This geodynamic model is supported by the results from the stations of the satellite geodesy network (Milukov et al., 2018).

After the Oni-II earthquake, researchers from the Schmidt Institute of Physics of the Earth RAS (IPE RAS) carried out studies in South Ossetia in the eastern part of the epicentral zone (Ovsyuchenko et al., 2011a, 2011b). The seismodislocations produced by this new seismic event were of secondary (seismogravitational) character and had very limited distribution in the area. The surface seismogenic rupture was absent. During the field works, paleoseismological studies of seismodislocations were conducted; these seismodislocations, although produced by ancient earthquakes, were spatially connected with the epicentral zone of the 1991 Racha earthquake, currently the strongest in the region. All paleoseismodislocations were also of seismogravitational character: landslides, landslides and rockfalls landslides and rockfalls, stone avalanches, and dammed lakes.

According to the obtained data, the previous strong earthquake (i.e., preceding the 1991 Racha one) in the Racha–Dzhava zone most likely occurred about 2000 years ago. Another earthquake was even more ancient—it occurred ca. 6000 years ago.

Greek specialists (Nikolaeva and Walter, 2016) used the data of Alos L-range of InSAR in order to render interferograms covering the time intervals before and after the 2009 earthquake. A local uplift of about 10 centimeters in amplitude was detected in the interferogram near the earthquake epicenter, while the field survey did not reveal any signs of surface ruptures. The mentioned authors compared the model surface of the fault plane in the source of the 2009 earthquake and the one of a “blind” rupture of the 1991 Racha earthquake to find that the rupture formed in 2009 was located near the rupture in the northern part of the source of 1991. In both cases, the ruptures were of thrust type dipping at about 30° in NNE direction beneath the axial zone of the Greater Caucasus. Such a solution is consistent with the deep position of the nodal plane describing the focal mechanism of NNE dip (Fig. 6).

GEODYNAMIC CHARACTERISTICS OF SEISMOGENERATING ZONES

The rates of horizontal movements in the zones of studied earthquakes can be assessed on the basis of the data from GPS observations (Milyukov et al., 2018) (Fig. 2). On the southern slope of the Greater Caucasus, in the zone of the Kakheti–Lechkhum Fault, the values of both longitudinal and transverse projections of horizontal movement rates abruptly change. High rates of horizontal shortening along the profile (3–4 mm/yr) reflect the approach Dzirula massif and the crystalline core of the Greater Caucasus along the zone of the Kakheti–Lechkhum Fault. In the most uplifted part of the Greater Caucasus located further north, to the north of the Gebi–Lagodekhi Fault, horizontal shortening rates noticeably reduce down to 0–1 mm/yr.

It should be recalled that the network including five GPS stations was installed in the epicentral zone of the 1991 Racha earthquake, after its occurrence, to record horizontal movements of the surface of the southern slope of the Greater Caucasus and in the Transcaucasian Plate; the network had been in operation for about four years (Prilepin et al., 1997). It was revealed that in this four-year period, the Dzirula middle massif and Okriba–Sachkhere zone of the Transcaucasian Plate moved northward at 4.2 ± 0.9 mm/yr, while movements of the tectonic zones of the southern slope, recorded immediately to the north of the Kakheti–Lechkhum Fault, were directed to the south and south-southwest and had rates of 6.8 ± 1.2 mm/yr. Hence, in the source zone of this earthquake, Earth’s surface contracted at the rate of about 1 cm/yr, and the northern (Caucasian) wing of the fault was characterized by considerably higher rate of horizontal movements.

In the studied region, the rate of displacements in recent years decreased almost twice, and rapid southward movements of the southern wing of the megaanticlinorium disappeared. However, the abrupt decrease (in fact, down to zero) in movement rates to the north of the zone of the Kakheti–Lechkhum Fault is the same as the tendency recorded after the 1991 Racha earthquake. Notably, the considerable decrease in rates of horizontal movements after the end of the aftershock process, predicted earlier by researchers (Prilepin et al., 1997), has been confirmed after both Oni earthquakes.

In the northern wing of the megaanticlinorium, the data of GPS observations (Milyukov et al., 2018) also demonstrate a small increase in mean rates of horizontal movements (1–2 mm/yr) in the area up to the zone of the Vladikavkaz Fault (Fig. 2). To the north of this fault, the rate of transverse shortening decreases down to 0–1 mm/yr.

Thus, it has been revealed that in the zone of the Vladikavkaz Fault, where the Khataldon earthquake occurred, and in the zone where the Racha and Oni ones occurred, modern rates of movements considerably decelerate in the submeridional direction (transversal to the Greater Caucasus).

These data can be considered a mid-term precursor of the strong earthquake being prepared in the central part of the Greater Caucasus. Moreover, the zone of the Vladikavkaz Fault demonstrates a clear seismic quiescence for many decades. With the exception of the 2008 Khataldon earthquake of moderate energy, no significant seismic events have occurred here recently (Milyukov et al., 2015).

Similar phenomena observed before strong earthquakes was documented in several regions of the world. When comparing the rates of horizontal movements in the epicentral zone of the preparing 2008 Wenchuan earthquake (in the Longmenshan Fault Zone) in the periods of 1991–2001 and 1999–2007 with those recorded after the event, small rates of movements were observed before the mainshock and anomalously high ones after it (Gu Guohua et al., 2009; Liu Xia et al., 2014). This gives us grounds to conclude that the 2008 Wenchuan earthquake activated the Longmenshan Fault Zone. The analysis of total seismic energy release during this earthquake suggests that the interval of 1977–2007 was the clear period of seismic quiescence before the strong earthquake, and the activation period began in 2008.

It should be noted that the situation with the directions and rates of Earth’s surface movements during the 2008 Wenchuan earthquake was similar to what was observed in the source zones of earthquakes such as the Simushir of 2006 and 2007 (Steblov et al., 2007), 2010 Chile (Moreno et al., 2010), and 2011 Tohoku-Oki (see the web links http://www.jishin.go.jp/ main/chousa/11mar_sanriku-oki/p01-e.htm and http://www.eri.u-tokyo.ac.jp/eng/ and publication by Rogozhin et al. (2014)); these were probably typical peculiarities of all zones of activated faults belonging to reverse-fault or thrust types. In this case, the anomalously low velocity of horizontal movements in the epicentral zones of preparing strong earthquakes can be thought to be a mid-term precursor and an indication of the future epicenter.

However, the considerable decrease in movement rates within the Racha–Oni zone seemingly indicates gradual lowering of the seismic activation that began there in the 1990s.

CONCLUSIONS

The comprehensive geological-geophysical, seismotectonic, and geodetic study of the zones of two remarkable earthquakes that occurred in the central sector of the Greater Caucasus, on its northern and southern slopes, in the early 21st century, has allowed us to infer the 3D structure and tectonic position of their sources. The Khataldon earthquake of May 11, 2008 was hosted in the southern branch of the Vladikavkaz Fault, characterized by listric structure and dipping northward. The slip in its source was a thrust of the northern wing of the fault. According to GPS observations, northward-directed rates of horizontal movements in the fault zone abruptly decrease down to almost zero, likely indicating the mid-term precursor of a strong earthquake that is to occur here in the future.

The tomographic analysis of the lateral and deep distribution of inhomogeneities in the field of latitudinal seismic wave velocities has allowed us to obtain new data on the time and space development of the source zones of the 1991 Racha and 2009 Oni earthquakes. The general correlation is notable between the depth position of the cloud of aftershocks’ hypocenters and the velocity jump corresponding to the cover–basement boundary. The structure of the epicentral zone at the base of the southern slope of the Greater Caucasus is notable: here, a narrow low-velocity inhomogeneity of about 3 kilometers thick, gently (5°–15°) dipping northwards, has been identified in the submeridional geophysical profile running along the surface of the Paleozoic crystalline basement, beneath the Alpine rock complex at the southern margin of the Chiauri zone.

An abrupt decrease in rates of modern horizontal tectonic movements of the Earth’s surface in the zones of the Racha earthquake and the Oni ones according to repeated GPS measurements for the last 25 years can be considered an indication of the end of seismic activation on the southern slope of the central sector of the Greater Caucasus that began in the 1990s.