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Crustal strain in the rogoun dam area
Jougrq^L oF OEODYNAM]CS10, 243--254 (1988)
243
CRUSTAL STRAIN IN THE ROGOUN DAM AREA
V. I. STARKOV, O. V. SOBOLEVA and E. YA. STARKOVA.
Institute of the Earth's Crust, Moscow, U.S.S.R. (Accepted August 5, 1988)
ABSTRACT Starker V. I., Soboleva O. V. and Starkova E. Ya., 1988. Crustal strain in the Rogoun Dam area. In: Yu. D, Boulanger, S. Holdahl and P. Vysko~il (Editors), Recent Crustal Movements. Journal of Geodynamics, 10: 243-254. In recent years much attention has been paid to the study of crustal movements within tectonically active regions. The present work is specially of interest since it deals with the site where one of the largest hydroelectric plants in the world is being built. The erection of the Rogoun HPP on the Vakhsh river involves the filling of a 320 m high dam using local materials and the construction of an undergroundhead hydroelectric unit with substations and auxiliary services. The work gives an evaluation of features and rates of seismotectonic deformations in two layers of the earth's crust (0.9 km and 10-25 kin), obtained for a period of 28 years, within an area of 3 thousand km z around the HPP. These data allowed us to determine the complex block structure of the region. Based on instrumental data concerning strain processes occurring within the construction site, an evaluation is given of recent movements in the areas adjoining the lonakhsh fault and the dislocations in its feather jointing.
The development of large industrial and hydrotechnical constructions in seismically active regions requires that much attention be given to the problems of recent crustal movements. In addition to traditional geological and geophysical methods of observation of crustal movements in areas where hydro-electric and other plants are under construction, certain instrumental methods have been widely used during the last decades. Such methods of conducting continous and highly accurate recordings allow of controling the stress-strain state of the rocks within the contours of the junction between the dam itself and the basement and the edges of the canyon. (Starkov, 1975; Starkov et. al., 1976a, 1976b, 1979a, 1979b, 1982). Beginning in 1972, instrumental tilt-strain measurements have been conducted at the Rogoun dam site. Information about block structure and quality and, if possible, quantitative estimations of separate tectonic block 0264-3707/88/$3.00
© 1988 Pergamon Press pie.
244
STARKOV. SOBOLEVA AND STARKOVA
movements have been obtained. The strain characteristics of deeper layers of the earth's crust became known from the regularities of fault-plane solutions. Earlier, in conducting detailed observations at the Dushanbe geodynamic test site, we compared the signs of the horizontal components of surface strain rates, obtained by means of quartz strain meters and crustal strain rates calculated from data on earthquake magnitudes and fault-plane solutions. The signs appeared to coincide in pratically all cases. Thus, we came to the conclusion that the strain in the surface layer of the earth's crust corresponds to that of deeper layers. This allowed us to use seismological data as well as data of continous crustal tilt-strain measurements conducted at the Rogoun test site, in view of the fact that all the data reflect the phenomena of the same tectonic process but all different scales.
PECULIARITIES OF THE GEOLOGICAL STRUCTURE AND TECTONIC DEVELOPMENT
The Rogoun site is situated in a quite complicated zone of contact between two different tectonic regions - - Gissar-Alay and the Tadjik Depression - - divided by the narrow zone of the Pregissar Depression. The region is crossed by the Illiak-Vakhsh and Gissar-Kokshal, the deep faults which have some controlling influence on the regional tectonics and essentially determine its seismicity. Multiple faults of higher orders are confined to the south and the south-east sides of the faults mentioned. The dominant mode of the faults is thrusting (Babaev, 1975). Such an inhomogeneous regional structure may be evidence of the fact that strain features in the earth's crust are quite complicated and different at various scales.
SEISMOTECTONIC DEFORMATION
The concepts of rock strain during earthquakes (so-called seismotectonic deformation (STD)) were developed in works by Kostrov (1974, 1975); Riznitchenko (1965, 1977); and Yunga (1979). The detailed method of calculating the STD parameter has been described in various publications (e.g. Riznitchenko, 1977; Yunga, 1979; Soboleva et. al., 1981; Riznitchenko et. al., 1982). In amounts to a summing up of seismic moment tensors and calculation of principal values for the summary tensor and its invariants, orientation of the maximum shear planes and other parameters, using well-known formulae from the theory of rock mechanics (Filin, 1975). To study the seismotectonic deformations within the Rogoun site, some
CRUSTAL STRAIN IN THE ROGOUN DAM AREA
245
28 years of observations (1955-1982) were used. More than 350 earthquakes with K = 9-13 were analysed. It is natural to consider the earthquakes with K = 13 to have had the most influence on the seismotectonic deformation of the earth's crust in the region studied, since they were the strongest during the period considered. Using the data concerning source mechanisms for such earthquakes, the STD parameters for the seismoactive layer of the earth's crust within the socalled Dushanbe-Vakhsh region, including the Rogoun site, were calculated (Soboleva, 1986). They appeared to be constant in space and time, and thus the tensor obtained can be considered as a regional one. These deformations were almost all of shear type (the ratio of the maximum compression to the maximum extension was close to one), the orientation of the maximum shortening was subhorizontal, in the azimuth 322 °, and the maximum lengthening is inclined at an angle of 57 ° to the horizontal in the azimuth 66 °. Maximum shear planes extent SW-NE and in sublatitudinal direction. In other words, the totality of displacements in earthquake sources leads to a reduction of the horizontal thickness of the seismoactive layer in the earth's crust in the direction SE-NW and to a nearly equivalent thickening with some negnigible diffluence towards SW-NE. Let us analyse the spacial structure of the STD fields at different depths within the Rogoun site and thereby reveal the areas of maximum distortion in the regional field of deformation. For this purpose, STD parameters have been calculated in some elementary volumes, selected in the following way. The total seismoactive (25 km thick) layer was divided into two levels, 0-9 km and 10-25 kin, in correspondence with the average depth of the sedimentary layer (Kulagin, 1968), and into cells 15'x15' in size throughout the area. At depths of more than 25 km only single shocks had occured and these were not taken into account. Each cell includes from 10 to 130 epicenters. Using the data on the orientation of the principal axis in each cell, the trajectories of shortening and lengthening were plotted (Fig. 1). The figure shows that the seismoactive layer consists of differently deformed volumes. The most complicated structure of deformation is represented in the upper level. Here, the trajectories of principal deformations coincide with the regional tensor orientation only in small parts situated, primarily, south of the Gissar-Kokshal fault line. In the middle of the site, there is a block where the maximum shortening has an anomalous north-east direction. The main part of the anomalously deformed area coincides with the Logour and Novobad blocks of Karategin, but the area continues southward, covering the zone where the Ionakhsh and Gulizindan faults are attached to the Gissar-Kokshal fault. The Rogoun dam site is situated at the border of this area. From Figure 1 it is seen that this is the most complicated area, where differently deformed volumes of the earth's
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CRUSTAL STRAIN IN THE ROGOUN DAM AREA
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crust adjoin, and is the reason why some contrasting heterodromous displacements can be expected along the geological dislocations crossing the area. Despite the fact that the geological faults only serve as the borders between homogeneously deformed volumes in some separate parts of the area, their role in the formation of deformation fields is rather important. The figure shows that approaching the geological fault lines, or crossing them, the principal deformation trajectories change their directions. At depths of more than 10 km, the STD field is somewhat different, more simple. The entire area of the site is divided essentially into three parts. In the western part, the regional deformations are very clear, in the eastern part the maximum shortenings are vertically oriented. Between them a submeridional zone passes where the direction of maximum shortening changes from the regional north-west direction towards the anomalous north-eastern one. This zone is a result of the depth of the anomalous deformed area beginning in the upper level of the earth's crust. Though the borders between those three differently deformed zones are clearly manifested, they do not correspond to any geological faults but rather they traverse the surface lines. It is not to be excluded that the picture presented reflects the difference between the structures of the sedimentary layers and of the cristalline basement. The strain rate in the seismoactive layer variable within a range of several orders of magnitude, but never exceeds the value of 0.5 x 10 -s year-i. The sign of its vertical component is the same (except in some cases) at different depths, and this is indicative of the total uplift of the earth's surface.
CHARACTERISTICS OF RECENT STRAIN PROCESSES OCCURRING AT THE ROGOUN DAM SITE ACCORDING TO THE DATA OF TILT-STRAIN MEASUREMENTS
From 1972 to 1982 measurements were made which allowed us to estimate the tilt rate and linear deformations of the earth's surface within the zone of the Ionakhsh fault and the feathering of local ruptures on it. The most important of them is the fault N35, situated in the immediate vicinity to the plant's main structures. The scheme of the geology is given in Figure 2a. Surface tilt was measured at nine points on the construction site. To obtain the most objective information about the spacial structure of deformations occurring within comparatively short time intervals, measurements should be made at all the points simultaneously, but this requirement was not always met. The first measurements of tilting of the earth's surface were made at the points TI and Tg installed in the tunnel 1001. The disposition
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CRUSTAL STRAIN IN THE ROGOUN DAM AREA
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and equipment of observational points, as well as the section along the tunnel axis, are given in figure 2b. The figure shows that tilting of the earth's surface was measured on the outer sides of the lonakhsh fault by means of pendulum tiltmeters. Every point was equipped with two cardinally oriented tiltmeters. The measurements were conducted during the years 1972-1975. At points P~3 and P~5 in the tunnel 1001a, tilt measurements were repeated in the zone of the Ionakhsh fault. At the point P~5 the total surface tilt for the four-year cycle of observations was 9 x 10 -4 radians and this appeared to be of the same order as the values obtained earlier in the tunnel 1001 at the points P~ and Ps. At the point Pt3, the value of the surface tilt was 3 x 10 -5 radians per year. Such a small value can apparently be explained by the fact that the observation point appeared to be within a "lens" limited by two sutures of the Ionakhsh fault. Tectonic activity of the fault N35 has been observed at points Pg, Ptl, P=6 and Pt8. As is seen in the general scheme of the geology (fig. 3), the point P9 is situated directly in the zone of the fault, where the resultant vector of surface tilt for 2 years was 17 x 10 -5 radians. The point P~t situated in the eastern wing of fault N35, is chosen as a base point for long-term observations. An observational dissection was excavated in the tunnel in quite strong, well-preserved sandstones without visible tectoni-
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cal cracks. Observations were carried out at this point during seven years. Measurements on the left side were made at two points, Pt6 and P~8. They are situated on the southern and the northern sides of one of the sutures in fault N35. Measurements of the inner "monolithic" block were made at the point Pt2. Based on the distribution of the values of the surface tilt vectors and their spacial and time directions (Fig. 3), we can present the following picture of the deformation processes occurring at the Rogoun dam site. The maximum values of surface tilt were observed in the active zone of the Ionakhsh fault and on the southern slope of the fault N35. On the basis of geodetic, tilt and direct measurements of stress in naturally bedding rocks, conducted on the left slope of the dam, Kolichko et. al. (1982), came to the conclusion that the block situated between the Ionakhsh fault and the fault N35 is monolithic. However, these data of tilt measurements show that only a part of the block situated to the north of a conditional line passing between the measuring points Pt6 and P,8, situated on the left side, and through the points P~2 and Ps, on the right side, is monolithic. Based on analysis of the velocities of surface tilt obtained at the points Pg, P ~ , Pt5 and Pt6, the Ionakhsh and Gulizindan faults should be considered as the boundaries of the lower (southern) block. If the transverse size of the "monolithic" block, squeezed between the Ionakhsh fault and the fault N35, is 500-1000 m, the annual velocities of the surface tilts obtained at the points P~2 and P,8 are equal to 3 x l0 -5 radians, and then we can calculate the velocity for vertical displacements of the "monolithic" block slopes. h=L.~, where h - is the vertical displacement per year, L - is the transverse size of the block, and ~P- is the resultant tilt per year. The calculated h is equal to 14-28 ram/year. The results obtained simultaneously at the points PI~ and PIs, separated by 500 m, should be specially mentioned. At these points, a simultaneous change of sign of motion is observed. But the fact that the velocities and tilt directions are the same at the two points does not allow us to consider the points to be within the same block, since the measured values of relative vertical displacements on the block slopes contradict the calculated data. In the example given, the calculated relative displacement of the Ionakhsh fault slopes is 82 mm/year, if the distance between the points P~ and PJ5 is 500m and the annual tilt velocities are 16x 10 -5 radian. Relative displacements of the slopes of the Ionakhsh fault and fault N35 are 1-4 mm/year (the values were obtained from direct geodetic measurements and using hydrostatic levels). These results disprove the concept of monolithic blocks. To obtain conformity between too high values of
CRUSTAL STRAIN IN THE R O G O U N DAM AREA
251
surface-tilt velocities and too low (a few ram/year) values of relative vertical displacements, the linear demensions of blocks should be taken as not more than a few tens of meters. In other words, the media, the southern block especially, should be assumed to be a small block structure. In the condition of horizontal compression, small blocks may be smoothed forming a certain fan shape on a free surface (Kolichko, 1982).
STRAIN MEASUREMENTS
Linear deformation of the earth's surface has been measured by means of strain meters at the point P~7 installed in the tunnel 1030 (see Fig. 3). This installation consists of two basal foundations fixed to the opposite slopes of one of the sutures in fault N35. The measurements were made by two parallel instruments (Starkov et. al., 1981). The instrument base (the distance between the basal foundations) was 10 m long. The installation's sensitivities to strain were 5.03 x 10-9 and 5.02 x 10-9 relative units per 1 m of a record. During the two-year period of observations, monotonous changes in strain were recorded with the constant annual rate, which was determined as an average value of the date from the two instruments. The value of the annual strain rate was 2.75 x 10-6 per 1 year. The mode of the observed strain corresponds to extension.
STUDY O F T H R E E - D I M E N S I O N A L STRAIN
Measurements of strain within the slopes of tectonic fractures in the block situated between the two ruptures of the Ionakhsh fault were made at the points P3, P4, P5 and P6 installed in the tunnel 1001. The measurements TABLE 1 Displacements along the cracks in the tectonic lens fault side displacements, mm NN observation points
along the axis of instrument base, horizontal
across the axis of instrument base, horizontal
across the axis of instrument base, vertical
3 4 5 6
- 0.012 - 0.260 -0.182 0.152
- 0.217 - 0.140 -0.170 - 0.280
0.078 0.330 0.160 0.120
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STARKOV. SOBOLEVA AND STARKOVA
were made by means of three-component strain meters set on a 1 m long base (Starkov, 1981); recording was not continuous. The displacement sensitivity of the measuring systems is 1-5 km. One of the main features of the strainmeters used is their insensitivity to tilts of the plate on which they are installed, so that they can only record shear deformations. Table I gives values of displacement of the sides of the tectonic fracture, recorded for 1.8 year (in 1973-1974). As is seen from the table, the sandstone block squeezed between the two ruptures of the Ionakhsh fault, is not a consolidated body, but it is broken by tectonic fractures into separate mobile blocks.
VERTICAL RELATIVE DISPLACEMENT OF FAULT SIDES
To measure relative vertical displacements of fault sides, hydrostatic levels are used. The levels are fixed on the opposite sides of the fault. Two observational points P2 and P7 (Fig. 2b) were installed in tunnel 1001 on the two ruptures of the Ionakhsh fault to measure its vertical displacements. The base under the installation at the point P2 is 6 m long, and 10.3 m long at point P7. The observations were conducted in 1973-1975. The value of relative displacement for the whole observational period was taken as an average from two parallel instruments. The relative displacement at the point P2 was 0.248+__0.1 mm per 1.7 years, and the average annual value was 0.146 __+0.065 mm/year: at point P7 it was 0 . 3 3 7 + 0 . 0 3 7 m m and 0.135 ___0.015 mm/year. Let us compare the results obtained from NSO strain meters and from hydrostatic levels at the points P,-P2 and P7-Ps. For this we express the relative displacement at the points P2 and P7 by the angle of tilt along the axis of the hydrostatic levels, and calculate the projections of annual tilt values at the point P, and Ps upon the direction of the level axis. At P2 the annual tilt was 24.2 × 10 ~6 rad/year, at P7 it was 13 x 10 -6 rad/year. The projection of the annual value for the tilt at P, was 364 x 10-6 rad/year and at Px it was 98 × 10-6 rad/year. As the comparison shows, the values of tilt on different sides of the fault do not coincide with the results given by the instruments with their axis crossing the fault. Probably, apart from the whole plate tilt, there is some relative displacement of the fault sides which may be reason for this "disagreement". Having the data of some simultaneous observations and assuming the motions to be independent, we can calculate the relative displacements of fault sides: h = ( ~T -+- tlVu) • L
when tilt directions are different
h = ( ~ T - - tFtL) • L
when tilt directions coincide
CRUSTAL STRAIN IN THE ROGOUN DAM AREA
253
where h is the relative displacement of fault sides; ~T is tilt, directed to the axis of the hydrostatic levels, obtained from pendulum strain meters; 9' L is tilt, obtained from hydrostatic levels; L is the base of the hydrostatic level; Thus, relative displacements for the Ionakhsh ruptures at P: and P7 and for one of the ruptures in fault N35 were calculated (Fig. 3). These were: - 2 mm/year at P2, - 1.15 mm/year at PT, - 1.1 mm/year at Pro. CONCLUSIONS
- The Rogoun dam site is situated in a zone of contact of variously deformed parts of the earth's crust; for this reason, some contrast antidromous displacements along the faults may be observed. The vertical component of deformation indicates some general uplift of the earth's crust. According to the results of instrumental observations, the dam site may be conditionally divided into two blocks with different rates of inner displacements. - T h e first block is squeezed between the Ionakhsh fault and fault N35 and is limited by a conditional line, crossing the observational points Pt2-Ps on the right bank and the points Pt6-Pl8 on the left bank of the Vakhsh river. The block has a low rate of inner displacement (a few angle seconds per year). - T h e second block is situated lower down the Vakhsh river. It is limited along the sublatitudinal direction by the Ionakhsh fault on one side and by the Gulizidan fault on the other. Inner strain rates were some tens of angle seconds per year. - T h e two blocks have extremely high rates of crustal tilts along their boundaries: about 110 angle seconds per year. Relative vertical displacements of the sides of the Ionakhsh fault and the fault N35 are 1-4 mm/year. This value is not correlated with such high values of tilts. This disagreement may be explained by an inhomogeneous structure of the blocks, the second block structure being more differentiated. and signs of deformations occurring within the site considered are changeable both in space and in time. -
-
- V a l u e s
REFERENCES Bavaev A. M. 1975: Recent tectonogenesis in the zone of conjunction of the Gissar-Alley and the Tadjik Depression. Dushanb¢: Donish, 150 p.
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Filin P. P. Applied mechanics of a solid deformed body. V. I., M. Nauka, 832 p. Gubin I. E. 1960: Regularities of seismicity in the territory of Tadjikistan, M. USSR AS Publishers, 463 p. Kolichko A. V., et al., 1982: Effect of geological structure and stress conditions of rock mass in Designing Underground Power Houses. JSRM Symposium (Aachen} 1982. Kostrov B. V. 1974: Seismic moment, earthquake magnitude and seismic flow of rock mass. USSR AS lzvestia, Earth Physics, NI, p. 23-41. Kostrov B. V. 1975: Source mechanics of tectonic earthquakes. M.: Nauka, 175 p. Kulagin V. K. 1968: The structure of the earth's crust in the central part of the Tadjik Depression and the southern slope of the Gissar ridge. In the book: Deep structure and earthquakes of Tadjikistan. Dushanbe: Donish, p. 5-47. Kuchay V. K. 1981: Zonal orogenesis and seismicity. M. Nauka, 162 p. Riznichenko Yu. V. 1965: On seismic flow of rock mass. In the book: Dynamics of the earth's crust. M. Nauka, p. 56-63. Riznichenko Yu. V. 1977: Calculation of strain rate for rock mass seismic flow. USSR AS Izvestia, Earth Physics, NI0, p. 3 4 - 4 7 . Riznichenko Yu. V., Soboleva O. V., Kuchay O. A., Mikhailova R. S. and Vasilieva O, N. 1982: Seismotectonic deformation of the earth's crust in the south of Central Asia. USSR AS [zvestia, Earth Physics. NI0, p. 90-105. Starkov V. I. 1975: Earth surface tilt changes. In the coll.: Induced seismicity within the Nurek dam site. Dushanbe: Donish. Starkov V. [. and Karmaleeva E. M. 1976a: On displacements and deformations of the earth's crust within the Nurek dam site. In the col.: Recent movements of the earth's crust. Novosibirsk. Starkov V. 1., Starkova E. Ya, and Kolichko A. V. 1976b. The results of tilt observations within the Nurek dam site. In the col.: Recent movements of the earth crust. Novosibirsk, Starkov V. I., Soboleva O. V. and Starkova E. Ya. 1981: Strain rate of the earth crust within the Dushanbe geodynamic test site. Petropav[ovsk-Kamchatskiy, p. 16. Starkov V. l. and Starkova E. Ya. 1982: Earth crust deformation in the Nurek reservoir area. VINITY N5431-82 dep. 3.XI.82. Starkov V. 1., Starkova E. Ya. and Fcel V. N. 1979a: Measurements of tectonic deformation in the Rogoun dam area by means of tiltmeters. In the co[.: Recent movements of the earth's crust. Stepanov. V. Ya. et al. 1979b: Stress state of rocks in the Rogoun dam site. in the col,: Stress-strain state and stability of rock slopes and quarry boris. Frunze, llim. Soboleva O, V. 1975: Earthquake source mechanism and seismotectonic deformation of the earth's crust in the Dushanbe-Vakhsh site. In the book: Geology and geophysics of Tadjikistan. N2, Dushanbe: Donish, 1986. Soboleva O, V., Bibarsova D. G., Vakhidova Z. M. 1981: Calculation of seismotectonic deformation parameters. Dep. VINITY N5402-81, 34p. Yunga S. L, 1979: On the deformation mechanism for seismoactive layer of the earth's crust. USSR AS lzvestia, Earth Physics, NI0, p. 14-23.
243
CRUSTAL STRAIN IN THE ROGOUN DAM AREA
V. I. STARKOV, O. V. SOBOLEVA and E. YA. STARKOVA.
Institute of the Earth's Crust, Moscow, U.S.S.R. (Accepted August 5, 1988)
ABSTRACT Starker V. I., Soboleva O. V. and Starkova E. Ya., 1988. Crustal strain in the Rogoun Dam area. In: Yu. D, Boulanger, S. Holdahl and P. Vysko~il (Editors), Recent Crustal Movements. Journal of Geodynamics, 10: 243-254. In recent years much attention has been paid to the study of crustal movements within tectonically active regions. The present work is specially of interest since it deals with the site where one of the largest hydroelectric plants in the world is being built. The erection of the Rogoun HPP on the Vakhsh river involves the filling of a 320 m high dam using local materials and the construction of an undergroundhead hydroelectric unit with substations and auxiliary services. The work gives an evaluation of features and rates of seismotectonic deformations in two layers of the earth's crust (0.9 km and 10-25 kin), obtained for a period of 28 years, within an area of 3 thousand km z around the HPP. These data allowed us to determine the complex block structure of the region. Based on instrumental data concerning strain processes occurring within the construction site, an evaluation is given of recent movements in the areas adjoining the lonakhsh fault and the dislocations in its feather jointing.
The development of large industrial and hydrotechnical constructions in seismically active regions requires that much attention be given to the problems of recent crustal movements. In addition to traditional geological and geophysical methods of observation of crustal movements in areas where hydro-electric and other plants are under construction, certain instrumental methods have been widely used during the last decades. Such methods of conducting continous and highly accurate recordings allow of controling the stress-strain state of the rocks within the contours of the junction between the dam itself and the basement and the edges of the canyon. (Starkov, 1975; Starkov et. al., 1976a, 1976b, 1979a, 1979b, 1982). Beginning in 1972, instrumental tilt-strain measurements have been conducted at the Rogoun dam site. Information about block structure and quality and, if possible, quantitative estimations of separate tectonic block 0264-3707/88/$3.00
© 1988 Pergamon Press pie.
244
STARKOV. SOBOLEVA AND STARKOVA
movements have been obtained. The strain characteristics of deeper layers of the earth's crust became known from the regularities of fault-plane solutions. Earlier, in conducting detailed observations at the Dushanbe geodynamic test site, we compared the signs of the horizontal components of surface strain rates, obtained by means of quartz strain meters and crustal strain rates calculated from data on earthquake magnitudes and fault-plane solutions. The signs appeared to coincide in pratically all cases. Thus, we came to the conclusion that the strain in the surface layer of the earth's crust corresponds to that of deeper layers. This allowed us to use seismological data as well as data of continous crustal tilt-strain measurements conducted at the Rogoun test site, in view of the fact that all the data reflect the phenomena of the same tectonic process but all different scales.
PECULIARITIES OF THE GEOLOGICAL STRUCTURE AND TECTONIC DEVELOPMENT
The Rogoun site is situated in a quite complicated zone of contact between two different tectonic regions - - Gissar-Alay and the Tadjik Depression - - divided by the narrow zone of the Pregissar Depression. The region is crossed by the Illiak-Vakhsh and Gissar-Kokshal, the deep faults which have some controlling influence on the regional tectonics and essentially determine its seismicity. Multiple faults of higher orders are confined to the south and the south-east sides of the faults mentioned. The dominant mode of the faults is thrusting (Babaev, 1975). Such an inhomogeneous regional structure may be evidence of the fact that strain features in the earth's crust are quite complicated and different at various scales.
SEISMOTECTONIC DEFORMATION
The concepts of rock strain during earthquakes (so-called seismotectonic deformation (STD)) were developed in works by Kostrov (1974, 1975); Riznitchenko (1965, 1977); and Yunga (1979). The detailed method of calculating the STD parameter has been described in various publications (e.g. Riznitchenko, 1977; Yunga, 1979; Soboleva et. al., 1981; Riznitchenko et. al., 1982). In amounts to a summing up of seismic moment tensors and calculation of principal values for the summary tensor and its invariants, orientation of the maximum shear planes and other parameters, using well-known formulae from the theory of rock mechanics (Filin, 1975). To study the seismotectonic deformations within the Rogoun site, some
CRUSTAL STRAIN IN THE ROGOUN DAM AREA
245
28 years of observations (1955-1982) were used. More than 350 earthquakes with K = 9-13 were analysed. It is natural to consider the earthquakes with K = 13 to have had the most influence on the seismotectonic deformation of the earth's crust in the region studied, since they were the strongest during the period considered. Using the data concerning source mechanisms for such earthquakes, the STD parameters for the seismoactive layer of the earth's crust within the socalled Dushanbe-Vakhsh region, including the Rogoun site, were calculated (Soboleva, 1986). They appeared to be constant in space and time, and thus the tensor obtained can be considered as a regional one. These deformations were almost all of shear type (the ratio of the maximum compression to the maximum extension was close to one), the orientation of the maximum shortening was subhorizontal, in the azimuth 322 °, and the maximum lengthening is inclined at an angle of 57 ° to the horizontal in the azimuth 66 °. Maximum shear planes extent SW-NE and in sublatitudinal direction. In other words, the totality of displacements in earthquake sources leads to a reduction of the horizontal thickness of the seismoactive layer in the earth's crust in the direction SE-NW and to a nearly equivalent thickening with some negnigible diffluence towards SW-NE. Let us analyse the spacial structure of the STD fields at different depths within the Rogoun site and thereby reveal the areas of maximum distortion in the regional field of deformation. For this purpose, STD parameters have been calculated in some elementary volumes, selected in the following way. The total seismoactive (25 km thick) layer was divided into two levels, 0-9 km and 10-25 kin, in correspondence with the average depth of the sedimentary layer (Kulagin, 1968), and into cells 15'x15' in size throughout the area. At depths of more than 25 km only single shocks had occured and these were not taken into account. Each cell includes from 10 to 130 epicenters. Using the data on the orientation of the principal axis in each cell, the trajectories of shortening and lengthening were plotted (Fig. 1). The figure shows that the seismoactive layer consists of differently deformed volumes. The most complicated structure of deformation is represented in the upper level. Here, the trajectories of principal deformations coincide with the regional tensor orientation only in small parts situated, primarily, south of the Gissar-Kokshal fault line. In the middle of the site, there is a block where the maximum shortening has an anomalous north-east direction. The main part of the anomalously deformed area coincides with the Logour and Novobad blocks of Karategin, but the area continues southward, covering the zone where the Ionakhsh and Gulizindan faults are attached to the Gissar-Kokshal fault. The Rogoun dam site is situated at the border of this area. From Figure 1 it is seen that this is the most complicated area, where differently deformed volumes of the earth's
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CRUSTAL STRAIN IN THE ROGOUN DAM AREA
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crust adjoin, and is the reason why some contrasting heterodromous displacements can be expected along the geological dislocations crossing the area. Despite the fact that the geological faults only serve as the borders between homogeneously deformed volumes in some separate parts of the area, their role in the formation of deformation fields is rather important. The figure shows that approaching the geological fault lines, or crossing them, the principal deformation trajectories change their directions. At depths of more than 10 km, the STD field is somewhat different, more simple. The entire area of the site is divided essentially into three parts. In the western part, the regional deformations are very clear, in the eastern part the maximum shortenings are vertically oriented. Between them a submeridional zone passes where the direction of maximum shortening changes from the regional north-west direction towards the anomalous north-eastern one. This zone is a result of the depth of the anomalous deformed area beginning in the upper level of the earth's crust. Though the borders between those three differently deformed zones are clearly manifested, they do not correspond to any geological faults but rather they traverse the surface lines. It is not to be excluded that the picture presented reflects the difference between the structures of the sedimentary layers and of the cristalline basement. The strain rate in the seismoactive layer variable within a range of several orders of magnitude, but never exceeds the value of 0.5 x 10 -s year-i. The sign of its vertical component is the same (except in some cases) at different depths, and this is indicative of the total uplift of the earth's surface.
CHARACTERISTICS OF RECENT STRAIN PROCESSES OCCURRING AT THE ROGOUN DAM SITE ACCORDING TO THE DATA OF TILT-STRAIN MEASUREMENTS
From 1972 to 1982 measurements were made which allowed us to estimate the tilt rate and linear deformations of the earth's surface within the zone of the Ionakhsh fault and the feathering of local ruptures on it. The most important of them is the fault N35, situated in the immediate vicinity to the plant's main structures. The scheme of the geology is given in Figure 2a. Surface tilt was measured at nine points on the construction site. To obtain the most objective information about the spacial structure of deformations occurring within comparatively short time intervals, measurements should be made at all the points simultaneously, but this requirement was not always met. The first measurements of tilting of the earth's surface were made at the points TI and Tg installed in the tunnel 1001. The disposition
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CRUSTAL STRAIN IN THE ROGOUN DAM AREA
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and equipment of observational points, as well as the section along the tunnel axis, are given in figure 2b. The figure shows that tilting of the earth's surface was measured on the outer sides of the lonakhsh fault by means of pendulum tiltmeters. Every point was equipped with two cardinally oriented tiltmeters. The measurements were conducted during the years 1972-1975. At points P~3 and P~5 in the tunnel 1001a, tilt measurements were repeated in the zone of the Ionakhsh fault. At the point P~5 the total surface tilt for the four-year cycle of observations was 9 x 10 -4 radians and this appeared to be of the same order as the values obtained earlier in the tunnel 1001 at the points P~ and Ps. At the point Pt3, the value of the surface tilt was 3 x 10 -5 radians per year. Such a small value can apparently be explained by the fact that the observation point appeared to be within a "lens" limited by two sutures of the Ionakhsh fault. Tectonic activity of the fault N35 has been observed at points Pg, Ptl, P=6 and Pt8. As is seen in the general scheme of the geology (fig. 3), the point P9 is situated directly in the zone of the fault, where the resultant vector of surface tilt for 2 years was 17 x 10 -5 radians. The point P~t situated in the eastern wing of fault N35, is chosen as a base point for long-term observations. An observational dissection was excavated in the tunnel in quite strong, well-preserved sandstones without visible tectoni-
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250
STARKOV. SOBOLEVA AND STARKOVA
cal cracks. Observations were carried out at this point during seven years. Measurements on the left side were made at two points, Pt6 and P~8. They are situated on the southern and the northern sides of one of the sutures in fault N35. Measurements of the inner "monolithic" block were made at the point Pt2. Based on the distribution of the values of the surface tilt vectors and their spacial and time directions (Fig. 3), we can present the following picture of the deformation processes occurring at the Rogoun dam site. The maximum values of surface tilt were observed in the active zone of the Ionakhsh fault and on the southern slope of the fault N35. On the basis of geodetic, tilt and direct measurements of stress in naturally bedding rocks, conducted on the left slope of the dam, Kolichko et. al. (1982), came to the conclusion that the block situated between the Ionakhsh fault and the fault N35 is monolithic. However, these data of tilt measurements show that only a part of the block situated to the north of a conditional line passing between the measuring points Pt6 and P,8, situated on the left side, and through the points P~2 and Ps, on the right side, is monolithic. Based on analysis of the velocities of surface tilt obtained at the points Pg, P ~ , Pt5 and Pt6, the Ionakhsh and Gulizindan faults should be considered as the boundaries of the lower (southern) block. If the transverse size of the "monolithic" block, squeezed between the Ionakhsh fault and the fault N35, is 500-1000 m, the annual velocities of the surface tilts obtained at the points P~2 and P,8 are equal to 3 x l0 -5 radians, and then we can calculate the velocity for vertical displacements of the "monolithic" block slopes. h=L.~, where h - is the vertical displacement per year, L - is the transverse size of the block, and ~P- is the resultant tilt per year. The calculated h is equal to 14-28 ram/year. The results obtained simultaneously at the points PI~ and PIs, separated by 500 m, should be specially mentioned. At these points, a simultaneous change of sign of motion is observed. But the fact that the velocities and tilt directions are the same at the two points does not allow us to consider the points to be within the same block, since the measured values of relative vertical displacements on the block slopes contradict the calculated data. In the example given, the calculated relative displacement of the Ionakhsh fault slopes is 82 mm/year, if the distance between the points P~ and PJ5 is 500m and the annual tilt velocities are 16x 10 -5 radian. Relative displacements of the slopes of the Ionakhsh fault and fault N35 are 1-4 mm/year (the values were obtained from direct geodetic measurements and using hydrostatic levels). These results disprove the concept of monolithic blocks. To obtain conformity between too high values of
CRUSTAL STRAIN IN THE R O G O U N DAM AREA
251
surface-tilt velocities and too low (a few ram/year) values of relative vertical displacements, the linear demensions of blocks should be taken as not more than a few tens of meters. In other words, the media, the southern block especially, should be assumed to be a small block structure. In the condition of horizontal compression, small blocks may be smoothed forming a certain fan shape on a free surface (Kolichko, 1982).
STRAIN MEASUREMENTS
Linear deformation of the earth's surface has been measured by means of strain meters at the point P~7 installed in the tunnel 1030 (see Fig. 3). This installation consists of two basal foundations fixed to the opposite slopes of one of the sutures in fault N35. The measurements were made by two parallel instruments (Starkov et. al., 1981). The instrument base (the distance between the basal foundations) was 10 m long. The installation's sensitivities to strain were 5.03 x 10-9 and 5.02 x 10-9 relative units per 1 m of a record. During the two-year period of observations, monotonous changes in strain were recorded with the constant annual rate, which was determined as an average value of the date from the two instruments. The value of the annual strain rate was 2.75 x 10-6 per 1 year. The mode of the observed strain corresponds to extension.
STUDY O F T H R E E - D I M E N S I O N A L STRAIN
Measurements of strain within the slopes of tectonic fractures in the block situated between the two ruptures of the Ionakhsh fault were made at the points P3, P4, P5 and P6 installed in the tunnel 1001. The measurements TABLE 1 Displacements along the cracks in the tectonic lens fault side displacements, mm NN observation points
along the axis of instrument base, horizontal
across the axis of instrument base, horizontal
across the axis of instrument base, vertical
3 4 5 6
- 0.012 - 0.260 -0.182 0.152
- 0.217 - 0.140 -0.170 - 0.280
0.078 0.330 0.160 0.120
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STARKOV. SOBOLEVA AND STARKOVA
were made by means of three-component strain meters set on a 1 m long base (Starkov, 1981); recording was not continuous. The displacement sensitivity of the measuring systems is 1-5 km. One of the main features of the strainmeters used is their insensitivity to tilts of the plate on which they are installed, so that they can only record shear deformations. Table I gives values of displacement of the sides of the tectonic fracture, recorded for 1.8 year (in 1973-1974). As is seen from the table, the sandstone block squeezed between the two ruptures of the Ionakhsh fault, is not a consolidated body, but it is broken by tectonic fractures into separate mobile blocks.
VERTICAL RELATIVE DISPLACEMENT OF FAULT SIDES
To measure relative vertical displacements of fault sides, hydrostatic levels are used. The levels are fixed on the opposite sides of the fault. Two observational points P2 and P7 (Fig. 2b) were installed in tunnel 1001 on the two ruptures of the Ionakhsh fault to measure its vertical displacements. The base under the installation at the point P2 is 6 m long, and 10.3 m long at point P7. The observations were conducted in 1973-1975. The value of relative displacement for the whole observational period was taken as an average from two parallel instruments. The relative displacement at the point P2 was 0.248+__0.1 mm per 1.7 years, and the average annual value was 0.146 __+0.065 mm/year: at point P7 it was 0 . 3 3 7 + 0 . 0 3 7 m m and 0.135 ___0.015 mm/year. Let us compare the results obtained from NSO strain meters and from hydrostatic levels at the points P,-P2 and P7-Ps. For this we express the relative displacement at the points P2 and P7 by the angle of tilt along the axis of the hydrostatic levels, and calculate the projections of annual tilt values at the point P, and Ps upon the direction of the level axis. At P2 the annual tilt was 24.2 × 10 ~6 rad/year, at P7 it was 13 x 10 -6 rad/year. The projection of the annual value for the tilt at P, was 364 x 10-6 rad/year and at Px it was 98 × 10-6 rad/year. As the comparison shows, the values of tilt on different sides of the fault do not coincide with the results given by the instruments with their axis crossing the fault. Probably, apart from the whole plate tilt, there is some relative displacement of the fault sides which may be reason for this "disagreement". Having the data of some simultaneous observations and assuming the motions to be independent, we can calculate the relative displacements of fault sides: h = ( ~T -+- tlVu) • L
when tilt directions are different
h = ( ~ T - - tFtL) • L
when tilt directions coincide
CRUSTAL STRAIN IN THE ROGOUN DAM AREA
253
where h is the relative displacement of fault sides; ~T is tilt, directed to the axis of the hydrostatic levels, obtained from pendulum strain meters; 9' L is tilt, obtained from hydrostatic levels; L is the base of the hydrostatic level; Thus, relative displacements for the Ionakhsh ruptures at P: and P7 and for one of the ruptures in fault N35 were calculated (Fig. 3). These were: - 2 mm/year at P2, - 1.15 mm/year at PT, - 1.1 mm/year at Pro. CONCLUSIONS
- The Rogoun dam site is situated in a zone of contact of variously deformed parts of the earth's crust; for this reason, some contrast antidromous displacements along the faults may be observed. The vertical component of deformation indicates some general uplift of the earth's crust. According to the results of instrumental observations, the dam site may be conditionally divided into two blocks with different rates of inner displacements. - T h e first block is squeezed between the Ionakhsh fault and fault N35 and is limited by a conditional line, crossing the observational points Pt2-Ps on the right bank and the points Pt6-Pl8 on the left bank of the Vakhsh river. The block has a low rate of inner displacement (a few angle seconds per year). - T h e second block is situated lower down the Vakhsh river. It is limited along the sublatitudinal direction by the Ionakhsh fault on one side and by the Gulizidan fault on the other. Inner strain rates were some tens of angle seconds per year. - T h e two blocks have extremely high rates of crustal tilts along their boundaries: about 110 angle seconds per year. Relative vertical displacements of the sides of the Ionakhsh fault and the fault N35 are 1-4 mm/year. This value is not correlated with such high values of tilts. This disagreement may be explained by an inhomogeneous structure of the blocks, the second block structure being more differentiated. and signs of deformations occurring within the site considered are changeable both in space and in time. -
-
- V a l u e s
REFERENCES Bavaev A. M. 1975: Recent tectonogenesis in the zone of conjunction of the Gissar-Alley and the Tadjik Depression. Dushanb¢: Donish, 150 p.
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Filin P. P. Applied mechanics of a solid deformed body. V. I., M. Nauka, 832 p. Gubin I. E. 1960: Regularities of seismicity in the territory of Tadjikistan, M. USSR AS Publishers, 463 p. Kolichko A. V., et al., 1982: Effect of geological structure and stress conditions of rock mass in Designing Underground Power Houses. JSRM Symposium (Aachen} 1982. Kostrov B. V. 1974: Seismic moment, earthquake magnitude and seismic flow of rock mass. USSR AS lzvestia, Earth Physics, NI, p. 23-41. Kostrov B. V. 1975: Source mechanics of tectonic earthquakes. M.: Nauka, 175 p. Kulagin V. K. 1968: The structure of the earth's crust in the central part of the Tadjik Depression and the southern slope of the Gissar ridge. In the book: Deep structure and earthquakes of Tadjikistan. Dushanbe: Donish, p. 5-47. Kuchay V. K. 1981: Zonal orogenesis and seismicity. M. Nauka, 162 p. Riznichenko Yu. V. 1965: On seismic flow of rock mass. In the book: Dynamics of the earth's crust. M. Nauka, p. 56-63. Riznichenko Yu. V. 1977: Calculation of strain rate for rock mass seismic flow. USSR AS Izvestia, Earth Physics, NI0, p. 3 4 - 4 7 . Riznichenko Yu. V., Soboleva O. V., Kuchay O. A., Mikhailova R. S. and Vasilieva O, N. 1982: Seismotectonic deformation of the earth's crust in the south of Central Asia. USSR AS [zvestia, Earth Physics. NI0, p. 90-105. Starkov V. I. 1975: Earth surface tilt changes. In the coll.: Induced seismicity within the Nurek dam site. Dushanbe: Donish. Starkov V. [. and Karmaleeva E. M. 1976a: On displacements and deformations of the earth's crust within the Nurek dam site. In the col.: Recent movements of the earth's crust. Novosibirsk. Starkov V. 1., Starkova E. Ya, and Kolichko A. V. 1976b. The results of tilt observations within the Nurek dam site. In the col.: Recent movements of the earth crust. Novosibirsk, Starkov V. I., Soboleva O. V. and Starkova E. Ya. 1981: Strain rate of the earth crust within the Dushanbe geodynamic test site. Petropav[ovsk-Kamchatskiy, p. 16. Starkov V. l. and Starkova E. Ya. 1982: Earth crust deformation in the Nurek reservoir area. VINITY N5431-82 dep. 3.XI.82. Starkov V. 1., Starkova E. Ya. and Fcel V. N. 1979a: Measurements of tectonic deformation in the Rogoun dam area by means of tiltmeters. In the co[.: Recent movements of the earth's crust. Stepanov. V. Ya. et al. 1979b: Stress state of rocks in the Rogoun dam site. in the col,: Stress-strain state and stability of rock slopes and quarry boris. Frunze, llim. Soboleva O, V. 1975: Earthquake source mechanism and seismotectonic deformation of the earth's crust in the Dushanbe-Vakhsh site. In the book: Geology and geophysics of Tadjikistan. N2, Dushanbe: Donish, 1986. Soboleva O, V., Bibarsova D. G., Vakhidova Z. M. 1981: Calculation of seismotectonic deformation parameters. Dep. VINITY N5402-81, 34p. Yunga S. L, 1979: On the deformation mechanism for seismoactive layer of the earth's crust. USSR AS lzvestia, Earth Physics, NI0, p. 14-23.