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Recent seismotectonic fault movements in the mountain regions of Middle Asia and their relation with earthquake magnitude
Tectonophysics, 29 (1975) 439-446 0 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
439
RECENT SEISMOTECTONIC FAULT MOVEMENTS IN THE MOUNTAIN REGIONS OF MIDDLE ASIA AND THEIR RELATION WITH EARTHQUAKE MAGNITUDE
A.A. NIKONOV Academy
of Sciences, Institute of Physics of the Earth, Moscow (U.S.S.R.)
(Revised version accepted July 11,1975)
ABSTRACT Nikonov, A.A., 1975. Recent seismotectonic fault movements in the mountain regions of Middle Asia and their relation with earthquake magnitude. In: N. Pavoni and R. Green (Editors), Recent Crustal Movements. Tectonophysics, 29 (l-4): 439-446. For the first time for Middle Asia, the data on twenty destructive earthquakes of 1885-1970 allow of pointing out quantitative correlations of the length of rupture on surface (I.), the magnitude of displacement along the fault (D), width (5) and depth (h) of the originating trenches and ruptures with the magnitude (M) of the earthquakes giving rise to them. In Middle Asia the occurrence of seismotectonic ruptures at the surface during earthquakes with M < 5.5-6 and displacements along the fractures during earthquakes with M < 6.5 are unknown. The maximum known length of a rupture zone (L = 50-200 km) is connected with an earthquake M = 8 (8.7 ?); maximum displacements (D = 5-10 m) are observed in the case of earthquakes with M = 7.5-8. Correlational dependence of L on M in the studied region is similar but not identical to that in other regions.
One important type of recent movement in seismically active areas is that represented by displacements along existing faults or else by new faults produced by earthquakes. These movements, which have their origin in the focal zones of large earthquakes, often come to the earth’s surface in the form of linear seismotectonic dislocations or residual deformation (Bogdanovitch et al., 1914; Richter, 1958; Solonenko et al., 1969). There have been special-purpose detailed geologic-geomorphological investigations of seismotectonic dislocations, carried out immediately after the occurrence of some of the more widely known Middle Asian earthquakes: those of 1887, M = 6.5-7.7 (Mouchketov, 1890); 1911, M = 8 (8.7?) (Bogdanovitch et al., 1914); 1949, M = 7.5-7.7 (Leonov, 1960), and others. Recently the author has studied, from the point of view here discussed, the epicentral zones of several catastrophic earthquakes of the past: the Belovodsk earthquake of 1885 (M = 6.5-7.5), the 1907 Karatag earthquake (M = 8), and the 1943 Faizabad earthquake
I
Ii)
(;U :-I6). We have discovered and investigated fresh seismotectonic dislocations at, the surface in the epicentral zones of these earthquakes. They appear as seismote~toni~ scarps and trenches with traces of movements, like those originating from some of the well-known earthquakes, e.g., Beludjistan, 1892 (see Richter, 1958); North Tyen-Shan, 1911, M = 8 (8.7?) (Bogdanovitch et al., 1914); the Iran earthquakes of 1962 (M = 7---7.5) and of 1968, M = 7.3 (Ambraseys, 1963; Ambraseys and Tchalenko, 1968); also large earthquakes in the Mongol--Baikal seismic belt (Solonenko et al., 1969), and in other seismic regions of the world (see Richter, 1958). Thus, in the epicentral zone of the 1907 Karatag earthquake, M = 8, there is a system of fresh seismodislocations on the southern slope of the Gissar ridge cutting various elements of the relief in a northeast direction, not less than 2.5 km in length, in the shape of en-echelon or interchanging trenches, scarps or cracks, and the seismogravitational forms connected with them. The heights of the scarps and the depth of the trenches are from 1 to 3 m, while the maximum vertical displacements recorded are from 0.5 to 2 m. These seismotectonic dislocations and displacements were not noticed in the field investigations during the winter of 1907 because of heavy snow and destruction of the mountain paths (Bronnikov, 1908). In spit.e of this, however, there is some indirect evidence (freshness of the forms and microforms, including those in the loess deposits, the lowering of slide forms into the bed of the Karatag river, the age of the trees that have since grown in the trenches, and so on) pointing to the dislocations in question being associated with just this 1907 earthquake. We have also discovered a few of the still earlier generations of the Holocene seismotectonic dislocations in this area. Besides the author’s own investigations in the epicentral regions of several large earthquakes, he has critically analysed all the literature available on the known large earthquakes in Middle Asia that occurred between 1885 and 1970. As a result of this analysis, we have succeeded in discovering traces of tectonic fault breaks with predominating dip-slip displacetnents for most of the earthquakes (thirteen out of the twenty earthquakes with 5.5 < fif < 8 (8.7?) or intensity J = VIII-X). 111t,he epicentral zones of 13 out of the 23 earthquakes for this period with 111LS6.5 occurring in the mountain regions of Soviet Middle Asia and the adjoining territory to the east of it (Gelfand et al., 1972), seismotectonic dislocations have been found; in three of the epicentral zones, there are large seismogravitational phenomena which may mask surface displacements; there is no information available for five epicentral zones (Fig. 1); it was only in the case of the 1887 Vernyi earthquake (M = 6.57.5) at the foot of the Zailiyskiy Alatau mountains that there were no surface fractures, although gravitational formations were rather numerous (Mouchketov, 1890). It cannot always be unambiguously inferred from the older descrip-
441
Fig. 1. Morphostructural scheme of Middle Asia showing epicentres of large earthquakes and major seismodislocations. Diagram scale is approx. 1 : 6,600,OOO. I. Elements of the morphostructure (after E.Ya. Rantsman, with the author’s modifications for the southern part). l-3. Longitudinal fault zones: 1 = boundaries of mountain regions and morphostructurat regions, 2 = boundaries of megabloeks, 3 = block boundaries. 4 = fault zones at oblique and right angles (block boundaries). II. Epicentres of large earthquakes with M 2 6.5 or energy class K = 16-18 (after Gelfand et al., 1972). 5:K=18;6:K=17;7:K=16.Thefiguresatthesidegivetheyearoftheearthquake. 8 = the hypothetical seat of the epicentre of the 1832 earthquake, after the author. III. Major seismodislocations and groups of seismodislocations at the surface (according to the data of the author, 0.1. Gushchenko, Sh.Sh. Denikayev, K.V. Kurdyukov, V.K. Kuchay, O.P. Sapov, and others). 9-l 1. Dislocations associated with known earthquakes and situated in their epicentral zones: 9 = seismotectonic dislocations (scarps, trenches, cracks in basement rocks), 10 = in epicentral zones there are known seismogravitational dislocations which may probably mask seismotectonic ones, 11 = the epicentral zones have not been investigated. 12-13. Dislocations caused by the earthquakes of the last thousands and tens of thousands of years: 12 = seismotectonic dislocations and groups of dislocations, 13 = major seismogravitational dislocations which may partly mask seismotectonic dislocations.
tions whether the authors observed primary seismotectonic features or secondary surface fractures produced by the action of seismic waves. We have therefore tended to exclude questionable cases from further treatment. Outside the epicentral zones (the highest isoseismal curves) no seismotectonic dislocations have been found, but only secondary or gravitational fractures at the surface. Most of the surface seismotectonic fractures lie along the major structural elements, such as in the 1911 earthquake, for instance, but there are others that lie at right and oblique angles to them (the 1907,1934, 1949, and 1970 earthquakes). Displacements along the faults, where they could be identified at the surface, are mostly dip-slip in type; however, in some cases strike-slip displacements have been found. Their existence is well established for the 1889,1902 (in Andizhan), and 1946 earthquakes. They may have failed to be noticed because of observational difficulties and also on account of the scanty attention they have received in the past. The data collected on seismotectonic fault dislocations allow us, despite their scarcity, to give some first-approximation quantitative relationships between earthquake magnitude (M) on the one hand and quantities characterizing surface fault breaks on the other: fault break length (I), length of the fault break zone (I,), value of the displacement along the fault breaks (D), depth (h) and width (6) of the trenches and cracks. The Mvalues have been taken for the most part from the book Seismic Zoning of the U.S.S.R. (Medvedev, 1968). The relationships are presented graphically (Fig. 2) by the well-known method due to Tocher (1958) and Chinnery (1969). The magnitude values have been plotted against the maximum values of fault breaks (max) and also against the mean values (m). Here only the graphs of the first type are given. The scatter seen in the graphs cannot be considered as surprising; it may be ascribed both to insufficient accuracy of the basic data, especially in the case of those for the older earthquakes, and to differences in the composition and layering conditions of the underlying rocks, and in the orientation of the fault breaks with respect to the structures and relief elements, and so on. However, there can be no doubt that the general character of the relationships permits obtaining average curves. Tectonic dislocations in basic rocks are perfectly retained during many decades (Kuchai, 1969). So erosion effect should not appreciably distort the dislocation value, even in the case of their measurement many years after the arising earthquakes. Dislocation dimension may greatly depend on source depth. But there is a lack of proper definitions of depth for making comparisons. When considering the length of the arising fault breaks, it has been necessary to distinguish between the values for separate, or elementary, fault breaks (1) and those for whole zones (L) consisting of elementary enechelon fault breaks, or ones following at intervals. Correspondingly, in
Fig. 2. Diagrams showing the relationship between the m~nitudes of major Middle Asian earthquakes (5.5 < M < 8.5) and maximum size of the associated seismotectonic fault breaks and displacements at the surface. The figures are the years of origin of the earthquakes. (a) Relationship between magnitude and the length of separate fault breaks (M vs. 1max) and of fault break zone length (M vs. L max). (b) Relationship between magnitude and the value of displacement along the fault breaks (M vs. D,) (circles stand for dip-slip and squares for strike-slip d~spla~men~). (c) Relationship between magnitude and the depth of cracks and trenches (84 vs. h,,,). (d) Relationship between magnitude and the width of cracks and trenches (M vs. b,.,).
place of the usual single graph M vs. L (Tocher, 1958; Chinnery, 1969), the author has plotted two graphs, M vs. 1 and M vs. L (see Fig. Za). The relationships obtained are :
11 I
;I/I = 7.62 + = 7.86 + *VI= 6.30 + IV?= 6.25 + M
0.83 lg 0.92 lg 0.97 lg 0.90 lg
I,,, E, L,,,, L,,,
(/in km) (I in km) (1, in km) (L in km)
u = 1.6 0 = 1.7 0 = 2.2 (J = 2.4
(1) (2) (3) (4)
When determining the relationship between the value of displacement along the fault breaks and earthquake magnitude (M vs. D), the averaging line has, on account of scarcity of the data, been drawn independently of whether the dominating component was the vertical or the horizontal one (see Fig. 2b). In spite of this, just this M vs. D relationship is the most exact as compared with the other relationships under consideration. The relationship obtained is characterized by the equations: M = 5.43 + 0.90 lg D,,,
(D in cm)
u = 1.1
(5)
M = 5.38 + 0.94 lg D,
(D in cm)
0 = 1.2
(6)
The relationships M vs.h and M vs.b are neither exact nor representative, as can be seen from the considerable scatter of the points in the graphs (Figs. 2c and d). Therefore, the relationships given below can be considered as approximate. M = 6.23 + 1.4 lg h,,, M = 6.16 + 1.67 lg h, M = 6.90 + 0.70 lg b,,,
(h in m) (h in m) (b in m)
e= 1.7 o = 2.5 a = 1.7
(7) (8) (9)
M = 6.97 + 1.13lg
(b in m)
(i = 1.1
(10)
b,
All the equalities are only valid in the range 6 < M < 8.5. The data obtained show that, just as in some other seismically active regions of the world, seismotectonic fault breaks begin to appear at the surface for earthquakes with 5.5 < M < 6, while displacements along these fault breaks appear for earthquakes with M Z 6.5. As might be expected, the magnitude of the fault breaks and displacements along them increases with the corresponding earthquake magnitude. The maximum known fault-zone length in the region (L = 50-200 km) is associated with the 1911 Tien Shan earthquake, largest of those on record, M = 8 (8.7?). The diagrams and formulas obtained for the Middle Asian earthquakes of the magnitude considered are similar to those for the other seismically active regions of the world. There is a common difference when compared, for example, with the Mongol-Baikal region (Solonenko et al., 1969), California and Nevada (Tocher, 1958) namely, for Middle Asia the dislocations and displacements appearing along them are much smaller for the same value of magnitude. It is hard to say with certainty whether this is due to differences in the character of the stress state, or in the properties of the crustal rocks or in
445
earthquake mechanisms. One thing only is fairly certain, that relationships found for separate seismically active regions, or even those found by averaging over different regions of the earth, cannot hold for any other region, in particular for Middle Asia, and vice versa. This proposition gains in importance when we turn to seismotectonic displacements along the faults associated with the Middle Asian earthquakes before 1885 that have not been recorded in written sources. There are numerous traces of such dislocations and displacements of Holocene or, more rarely, of Late Pleistocene age, in the mountain regions of Middle Asia (see Fig. 1). The maximum displacements supposedly caused by large earthquakes of the past do not exceed those associated with the known earthquakes of the last ninety years. ACKNOWLEDGMENTS
The author is grateful to Dr. V.I. Bune, Dr. A.L. Levshin, Institute of Physics of the Earth, Academy of Sciences, U.S.S.R.; to Dr. M. Bath, Seismological Institute, Uppsala, Sweden, who provided useful advice.
REFERENCES Ambraseys, N.N., 1963. The Buyin-Zara (Iran) earthquake of September, 1962. Bull. Seismol. Sot. Am., 53 (4): 705-740. Ambraseys, N.N. and Tchalenko, J.S., 1968. Dashti Biaz, Iran, earthquake of August 1968. Nature, 220 (5170): 1751-1792. Bogdanovitch, Ch., Kark, J., Korolkov, B. and Mouchketov, D., 1914. Tremblement de terre du 22 decembre 1910 (janvier 1911) dans les districts septentrionaux du Tien-Chan. Memoires du Comite gkologique, Nouvelle serie. Livraison 89, St.-Petersburg: 270 pp. (in Russian with abstract in French). Bronnikov, M., 1908. Le tremblement de terre de Karatag. Bull. Corn. Geol., XXVII, (147): 475-515 (in Russian with abstract in French). Chinnery, M.A., 1969. Earthquake magnitude and source parameters. Bull. Seismoi. Sot. Am., 59 (5): 1969-1982. Gelfand, J.M., Guberman, Sh.J., Izvekova, M.L., Keilis-Borok, V.I. and Ranzman, E.Ja., 1972. Criteria of high seismicity determined by pattern recognition. Tectonophysics, 13 (l-4): 415-422. Kuchai, V.K., 1969. Results of reobservation of the rest deformations in the pleistoseismic zone of Kebin earthquake. Geol. Geophys., 8: 101-108 (in Russian). Leonov, N.N., 1960. The 1949 Khait earthquake and the geological conditions of its origin. Izvestia Akad. Nauk SSSR, Ser. Geophys., 3: 409-424 (in Russian). Medvedev, S.V. (Editor), 1968. Seismic Zoning of the U.S.S.R. Nauka, Moscow, 476 pp. (in Russian). Mouchketov, I.V., 1890. Das Erdbeben von Vernyj vom 28 Mai (9 Juni) 1887. Memoires du Comite gkologique, 10 (1): 154 pp., St. Petersburg (in Russian with abstract in German). Richter, C.F., 1958. Elementary Seismology. San Francisco, 768 pp. Solonenko, V.P., Kurushin, R.A., Pavlov, O.V., Hilko, S.D., Khromovskikh, V.S. and
Shmotov, A.P., 1969. Recent catastrophic earth-crust movements in the MongoliaBaikal seismic regions. In: Yu.D. Boulanger and Ju.A. Mescherikov (Editors), Problems of Recent Crustal Movements. 3rd Int. Symp., Leningrad, 1968. Moscow, pp. 377-385 (in Russian with English summary). Tocher, D., 1958. Earthquake energy and breakage. Bull. Seismol. Sot. Am., 48 (2): 147.-153.
439
RECENT SEISMOTECTONIC FAULT MOVEMENTS IN THE MOUNTAIN REGIONS OF MIDDLE ASIA AND THEIR RELATION WITH EARTHQUAKE MAGNITUDE
A.A. NIKONOV Academy
of Sciences, Institute of Physics of the Earth, Moscow (U.S.S.R.)
(Revised version accepted July 11,1975)
ABSTRACT Nikonov, A.A., 1975. Recent seismotectonic fault movements in the mountain regions of Middle Asia and their relation with earthquake magnitude. In: N. Pavoni and R. Green (Editors), Recent Crustal Movements. Tectonophysics, 29 (l-4): 439-446. For the first time for Middle Asia, the data on twenty destructive earthquakes of 1885-1970 allow of pointing out quantitative correlations of the length of rupture on surface (I.), the magnitude of displacement along the fault (D), width (5) and depth (h) of the originating trenches and ruptures with the magnitude (M) of the earthquakes giving rise to them. In Middle Asia the occurrence of seismotectonic ruptures at the surface during earthquakes with M < 5.5-6 and displacements along the fractures during earthquakes with M < 6.5 are unknown. The maximum known length of a rupture zone (L = 50-200 km) is connected with an earthquake M = 8 (8.7 ?); maximum displacements (D = 5-10 m) are observed in the case of earthquakes with M = 7.5-8. Correlational dependence of L on M in the studied region is similar but not identical to that in other regions.
One important type of recent movement in seismically active areas is that represented by displacements along existing faults or else by new faults produced by earthquakes. These movements, which have their origin in the focal zones of large earthquakes, often come to the earth’s surface in the form of linear seismotectonic dislocations or residual deformation (Bogdanovitch et al., 1914; Richter, 1958; Solonenko et al., 1969). There have been special-purpose detailed geologic-geomorphological investigations of seismotectonic dislocations, carried out immediately after the occurrence of some of the more widely known Middle Asian earthquakes: those of 1887, M = 6.5-7.7 (Mouchketov, 1890); 1911, M = 8 (8.7?) (Bogdanovitch et al., 1914); 1949, M = 7.5-7.7 (Leonov, 1960), and others. Recently the author has studied, from the point of view here discussed, the epicentral zones of several catastrophic earthquakes of the past: the Belovodsk earthquake of 1885 (M = 6.5-7.5), the 1907 Karatag earthquake (M = 8), and the 1943 Faizabad earthquake
I
Ii)
(;U :-I6). We have discovered and investigated fresh seismotectonic dislocations at, the surface in the epicentral zones of these earthquakes. They appear as seismote~toni~ scarps and trenches with traces of movements, like those originating from some of the well-known earthquakes, e.g., Beludjistan, 1892 (see Richter, 1958); North Tyen-Shan, 1911, M = 8 (8.7?) (Bogdanovitch et al., 1914); the Iran earthquakes of 1962 (M = 7---7.5) and of 1968, M = 7.3 (Ambraseys, 1963; Ambraseys and Tchalenko, 1968); also large earthquakes in the Mongol--Baikal seismic belt (Solonenko et al., 1969), and in other seismic regions of the world (see Richter, 1958). Thus, in the epicentral zone of the 1907 Karatag earthquake, M = 8, there is a system of fresh seismodislocations on the southern slope of the Gissar ridge cutting various elements of the relief in a northeast direction, not less than 2.5 km in length, in the shape of en-echelon or interchanging trenches, scarps or cracks, and the seismogravitational forms connected with them. The heights of the scarps and the depth of the trenches are from 1 to 3 m, while the maximum vertical displacements recorded are from 0.5 to 2 m. These seismotectonic dislocations and displacements were not noticed in the field investigations during the winter of 1907 because of heavy snow and destruction of the mountain paths (Bronnikov, 1908). In spit.e of this, however, there is some indirect evidence (freshness of the forms and microforms, including those in the loess deposits, the lowering of slide forms into the bed of the Karatag river, the age of the trees that have since grown in the trenches, and so on) pointing to the dislocations in question being associated with just this 1907 earthquake. We have also discovered a few of the still earlier generations of the Holocene seismotectonic dislocations in this area. Besides the author’s own investigations in the epicentral regions of several large earthquakes, he has critically analysed all the literature available on the known large earthquakes in Middle Asia that occurred between 1885 and 1970. As a result of this analysis, we have succeeded in discovering traces of tectonic fault breaks with predominating dip-slip displacetnents for most of the earthquakes (thirteen out of the twenty earthquakes with 5.5 < fif < 8 (8.7?) or intensity J = VIII-X). 111t,he epicentral zones of 13 out of the 23 earthquakes for this period with 111LS6.5 occurring in the mountain regions of Soviet Middle Asia and the adjoining territory to the east of it (Gelfand et al., 1972), seismotectonic dislocations have been found; in three of the epicentral zones, there are large seismogravitational phenomena which may mask surface displacements; there is no information available for five epicentral zones (Fig. 1); it was only in the case of the 1887 Vernyi earthquake (M = 6.57.5) at the foot of the Zailiyskiy Alatau mountains that there were no surface fractures, although gravitational formations were rather numerous (Mouchketov, 1890). It cannot always be unambiguously inferred from the older descrip-
441
Fig. 1. Morphostructural scheme of Middle Asia showing epicentres of large earthquakes and major seismodislocations. Diagram scale is approx. 1 : 6,600,OOO. I. Elements of the morphostructure (after E.Ya. Rantsman, with the author’s modifications for the southern part). l-3. Longitudinal fault zones: 1 = boundaries of mountain regions and morphostructurat regions, 2 = boundaries of megabloeks, 3 = block boundaries. 4 = fault zones at oblique and right angles (block boundaries). II. Epicentres of large earthquakes with M 2 6.5 or energy class K = 16-18 (after Gelfand et al., 1972). 5:K=18;6:K=17;7:K=16.Thefiguresatthesidegivetheyearoftheearthquake. 8 = the hypothetical seat of the epicentre of the 1832 earthquake, after the author. III. Major seismodislocations and groups of seismodislocations at the surface (according to the data of the author, 0.1. Gushchenko, Sh.Sh. Denikayev, K.V. Kurdyukov, V.K. Kuchay, O.P. Sapov, and others). 9-l 1. Dislocations associated with known earthquakes and situated in their epicentral zones: 9 = seismotectonic dislocations (scarps, trenches, cracks in basement rocks), 10 = in epicentral zones there are known seismogravitational dislocations which may probably mask seismotectonic ones, 11 = the epicentral zones have not been investigated. 12-13. Dislocations caused by the earthquakes of the last thousands and tens of thousands of years: 12 = seismotectonic dislocations and groups of dislocations, 13 = major seismogravitational dislocations which may partly mask seismotectonic dislocations.
tions whether the authors observed primary seismotectonic features or secondary surface fractures produced by the action of seismic waves. We have therefore tended to exclude questionable cases from further treatment. Outside the epicentral zones (the highest isoseismal curves) no seismotectonic dislocations have been found, but only secondary or gravitational fractures at the surface. Most of the surface seismotectonic fractures lie along the major structural elements, such as in the 1911 earthquake, for instance, but there are others that lie at right and oblique angles to them (the 1907,1934, 1949, and 1970 earthquakes). Displacements along the faults, where they could be identified at the surface, are mostly dip-slip in type; however, in some cases strike-slip displacements have been found. Their existence is well established for the 1889,1902 (in Andizhan), and 1946 earthquakes. They may have failed to be noticed because of observational difficulties and also on account of the scanty attention they have received in the past. The data collected on seismotectonic fault dislocations allow us, despite their scarcity, to give some first-approximation quantitative relationships between earthquake magnitude (M) on the one hand and quantities characterizing surface fault breaks on the other: fault break length (I), length of the fault break zone (I,), value of the displacement along the fault breaks (D), depth (h) and width (6) of the trenches and cracks. The Mvalues have been taken for the most part from the book Seismic Zoning of the U.S.S.R. (Medvedev, 1968). The relationships are presented graphically (Fig. 2) by the well-known method due to Tocher (1958) and Chinnery (1969). The magnitude values have been plotted against the maximum values of fault breaks (max) and also against the mean values (m). Here only the graphs of the first type are given. The scatter seen in the graphs cannot be considered as surprising; it may be ascribed both to insufficient accuracy of the basic data, especially in the case of those for the older earthquakes, and to differences in the composition and layering conditions of the underlying rocks, and in the orientation of the fault breaks with respect to the structures and relief elements, and so on. However, there can be no doubt that the general character of the relationships permits obtaining average curves. Tectonic dislocations in basic rocks are perfectly retained during many decades (Kuchai, 1969). So erosion effect should not appreciably distort the dislocation value, even in the case of their measurement many years after the arising earthquakes. Dislocation dimension may greatly depend on source depth. But there is a lack of proper definitions of depth for making comparisons. When considering the length of the arising fault breaks, it has been necessary to distinguish between the values for separate, or elementary, fault breaks (1) and those for whole zones (L) consisting of elementary enechelon fault breaks, or ones following at intervals. Correspondingly, in
Fig. 2. Diagrams showing the relationship between the m~nitudes of major Middle Asian earthquakes (5.5 < M < 8.5) and maximum size of the associated seismotectonic fault breaks and displacements at the surface. The figures are the years of origin of the earthquakes. (a) Relationship between magnitude and the length of separate fault breaks (M vs. 1max) and of fault break zone length (M vs. L max). (b) Relationship between magnitude and the value of displacement along the fault breaks (M vs. D,) (circles stand for dip-slip and squares for strike-slip d~spla~men~). (c) Relationship between magnitude and the depth of cracks and trenches (84 vs. h,,,). (d) Relationship between magnitude and the width of cracks and trenches (M vs. b,.,).
place of the usual single graph M vs. L (Tocher, 1958; Chinnery, 1969), the author has plotted two graphs, M vs. 1 and M vs. L (see Fig. Za). The relationships obtained are :
11 I
;I/I = 7.62 + = 7.86 + *VI= 6.30 + IV?= 6.25 + M
0.83 lg 0.92 lg 0.97 lg 0.90 lg
I,,, E, L,,,, L,,,
(/in km) (I in km) (1, in km) (L in km)
u = 1.6 0 = 1.7 0 = 2.2 (J = 2.4
(1) (2) (3) (4)
When determining the relationship between the value of displacement along the fault breaks and earthquake magnitude (M vs. D), the averaging line has, on account of scarcity of the data, been drawn independently of whether the dominating component was the vertical or the horizontal one (see Fig. 2b). In spite of this, just this M vs. D relationship is the most exact as compared with the other relationships under consideration. The relationship obtained is characterized by the equations: M = 5.43 + 0.90 lg D,,,
(D in cm)
u = 1.1
(5)
M = 5.38 + 0.94 lg D,
(D in cm)
0 = 1.2
(6)
The relationships M vs.h and M vs.b are neither exact nor representative, as can be seen from the considerable scatter of the points in the graphs (Figs. 2c and d). Therefore, the relationships given below can be considered as approximate. M = 6.23 + 1.4 lg h,,, M = 6.16 + 1.67 lg h, M = 6.90 + 0.70 lg b,,,
(h in m) (h in m) (b in m)
e= 1.7 o = 2.5 a = 1.7
(7) (8) (9)
M = 6.97 + 1.13lg
(b in m)
(i = 1.1
(10)
b,
All the equalities are only valid in the range 6 < M < 8.5. The data obtained show that, just as in some other seismically active regions of the world, seismotectonic fault breaks begin to appear at the surface for earthquakes with 5.5 < M < 6, while displacements along these fault breaks appear for earthquakes with M Z 6.5. As might be expected, the magnitude of the fault breaks and displacements along them increases with the corresponding earthquake magnitude. The maximum known fault-zone length in the region (L = 50-200 km) is associated with the 1911 Tien Shan earthquake, largest of those on record, M = 8 (8.7?). The diagrams and formulas obtained for the Middle Asian earthquakes of the magnitude considered are similar to those for the other seismically active regions of the world. There is a common difference when compared, for example, with the Mongol-Baikal region (Solonenko et al., 1969), California and Nevada (Tocher, 1958) namely, for Middle Asia the dislocations and displacements appearing along them are much smaller for the same value of magnitude. It is hard to say with certainty whether this is due to differences in the character of the stress state, or in the properties of the crustal rocks or in
445
earthquake mechanisms. One thing only is fairly certain, that relationships found for separate seismically active regions, or even those found by averaging over different regions of the earth, cannot hold for any other region, in particular for Middle Asia, and vice versa. This proposition gains in importance when we turn to seismotectonic displacements along the faults associated with the Middle Asian earthquakes before 1885 that have not been recorded in written sources. There are numerous traces of such dislocations and displacements of Holocene or, more rarely, of Late Pleistocene age, in the mountain regions of Middle Asia (see Fig. 1). The maximum displacements supposedly caused by large earthquakes of the past do not exceed those associated with the known earthquakes of the last ninety years. ACKNOWLEDGMENTS
The author is grateful to Dr. V.I. Bune, Dr. A.L. Levshin, Institute of Physics of the Earth, Academy of Sciences, U.S.S.R.; to Dr. M. Bath, Seismological Institute, Uppsala, Sweden, who provided useful advice.
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