Dynamics of North Anatolian Fault Zone
 

Dynamics of North Anatolian Fault Zone

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    Dynamics of North Anatolian Fault Zone Dynamics of North Anatolian Fault Zone Document Transcript

    • DYNAMICS OF THE NORTH ANATOLIAN FAULT N. Canitez Department of Earth and Planetary Sciences Massachusetts Institute of Technology Cambridge, Massachusetts 02139 (On leave from the Technical University of Istanbul, Turkey) Abstract. Recent progress on the genetic and dynamic proper­ ties of the North Anatolian fault has been reviewed. Detailed geological studies show that the fault started its activity in Miocene times, and had a 5-6 mm/year average slip-rate for the last 15 m.y. The slip-rate for the last 1/2 m.y. is about 7 mm/ year. The recent slip-rate associated with earthquakes is between 1.2 and 2.4 cm/year depending on the vertical extension of the fault. The stress-drop due to earthquakes is not strongly depen­ dent on magnitude for M > 7. Studies of the dependence of the surface area of faulting on the seismic moment of earthquakes give different relationships for large and small earthquakes. Necessary future studies related to the North Anatolian fault are discussed at the end of the paper. INTRODUCTION The North Anatolian fault has attracted wide attention because of a series of large earthquakes that began with the Erzincan earthquake in 1939. The fault is quite similar to the famous active strike-slip faults of the earth such as the San Andreas fault of California and the Alpine fault of New Zealand. The rift topography and characteristic features of Quaternary fault­ ing (sag ponds, offset streams, scarps in alluvium) mark the fault throughout its extent. The number of earthquakes with magnitude equal to or larger than 6 in the fault zone is about 40 since 1900, Only 8 of the major earthquakes have been investigated in detail in the field (Ketin, 1969; Ambraseys, 1970), The average displacement associated with these earthquakes varied from 30 to 430 cm. The fault shows a continuous creep around Gerede (Ismetpasa station). These are almost the only wellknown characteristics of the fault. More information is needed for a better under­ standing of static and dynamic characteristics of the fault. In this paper an attempt has been made to summarize the recent progress on the genetic and dynamic properties of the North Anatolian fault. GEOLOGICAL HISTORY OF THE NORTH ANATOLIAN FAULT The verifiable length of the North Anatolian fault is about 1300 km. The fault shows an interruption in the region of Marma­ ra in northwestern Turkey. In the eastern part, the fault is 50 Special Publications Geodynamics: Progress and Prospects Vol. 5 Copyright American Geophysical Union
    • continuous and the earthquake source mechanisms are consistent­ ly strike-slip. The western part, however, is much more compli­ cated both, in structure and fault mechanism. There are different opinions about the development of the fault. According to Pavoni (1961), the fault ruptured at the beginning of Tertiary time and the total slip since that time is 350-400 km. According to Ketin (1969), however, the fault started its activity after the Miocene or probably during the PIio-Quaternary period. Since that time there has been a total displacement of tens of kilometers across the fault. The North Anatolian fault is the boundary between the Pontid and Anatolid tectonic units. There is almost no lithologic cor­ relation between the two sides of the fault. The Mesozoic and Tertiary development of the eastern Pontids has been investiga­ ted by Tokel (1973). According to him, the Pontid mountains are assumed as massifs which were tectonically active and upheaved during Mesozoic and Tertiary times. The granites and the meta- morphic rocks of the Pontids were affected by the Hercynian orogeny. The region was above sea-level during Dogger and Lower Cretaceous times. The development of the Upper Cretaceous trough in the Pontids, according to Tokel (1973), is attributed to a subduction zone in the North Anatolian Tethys. After the Cretaceous period, the North Anatolian Tethys closed as a result of the northward movements of the African and Arabian plates, and probably a trench developed near the southern boundary of the Pontids. The closing of the North Anatolian Tethys was completed at the Miocene as the two continents moved towards each other. The boundary between the Anatolian and the Pontid plates is the North Anatolian fault. A part of the North Anatolian fault (Resadiye region) has been investigated by Seymen (1974). According to him, the Pontid unit does not contain the Alpine ofiolites and epimetamorphites. On the contrary, the Anatolids are composed of shelf and deep sea deposits accumulated on the pre-Alpine metamorphic basement, and of an allochthonous ofiolite belt and o1istostroms. The emplacement of these ofiolitic rocks bearing the Alpine epi- metamorphites occurred in the subduction zone which was de­ veloped in the northern branch of the Tethys at the beginning of Upper Cretaceous time. According to Seymen (1974) the paroxism of the Alpine mountain building (Pontian-phase) within the Niksar- Resadiye region was caused by the continent/is1and arc collision during the passage from Upper Paleocene to Eocene. This phase of the Alpine movements created the border folds trending in an E-W direction. Seymen is of the opinion that the second episode of the Alpine orogeny together with the calc-alkaline volcanism and plutonism of the eastern Pontids took place since the trench zone in the northern Tethys rejuvenated during Late Lutetian-Early Priabonian time. The last Alpine movements, according to him, occurred due to the continent/continent (i.e. Pont id/Anato1 id) collision in Lower-Middle Miocene times. In this period, the North Anatolian fault zone was also developed, as a result of the Anatolian plate being gradually pushed westwards by the Arabian block, so that this fault cut the regional trends and the boundary between the Pontids and Anatolids with an acute angle. Thus, the right-lateral movements along the fault dis­ placed the boundary of the Anato1id-Pontid in the vicinity of Susehri (Sivas) and Refahiye (Erzincan) by 90 + 5 km. Seymen (.1974) states that the average slip-rate along the North-Anato­ lian fault is about 0.5-0.6 cm/year since Middle Miocene times. 51 Special Publications Geodynamics: Progress and Prospects Vol. 5 Copyright American Geophysical Union
    • SLIP-RATE AND STRESS DROP ALONG THE NORTH ANATOLIAN FAULT The slip-rate of the right-handed motion along the North Anatolian fault has been investigated by several authors (Brune, 1968; Ambraseys, 1970; McKenzie, 1972; Canitez and Ezen, 1973; Seymen, 1974). Brune (1968) found a value of 11 cm/year for the period 1939-1967. As it was also pointed out by Brune himself, this value might be overestimated because this time interval has an episode of large dislocations. The average slip for the whole length of the fault since 1939 was reported to be about 90 cm by Ambraseys (1970). This value corresponds to an average slip-rate of about 3 cm/year. Even though the evidence for a 90 cm average displacement is not reported, a slip-rate of 3 cm/year seems more reasonable. According to McKenzie (1970, 1972), the sense of the motion of the Anatolian and the Aegean plates is almost the same. Thus, in order for a tensional zone to develop in western Turkey, the slip-rate of the Aegean plate has to be higher than that of the Anatolian plate. The deepest point of the sinking slab in the Aegean area is only 200 km deep (Caputo et al., 1970; Galanopou- los, 1972; Papzachos, 1973). One can think about two possibilities: 1) The Aegean lithosphere is assimilated by the mantle, and 2) the structure is quite young. If the first one is the case, and 10 m.y. is enough for the assimilation (Isaacs et al., 1968), the convergence rate in the Aegean area is 2 cm/year. If the second possibility is the case, and furthermore if the Aegean plate started moving as early as the Anatolian fault started, say 15 m.y. ago in the middle of the Miocene (Seymen, 1974), the convergence rate is 1.3 cm/year. McKenzie (1972) suggests a 4 cm/year slip-rate for the Turkish plate. Some recent tectonic events in Turkey have been investigated by Arpat and Saroglu (1975) using aerial photographs and field observations. The East Anatolian fault crossing the North Anatolian fault zone has been mapped. This left-handed fault is connected with the Dead Sea fault system. They also mapped some parts of the North Anatolian fault, and they showed some evidence indicating an average slip-rate greater than 7 mm/ year for the last 1/2 m.y. Another study of the static and dynamic characteristics of the North Anatolian fault has been conducted by Canitez and Ezen (1973). Using the static parameters (fault lengths and average dislocations) observed in the field, they found the relation: log M = 1.64 M + 14.75 for 6 < M < 8 (1) O S s between seismic moment and magnitude from which the total seismic moment for the period 1900 - 1971 has been obtained as 1.77 x l o z o dyne cm. Using this value, and following Brune (1968), they calculated the average slip-rate for the period of interest with different fault-depth assumptions. For instance, they found 2.4 cm/year for W = 20 km, 1.6 cm/year for W = 30 km, and 1.2 cm/year for W = 40 km. High-precision triangulat ion and trilateration measurements were started in 1972 in the western part (Gerede-Cerkes region) of the North Anatolian fault zone. The comparison of the 1946 (after the 1944 earthquake) and 1972 measurements showed a 75 cm horizontal displacement in the eastern end of the 1944 Gerede-Bolu earthquake fault. The total relative displacement 52 Special Publications Geodynamics: Progress and Prospects Vol. 5 Copyright American Geophysical Union
    • in the western portion, however, is 20 cm for the same period (Ugur, 1 974) . Stress drops for some earthquakes in the North Anatolian fault zone have been investigated by Chinnery (1969), Hanks and Wyss (1972) and Canitez and Ezen (1973). The general con­ clusion is that the stress drop due to earthquakes in the area is less than 50 bars. Canitez and Ezen (1973) concluded that the stress drop in the North Anatolian fault zone is between 10 and 15 bars for M > 7 and does not depend on magnitude as Aki (1972) expressed. For M < 7, however, they found the stress drop less than 10 bars. Using the observed fault lengths and relative displacements, and assuming that the vertical extension of the fault is 20 km, Canitez and Ezen (1973) compared the seismic moments with the fault areas. They found different relations for large and small earthquakes: log M = 26.76 + 1.66 x lo~4 S for M > 7 (2) o s and log M n = 20.5 + 7.5 x 1 0 ~ 3 S for M < 7 (3) 0 s 2 where, M is in dyne cm and S is in km . The accuracy in these relations is debatable because of the uncertainties in the vertical extension of the fault. Geological and seismological investigations undertaken so far show that the North Anatolian fault is very similar in character to the San Andreas fault. In summing up, we can say that right- handed motion of the fault started 15 m.y. ago in the middle of Miocene times with a long term slip-rate of 0.5 - 0.7 cm/ year. The average rate of the recent movements associated with earthquakes, however, is about 1-2 cm/year. The stress drop due to earthquakes is not dependent on magnitude for M > 7. NECESSARY FUTURE STUDIES The significance of the North Anatolian fault comes from its position as a very clear plate boundary in the eastern Mediterra­ nean area. Not only studied for recent tectonics, the fault zone is also a very interesting place to investigate the paleo- tectonics of the eastern Mediterranean region. However, the properties given in the previous sections are almost the only ones known presently. The following investigations are still pending: a) Only some small parts of the fault have been investigated so far in some detail. More field work is needed for detailed mapping of th.e fault zone. b) The paleogeography of Permian and Triassic times should be constructed using sedimentological and biostratigraphical methods. The result of this study will shed new light on the early development of the Tethys ocean in Anatolia. c) Geochemical, petrological, geochronological and bio- stratigraphical correlations must be made between the two sides of the fault. The questions of "What was the total amount of displacement during geological times?" and "Was the motion reversed anytime in its geological history?" might be answered using these data. 53 Special Publications Geodynamics: Progress and Prospects Vol. 5 Copyright American Geophysical Union
    • d) The one thing we know about the crustal structure is that the 36 km thickness of the crust in Central Anatolia is diminshing toward the Black Sea in the western part of Turkey (Canitez, 1962). There is no information about the velocity structure and lateral inhomogeneities within the crust and upper mantle. An explosion study program has to be started as soon as possible. e) The earthquake epicenters along the fault zone are quite scattered. Because of the unsatisfactory azimuthal distribu­ tions of seismological stations, it is very difficult to make high-precision relocation. A dense, local, short-period seismic network is necessary for detailed seismicity and seismo-tectonic study in the area. A 6-year program for micro-earthquake study was begun in 1975. f) The area is quite convenient for earthquake prediction studies. Although high-precision geodetic measurements have been started, strain, creep, tilt and magneto-telluric measure­ ments would be very helpful for this purpose. A dense seismic network also might be very useful to investigate the velocity changes and abnormal velocity zones in the area. Acknowledgement. The author wishes to express his sincere thanks to Professor M. N. Toksbz who read the manuscript. The study is supported by NATO Science Grant 4=f 568 . REFERENCES Aki, K. , Earthquake mechanism, Tectonophys ics, 1 3, 423-446 , 1 972 . Ambraseys, N. N., Some characteristic features of the Anatolian fault zone. Tectonophysics, 9, 143-165, 197o . Arpat, E. and Saroglu, F., Some recent tectonic events in Turkey (in Turkish), Bull. Geol. Soc. Turkey, 18, 91-1o1, 1 975 . Brune, J. N., Seismic moment, seismicity and rate of slip along major fault zones. J. Geophys. Res., 73, 777-784, 1968. Canitez, N., Crustal structure of the earth in North Anatolia from gravity and seismological data. Published Sc. D. Thesis, Mining Engineering Faculty, Technical University of Istanbul, Turkey (in Turkish), 1962 Canitez, N. and Ezen, tl. , Slip rate and stress drop along the North Anatolian fault. Symposium on Earthquake Statistics, UNESCO Balkan Project, Istanbul. (To be published elsewhere). Caputo, M. , Panza, G. F., and Postpisch_l , D M Deep structure of the Mediterranean basin. J. Geophys. Res., 75, 4919-4923, 197o. Chinnery, M., Theoretical fault models, a symposium on processes in the focal region. Publ. Pom. Obs. , Ottawa, Canada (37), 7, 211-223, 1969. Galanopoulos, A. G., Plate tectonics in the area of Greece as reflected in the deep focus seismicity. Bull. Geol. Soc. Greece, 9_, 266-285 , 1 972 Hanks, T. C., and Wyss, M., The use of body-wave spectra in the determination of seismic-source parameters. Bull. Seism. Soc. Am. , 6_2, 56 1-589 , 1 972 . Isaacs, B. , Oliver, J., and Sykes, L. R., Seismology and the new global tectonics. J. Geophys. Res., 73, 5855-5899, 1968. Ketin, I., Uber die nordanato1ische Horizontalverschiebung. 54 Special Publications Geodynamics: Progress and Prospects Vol. 5 Copyright American Geophysical Union
    • Bull. Mineral Res. Exploration Inst. Turkey, Foreign Ed., 7_2, 1-28 , 1 969. McKenzie, D. P., Plate tectonics of the Mediterranean region. Nature, 226, 239-243, 1970. McKenzie, D. P., Active tectonics of the Mediterranean region. Geophys. J. R. Astr. S o c , 30, 109-185 , 1972. Papazachos, B. C., Distribution of seismic foci in the Me­ diterranean and surrounding area and its tectonic implica­ tions. Geophys. J. R. Astr. S o c , 33, 42 1-430 , 1 973. Pavoni, N., Die nordanato1ische Horizontalverschiebung, Geo 1. Rundschau, 51, 1961. Seymen, I., Tectonic features of the North Anatolian fault zone along the Kelkit Valley. Published Ph. D. Thesis, I.T.U. Maden Fakultesi, Istanbul (in Turkish), 1974. Tokel, S., Mesozoic and Tertiary development of the eastern Pontids and its relation to the North Anatolian seismic zone. Proc. Cong. Earth Sci. on the occasion of the 50th anniversary of the Republic of Turkey, 1973. Ugur, E,, Recent crustal movements in the Gerede-Cerkes region of the North Anatolian fault zone by geodetic methods. Published Ph. D. Thesis, Istanbul Teknik Universitesi (in Turkish), 1974. 55 Special Publications Geodynamics: Progress and Prospects Vol. 5 Copyright American Geophysical Union