Document Type : Original Research Paper


1 Department of Geology, Faculty of Sciences, Golestan University, Gorgan, Iran

2 Department of Geology, Faculty of Sciences and Earthquake Research Group, Shahid Bahonar University of Kerman, Kerman, Iran

3 Department of Civil Engineering, Faculty of Engineering, Golestan University, Gorgan, Iran


In this research, to understand the present-day tectonic situation of Gorgan-plain in the east of the South Caspian basin, the tectonic stress regime of this area and its adjacent areas was estimated using the inversion of the data of the earthquake focal mechanisms. For this aim, in addition to analyzing focal mechanisms solved by different sources, the focal mechanisms were solved for several earthquakes with appropriate and sufficient information. The results indicate the presence of various mechanisms, including thrust, normal, and strike-slip, and as a result, various orientations of kinematic P and T axes, which can indicate the complexity of the region. By analyzing the earthquakes with a minimum magnitude of 4 in the Gorgan-plain, the Kopeh-Dagh in the east of the plain, and the entire eastern region of the South Caspian, a compression regime with a NNW-SSE trend was obtained. However, by analyzing all of the earthquakes (smaller and larger earthquakes) in the Gorgan-plain, a strike-slip regime was obtained, in which the trend of the P and T axes were calculated NE-SW and NW-SE, respectively. The results indicate a local strike-slip regime in addition to the regional compressional regime in the region.


Main Subjects

Agh-Atabai, M., 2014. Multifractal Analysis of Earthquake Epicenters in the Golestan, Scientific Quarterly Journal of Geosciences, 23(92), 123-128. doi:10.22071/gsj.2014.43691 (in Persian). 
Aki, K., and Richards, P.G., 1980. Quantative seismology: Theory and methods.  San Francisco: Freeman,Vol. I, 557 p.
Allen, M.B., Jones, S., Ismail-Zadeh, A., Simmons, M., and Anderson, L., 2002. Onset of subduction as the cause of rapid Pliocene-Quaternary subsidence in the South Caspian basin, Geology, 30(9), 775-778. doi: 10.1130/0091-7613(2002)030<0775:OOSATC>2.0.CO;2.
Allen, M.B., Vincent, S.J., Alsop, G.I., Ismail-zadeh, A., and Flecker, R., 2003. Late Cenozoic deformation in the South Caspian region: effects of a rigid basement block within a collision zone. Tectonophysics, 366(3-4), 223-239. doi:10.1016/S0040-1951(03)00098-2.
Ambraseys, N., and Melville, C., 1982. A history of persian earthquakes, Cambridge University Press, New York, 219 p.
Angelier, J., 1991. Inversion directe et recherche 4-D: comparaison physique et mathématique de deux modes de détermination des tenseurs des paléocontraintes en tectonique de failles. Comptes rendus de l’Académie des sciences. Série 2, Mécanique, Physique, Chimie, Sciences de l’univers, Sciences de la Terre, 312(10), 1213-1218.
Angelier, J., and Mechler, P., 1977. Sur une methode graphique de recherche des contraintes principales egalement utilisables en tectonique et en seismologie: la methode des diedres droits. Bulletin de la Société géologique de France, 7(6), 1309-1318. doi:10.2113/gssgfbull.
Berberian, M., 1976a. Seismotectonic map of Iran, 1:250 000. In Contribution to the seismotectonics of Iran (Part II). Geological Survey of Iran, No. 39, 516 p. 
Berberian, M., 1981. Active faulting and tectonics of Iran. Zagros Hindu Kush Himalaya Geodynamic Evolution, 3, 33-69. 
Berberian, M., 1983. The southern Caspian: a compressional depression floored by a trapped, modified oceanic crust, Canadian Journal of Earth Sciences, 20(2), 163-183. doi: 10.1139/e83-015
Berberian, M., and Walker, R., 2010. The Rudbār Mw 7.3 earthquake of 1990 June 20; seismotectonics, coseismic and geomorphic displacements, and historic earthquakes of the western ‘High-Alborz’, Iran, Geophysical Journal International, 182(3), 1577-1602. 
doi: 10.1111/j.1365-246X.2010.04705.x.
Bott, M.H.P., 1959. The mechanics of oblique slip faulting. Geological magazine, 96(2), 109-117.
Carey-Gailhardis, E., and Mercier, J.L., 1987. A numerical method for determining the state of stress using focal mechanisms of earthquake populations: application to Tibetan teleseisms and microseismicity of Southern Peru, Earth and Planetary Science Letters, 82(1-2), 
165-179. doi:10.1016/0012-821X(87)90117-8.
Delvaux, D., and Sperner, B., 2003. New aspects of tectonic stress inversion with reference to the TENSOR program, Geological Society, London, Special Publications, 212(1), 75-100. doi: 10.1144/GSL.SP.2003.212.01.06.
Delvaux, D., Moeys, R., Stapel, G., Petit, C., Levi, K., Miroshnichenko, A., Ruzhich, V., and San’kov, V., 1997. Paleostress reconstructions and geodynamics of the Baikal region, central Asia, Part 2. Cenozoic rifting. Tectonophysics, 282(1-4), 1-38. 
Etchecopar, A., Vasseur, G., and Daignieres, M., 1981. An inverse problem in microtectonics for the determination of stress tensors from fault striation analysis, Journal of Structural Geology, 3(1), 51-65. doi:10.1016/0191-8141(81)90056-0.
Gephart, J.W., 1990b. FMSI: A FORTRAN program for inverting fault/slickenside and earthquake focal mechanism data to obtain the regional stress tensor, Computers & Geosciences, 16(7), 953-989. doi:10.1016/0098-3004(90)90105-3.
Ghassemi, M.R., Mohammadkhani, H., and Abdollahi, A., 2007. The transition from uplift in the Kopeh-Dagh to subsidence in the South Caspian, introducing the newly known Gonbad-e-Kavous and Yasaghi faults, Geological Survey and Mineral Exploration of Iran 26th symposium of Geosciences (in Persian).
Hamidi, R., 2021. Investigation on tectonic deformation patterns in western Kopeh Dagh, Ph.D. thesis, Univ. of Golestan, 277 p (in Persian).
Hardcastle, K.C., and Hills, L.S., 1991.  BRUTE3 and SELECT: Quickbasic 4 programs for determination of stress tensor configurations and separation of heterogeneous populations of fault-slip data, Computers & Geosciences, 17(1), 23-43. doi:10.1016/0098-3004(91)90078-R
Havskov, J., and Ottemöller, L., 1999. SEISAN earthquake analysis software, Seismological Research Letters, 70(5), 532-534. 
Heidbach, O., Reinecker, J., Tingay, M., Müller, B., Sperner, B., Fuchs, K., and Wenzel, F., 2007. Plate boundary forces are not enough: Second and third order stress patterns highlighted in the World Stress Map database, Tectonics, 26(6). doi: 10.1029/2007TC002133.
Hollingsworth, J., Jackson, J., Walker, R., Reza Gheitanchi, M., and Javad Bolourchi, M., 2006. Strike-slip faulting, rotation, and along-strike elongation in the Kopeh Dagh mountains, NE Iran, Geophysical Journal International, 166(3), 1161-1177. doi: 10.1111/j.1365-246X.2006.02983.x.
Huber, H., 1977. Geological map of Iran, 1: 1,000,000 with explanatory note. National Iranian Oil Company. Exploration and Production Affairs, Tehran.
IIEES, International Institute of Earthquake Engineering and Seismology,
IRSC, Iranian Seismological Center, 
ISC, International Seismological Centre, 
Jackson, J., Priestley, K., Allen, M., and Berberian, M., 2002. Active tectonics of the south Caspian basin,  Geophysical Journal International, 148(2), 214-245. doi:10.1046/j.1365-246X.2002.01588.x
Lay, T., and Wallace, T.C., 1995. Modern global seismology, San Diego, Academic Press, 521 p.
Lyberis, N., and Manby, G., 1999. Oblique to orthogonal convergence across the Turan block in the post-Miocene, American Association of Petroleum Geologists bulletin, 83(7), pp.1135-1160. doi:10.1306/E4FD2E97-1732-11D7-8645000102C1865D.
Nemati, M., Hollingsworth, J., Zhan, Z., Bolourchi, M.J., and Talebian, M., 2013. Microseismicity and seismotectonics of the South Caspian Lowlands, NE Iran, Geophysical Journal International, 193(3), 1053-1070. doi:10.1093/GJI/GGS114.
Priestley, K., Baker, C., and Jackson, J., 1994. Implications of earthquake focal mechanism data for the active tectonics of the South Caspian Basin and surrounding regions, Geophysical Journal International, 118(1), 111-141. doi:10.1111/j.1365-246X.1994.tb04679.x.
Radfar, A., Chakdel, A.R., Nejati, A., and Soleimani, M., 2019. New insights into the structure of the South Caspian Basin from seismic reflection data, Gorgan Plain, Iran, International Journal of Earth Sciences, 108(2), 379-402. doi:10.1007/s00531-018-1659-x.
Ritz, J.F., Nazari, H., Ghassemi, A., Salamati, R., Shafei, A., Solaymani, S., and Vernant, P., 2006. Active transtension inside central Alborz: A new insight into northern Iran–southern Caspian geodynamics, Geology, 34(6), 477-480. doi: 10.1130/G22319.1.
Robert, A.M., Letouzey, J., Kavoosi, M.A., Sherkati, S., Müller, C., Vergés, J., and Aghababaei, A., 2014. Structural evolution of the Kopeh Dagh fold-and-thrust belt (NE Iran) and interactions with the South Caspian Sea Basin and Amu Darya Basin, Marine and Petroleum Geology, 57, 68-87. doi:10.1016/J.MARPETGEO.2014.05.002.
Sperner, B., Ratschbacher, L., and Ott, R., 1993. Fault-striae analysis: a Turbo Pascal program package for graphical presentation and reduced stress tensor calculation, Computers & Geosciences, 19(9), 1361-1388. doi: 10.1016/0098-3004(93)90035-4.
Stöcklin, J., 1974. Northern iran: Alborz mountains, Geological Society, London, Special Publications, 4(1), 213-234. 
Unruh, J.R., Twiss, R.J., and Hauksson, E., 1996. Seismogenic deformation field in the Mojave block and implications for tectonics of the eastern California shear zone, Journal of Geophysical Research: Solid Earth, 101(B4), 8335-8361. doi:10.1029/95JB03040.
Wallace, R.E., 1951. Geometry of shearing stress and relation to faulting. The Journal of geology, 59(2), 118-130. doi:10.1086/625831.
Zamani, B., Angelier, J., and Zamani, A., 2008. State of stress induced by plate convergence and stress partitioning in northeastern Iran, as indicated by focal mechanisms of earthquakes, Journal of Geodynamics, 45(2-3), 120-132. doi:10.1016/j.jog.2007.07.003.