Document Type : Original Research Paper


International Institute of Earthquake Engineering and Seismology (IIEES), Tehran, Iran


           A moderate earthquake (Ms=6.3) struck the coastal region of north of Iran and the central Alborz on 28 May, 2004 was responsible of several damages and about 35 casualties. The mainshock was followed by a large number of aftershocks, the largest one reaching Ml=4.8, based on the analysis of local waveforms. We study the mainshock, first major aftershock, and about 240 aftershocks recorded by Iranian National Seismic Network (INSN), Tehran Telemetry Seismic Network (Institute of Geophysics, Tehran University), and our temporary local seismological stations, which were installed on 30 May, around the epicentral area of this earthquake. Using waveforms of all permanent stations, the coordinates of the mainshock was determined as 36.30 °N for latitude and 51.60 °E for longitude. The analysis of aftershocks indicates that the seismic activity migrate  from east where the mainshock occurred toward west close to the location of the largest aftershock  (36.36 °  N,   51.45 ° E). 140 selected aftershocks recorded at a minimum of 6 stations, having rms less than 0.15 sec and uncertainties less than 2 km, were used to infer a precise geometry of the fault region. The aftershocks distribution has 30 km long and trends NW-SE parallel to the North Alborz and Khazar faults. The focal depths comprised between 10 and 28 km, unusually deep for Iran. Distribution of aftershocks cluster on cross-section defines a fault plane which dips at 40-50 degree south-westward. Its upward continuation can be related to either North Alborz or Khazar faults.  Most of the focal mechanisms are consistent with reverse faulting on NW-SE trending faults, parallel to the main active structures of the region. Well constraint focal mechanisms which dip gently at a rate of 25-40ْ indicate the activity of the second mentioned faults during the Firozabad-Kojour earthquake. Existence of focal depths up to 28 km indicates an unusual brittle lower crystalline crust in this part of central Alborz.


Alavi, M., 1996- Tectonostratigraphic synthesis and structural style of the Alborz mountain system in northern Iran. J. Geodyn. 21, 1 – 33.
Allen, M. B., Jones, S., Ismail-Zadeh, A., Simmons, M. D. & Anderson, L., 2002- Onset of subduction as the cause of rapid Pliocene-Quaternary subsidence in the South Caspian Basin, Geology, 30, 775 – 778.
Allen, M.B., Ghassemi, M.R., Sharabi, M. & Qoraishi, M., 2003- Accommodation of late Cenozoic oblique shortening in the Alborz range, northern Iran. J. Struct. Geol., 25, 659– 672.
Ambraseys, N. N. & Melville, C. P., 1982- A history of Persian Earthquakes, Cambridge Earth Science Series, Cambridge University Press, London.
Ashtari, M., Hatzfeld, D. & Kamalian, N., 2005- Microseismicity in the region of Tehran, Tectonophysics, 395, 193-208.
Berberian, M., 1983- The southern Caspian: a compressional depression floored by a trapped, modified oceanic crust. Can. J. Earth Sci., 20, 163– 183.
Berberian, M., Qorashi, M., Jackson, J.A., Priestley, K. & Wallace, T., 1992- The Rudbar–Tarom earthquake of 20 June 1990 in NW Persia: preliminary field and seismological observations, and its tectonic significance, Bull. seism. Soc. Am., 82, 1726–1755.
Berberian, M. & Yeats, R.S., 2001- Contribution of archaeological data to studies of earthquake history in the Iranian plateau. J. Struct. Geol. 23, 563– 584.
Berberian, M., Ghoraishi, M., Shoja-Taheri, J., Talebian, M., 1996- Seismotectonic and earthquake-fault hazard investigations in the Semna region. Geological Survey of Iran, Publication no. 63.
Brunet, M.-F., Korotaev, M.V., Ershov, A., Nikishin, A.M., 2003- The south Capian basin: a review of its evolution form subsidence modelling. Sediment. Geol. 156, 119– 148.
Engdahl, E. R., Van Der Hilst, R. & Buland, R., 1998- Global teleseismic earthquake relocation with improved travel times and procedures for depth determination, Bull. Seism. Soc. Am., 88, 722-743.
Geological Survey of Iran, 1991- Amol. Geological Survey of Iran, Tehran, scale 1:250,000.
Geological Survey of Iran, 1991- Gorgan. Geological Survey of Iran, Tehran, scale 1:250,000.
Geological Survey of Iran, 1991- Sari. Geological Survey of Iran, Tehran, scale 1:250,000.
Ghitanchi, M.R., 2005- Source characteristics of the 28 May 2004 Baladeh-Kojour destructive Earthquake in Central Alborz, revealed from far-field waveform data. Geosciences, GSI, 55, 154-163.
HarvardUniversity, Department of Geological Sciences, 2006- Centroid Moment Tensor catalogue, available online at: http://
Hessami, K.  & Jamali, F., 2006- Explanatory Notes to the Map of Major Active Faults of Iran, J. of Seismology and Earthquake Engineering, 8, No. 1.
Jackson, J., Priestley, K., Allen, M. & Berberian, M., 2000- Active tectonics of the south Caspian basin. Geophys. J. Int. 148, 214– 245.
Kissling, E. ,1988- Geotomography with local earthquake data, Rev. Of  Geophys., 26, 659-698.
Maggi, A., Jackson, J.A., Priestley, K. & Baker, C., 2000b- A re-assessment of focal depth distributions in southern Iran, the Tien Shan and northern India: do earthquakes really occur in the continental mantle?, Geophys. J. Int., 143, 629–661.
Nazari, H., 2006- Analyse de la tectonique récente et active dans l’Alborz Central et la région de Téhéran : Approche morphotectonique et paléoseismologique, Ph.D. theis, Université Montpellier II.
Priestley, K., Baker, C. & Jackson, J., 1994- Implications of earthquake focal mechanism data for the active tectonics of the south Caspian basin and surrounding regions. Geophys. J. Int. 118, 111– 141.
Reasenberg, P.A. & Oppenheimer, D., 1985- FPFIT, FPPLOT and FPPAGE, Fortran computer programs for calculating and displaying earthquake fault-plane solutions, USGS Open-File Report, no 85–739.
Sengo¨r, A.M.C., Altiner, D., Cin, A., Ustaomer, T. & Hsu, K.J., 1988- Origin and assembly of the Tethyside orogenic collage at the expense of GondwanaLand. Gondwana and Tethys. In: Audley-Charles, M.G., Hallam, A. (Eds.), Geological Society Special Publication, 37, 119–181.
Tatar, M., 2001- Etude Seismotectonique de deux zones de collision continentale: le Zagros Central et l’Alborz (Iran). PhD thesis, University de Joseph Fourier.
Tatar, M., Hatzfeld, D. & Ghafory-Ashtiany, M., 2004- Tectonics of the Central Zagros (Iran) deduced from microearthquake seismicity, Geophys. J. Int., 156, 255–266.
Trifonov, V.G., Hessami, K.T. & Jamali, F., 1996- West-Trending Oblique Sinitral–Reverse Fault system in Northern Iran. IIEES Special Pub., vol. 75. Tehran, Iran.
Vernant, P., Nilforoushan, F.,  Hatzfeld, D.,  Abbasi, M. R.,  Vigny, C., Masson, F., Nankali, H.,  Martinod, J.,  Ashtiany, A., Bayer, R.,  Tavakoli, F.,  Chéry, J., 2004a- Present day crustal deformation and plate kinematics in the middle east constrained by GPS measurements in Iran and Northern Oman., Geophys, J. Int. 157, 381-398.
Vernant,Ph., Nilforoushan,F., Chery,J.,  Bayer,R., Djamour,Y., Masson,F., Nankali,H., Ritz,J.F., Sedighi,M. & Tavakoli,F., 2004b- Deciphering oblique shortening of central Alborz in Iran using geodetic data, Earth and Planetary Science Letters, 223, 177– 185.