Rates of crustal deformation in western Turkey as deduced from major earthquakes

Abstract

Western Turkey has undergone dominantly N-S extension, probably since the late Miocene. This extensional regime caused the formation of E-W trending grabens and similarly orientated normal faults. The rates of deformation of the seismogenic layer caused by the major earthquakes ( M s ⩾5.5) during the period between 1943 and 1983 are calculated for western Anatolia. This region is first analysed as a single system and then divided into two provinces, namely the Marmara and southwestern Anatolian provinces, which are investigated as two separate seismotectonic units. The dominant mode of deformation in the Marmara province is amounting to right-lateral displacement 24.0 mm/yr. Other modes of upper crustal deformation in this province can be explained by an average 7.1 mm/yr N-S extension, 10.0 mm/yr E-W contraction, and 0.13 mm/yr thinning of a 10 km seismogenic layer. The dominant mode of deformation in southwestern Anatolia is about 13.5 mm/yr extension in the N-S direction. This region is also undergoing E-W extension of about 3.6 mm/yr, justifying the separation of Marmara and southwestern Anatolian provinces as two different seismotectonic provinces. The average rate of thinning of the seismogenic layer in this area is 0.5 mm/yr which is consistent with large-scale normal faulting observed here. It is found that N-S extension in western Anatolia increases from north to south, while E-W compression in the Marmara province changes to E-W extension in southwestern Anatolia. It is suggested that these kinematics may explain the present bending of the North Anatolian Transform, forming a transtensional basin between the Marmara and southwestern Anatolian provinces.