Palaeoseismology of the Havran-Balkesir Fault Zone: evidence for past earthquakes in the strike-slip-dominated contractional deformation along the southern branches of the North Anatolian fault in northwest Turkey

The Havran-Balkesir Fault Zone (HBFZ) is one of the major active structures of the Southern Marmara Region, which has been shaped by the southern branch of North Anatolian fault since the Pliocene. HBFZ is a 10-12km wide, 120km long, right-lateral strike-slip fault zone that consists of two ENE-striking main faults, namely, the Havran-Balya and Balkesir faults. The 90-km-long Havran-Balya fault exhibits right-stepping en echelon geometry and is made up of (1) Havran, (2) Osmanlar, (3) Turplu and (4) Ovack fault segments. On the eastern part, the 70-km-long Balkesir fault is divided into two fault segments; (1) Gokceyaz and (2) Kepsut. We estimated the long-term slip rate between 3.59 and 3.78mm/yr using river offset. The Kepsut, Gokceyaz and Ovack fault segments are capable of generating an earthquake with a moment magnitude of up to 7.2. Detailed palaeoseismological studies show that the HBFZ is responsible for some surface faulting earthquakes with an average recurrence interval of 1000-2000years during the late Holocene. Considering the fact that there was no evidence of a surface-ruptured earthquake for 2000years, it can be stated that there is a seismic gap on the Gokceyaz fault segment.