The June 6, 2000, Orta (Çankırı, Turkey) Earthquake: Sourced from a New Antithetic Sinistral Strike-slip Structure of the North Anatolian Fault System, the Dodurga Fault Zone

Authors: ALİ KOÇYİĞİT, BORA ROJAY, MUSTAFA CİHAN, ARDA ÖZACAR

Abstract: The İsmetpaşa-Kargı section of the North Anatolian Fault System (NAFS) consists of six subfault zones, namely the Eskipazar, the Ulusu, the Tosya, the Çerkeş-Kurşunlu, the Devrez and the newly detected Dodurga fault zone (DFZ). Together these fault zones form a well-developed dextral strike-slip-faulting pattern, in which the DFZ is an antithetic secondary strike-slip component, indicated by focal-mechanism solutions of moderate and large earthquakes that have occurred in the İsmetpaşa-Kargı section of the NAFS. The DFZ is a ~36-km-long, ~N-S-trending strike-slip structure located in the area between Saçak village in the north and Kösrelik village in the south. Pre-Upper Pliocene rocks are cut and tectonically offset for a distance of about 6 km. Upstream tributaries of the Devrez River are deviated in the shape of an \"S\" and are offset sinistrally up to 2.5 km. These values explain a 2.3 mm/yr rate of slip on the DFZ. In addition, several Plio-Quaternary pull-apart basins occur within the DFZ. An intermediate-magnitude (Mw= 6.0), shallow-focus earthquake, the Orta earthquake, struck on Tuesday, June 6, 2000, at 5:42 (local time) in the Orta area, and resulted in two deaths and severe damage to a total of 4842 structures, almost all of rural-style construction. Most of severe damage was confined to a narrow area along the Dodurga fault; that is, the master strand of the DFZ. Isoseismal lines display an ellipsoidal pattern with a long axis that parallels the Dodurga fault. Both fore- and after-shocks of the June 6, 2000 Orta earthquake form a highly concentrated, N-S-trending distribution pattern that parallels the DFZ. Focal-mechanism solutions of the main shock carried out by various seismographic stations, except for DER-DDR station, indicate sinistral strike-slip faulting with a normal-slip component. Consequently, taken together these field observations and seismological data indicate that the Dodurga fault is an antithetic sinistral strike-slip structure included in the NAFS, and that the June 6, 2000 Orta earthquake resulted from its activation.

Keywords: North Anatolian Fault System, Antithetic Fault, Sinistral Strike-slip Fault, Orta Earthquake

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