Source Characteristics of the 6 June 2000 Orta–C¸ Ankırı (Central Turkey

Source characteristics of the 6 June 2000 Orta–Ç ankırı (central Turkey) earthquake: a synthesis of seismological, geological and geodetic (InSAR) observations, and internal deformation of the Anatolian plate T. TAYMAZ1, T. J. WRIGHT2, S. YOLSAL1, O. TAN3, E. FIELDING4 & G. SEYI˙TOGˇ LU5 1Department of Geophysics, I˙stanbul Technical University, Maslak-TR 34469, I˙stanbul, Turkey (e-mail: [email protected]) 2Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, UK 3TU¨ BI˙TAK-Marmara Research Centre, 41470 Gebze, Kocaeli, Turkey 4Jet Propulsion Laboratory, 4800 Oak Grove Drive, Caltech, Pasadena, CA 91109, USA 5Department of Geological Engineering, Ankara University, 06100 Ankara, Turkey Abstract: This paper is concerned with the seismotectonics of the North Anatolian Fault in the vicinity of the Orta–Ç ankırı region, and consists of a study of a moderate-sized (Mw ¼ 6. 0) earthquake that occurred on 6 June 2000. The instrumental epicentre of this earthquake is far from the North Anatolian Fault Zone (NAFZ), and rapid focal mechanism solutions of USGS–NEIC and Harvard-CMT also demonstrate that this earthquake is not directly related to the right-lateral movement of the North Anatolian Fault. This earthquake is the only instrumentally recorded event of magnitude (Mw) .5.5 since 1900 between Ankara and Ç ankırı, and therefore provides valuable data to improve our understanding of the neotectonic framework of NW central Anatolia. Field observations carried out in the vicinity of Orta town and neighbouring villages immediately after the earthquake indicated no apparent surface rupture, but the reported damage was most intense in the villages to the SW of Orta. We used teleseismic long-period P- and SH-body waveforms and first-motion polarities of P-waves, broadband P-waves, and InSAR data to determine the source parameters of the 6 June 2000 (Orta–Ç ankırı, to ¼ 02:41:53.2, Mw ¼ 6. 0) earthquake. We compared the shapes and amplitudes of long-period P- and SH-waveforms recorded by GDSN stations in the distance range 30–908, for which signal amplitudes were large enough, with syn- thetic waveforms. The best-fitting fault-plane solution of the Orta–Ç ankırı earthquake shows normal faulting with a left-lateral component with no apparent surface rupture in the vicinity of the epicentre. The source parameters and uncertainties of this earthquake were: Nodal Plane 1: strike 28+58, dip 468+58, rake –298+58; Nodal Plane 2: strike 1138, dip 708, rake –1328; principal axes: P ¼ 3388 (488), T ¼ 2328 (148), B ¼ 1318 (398); focal depth 8+2 km (though this does not include uncertainty related to velocity structure), and seismic moment 16 Mo ¼ (140–185) Â 10 N m. Furthermore, analysis of a coseismic interferogram also allows the source mechanism and location of the earthquake to be determined. The InSAR data suggest that the north–south fault plane (Nodal Plane 1 above) was the one that ruptured during the earthquake. The InSAR mechanism is in good agreement with the minimum misfit solution of P- and SH-waveforms. Although the magnitude of slip was poorly constrained, trade-off with the depth range of faulting accurred such that solutions with a large depth range had small values of slip and vice versa. The misfit was small and the geodetic moment constant for fault slips greater than c. 1 m. The 6 June 2000 Orta–Ç ankırı earthquake occurred close to a restraining bend in the east–west-striking rightlateral strike-slip fault that moved in the much larger earthquake of 13 August 1951 (Ms ¼ 6.7). The faulting in this anomalous earthquake could be related to the local geometry of the main strike-slip system, and may not be a reliable guide to the regional strain field in NW central Turkey. We tentatively suggest that one possible explanation for the occurrence of the 6 June 2000 Orta–Ç ankırı earthquake could be localized clockwise rotations as a result of shear of the lower crust and lithosphere. The epicentre of the 6 June 2000 Orta earthquake demonstrate that this earthquake is not directly (Mw ¼ 6.0) is far from the North Anatolian Fault related to the right-lateral movement of the North Zone (Figs 1 and 2). Rapid focal mechanism sol- Anatolian Fault (Fig. 2; Taymaz et al. 2002a, b). utions of USGS–NEIC and Harvard-CMT also Field observations carried out in Orta town and From:TAYMAZ, T., YILMAZ,Y.&DILEK, Y. (eds) The Geodynamics of the Aegean and Anatolia. Geological Society, London, Special Publications, 291, 259–290. DOI: 10.1144/SP291.12 0305-8719/07/$15.00 # The Geological Society of London 2007. 260 T. TAYMAZ ET AL. Fig. 1. Summary sketch map of the faulting and bathymetry in the Eastern Mediterranean region, compiled from our observations and those of Le Pichon et al. (1984, 2001), S¸engo¨r et al. (1985), Mercier et al. (1989), Taymaz et al. (1990, 1991a, b, 2002a, b), S¸arog˘lu et al. (1992), Taymaz & Price (1992), Jackson (1994), Price & Scott (1994), Alsdorf et al. (1995), Papazachos et al. (1998), Kurt et al. (1999, 2000), McClusky et al. (2000), Taymaz & Tan (2001), and Demirbag˘ et al. (2003). NAF, North Anatolian Fault; EAF, East Anatolian Fault; DSF, Dead Sea Fault; EPF, Ezinepazarı Fault; PTF, Paphos Transform Fault; CTF, Cephalonia Transform Fault; G, Go¨kova Fault; BMG, Buÿu¨k Menderes Graben; Ge, Gediz Graben; Si, Simav Graben; BuF, Burdur Fault; BGF, Beysehir Go¨lu¨ Fault; TF, Tatarlı Fault; SF, Sultandag˘ Fault; TGF, Tuz Go¨lu¨ Fault; EcF, Ecemis¸ Fault; ErF, Erciyes Fault; DF, Deliler Fault; EF, Elbistan Fault; MF, Malatya Fault; KFZ, Karatas¸–Osmaniye Fault Zone. the neighbouring villages immediately after the main tectonic element is the Kırıkkale–Erbaa earthquake indicated no apparent surface rupture, Fault Zone (KEFZ), which is responsible for the but the damage was most intense in the villages to 10 June 1985, 14 February 1992 and 14 August the SW of Orta. To explain the cause of this earth- 1996 earthquakes. The third tectonic element is quake the regional neotectonic framework should a NNE–SSW-trending pinched crustal wedge be taken into account. There are three main neo- between Ankara and Ç ankırı (see below). Its tectonic elements in NW central Anatolia that neotectonic importance was not accurately recog- correspond to the regional earthquake epicentre dis- nized until recently (Seyitog˘lu et al. 2000, 2001). tribution (M . 2) between 1964 and 2000 (Fig. 2). The closer distribution of epicentre locations The first is the well-known North Anatolian Fault (Fig. 2) along the line of Ankara and Ç ankırı corre- Zone (NAFZ), which has the ability to generate sponds to the NNE–SSW-trending I˙zmir–Ankara earthquakes with magnitudes .5. The second suture zone. INTERNAL DEFORMATION OF ANATOLIAN PLATE 261 Fig. 2. Seismicity of NW Turkey and the character of the North Anatolian Fault (NAF; after S¸arog˘lu et al. 1992) reported by ISC during 1964–2000 for M .2 superimposed on a shaded relief map derived from the GTOPO-30 Global Topography Data from USGS. Bathymetry data are from Smith & Sandwell (1997). Geographical location of the study area is outlined by a rectangular box. The anomalous seismic activity (Kandilli Observatory) orthogonal to the trace of the NAF should be noted. Star indicates the instrumental epicentre of 6 June 2000 Orta–Ç ankırı earthquake reported by the USGS. 262 T. TAYMAZ ET AL. The 6 June 2000 Orta earthquake is the only Koçyig˘it (1991a, b, 1992) and Koçyig˘it et al. instrumentally recorded event that has magnitude (1995) proposed that intracontinental convergence .5 since 1900 between Ankara and Ç ankırı, and related to the closure of the Neo-Tethyan ocean therefore provides valuable data on the neotectonic continued until the Late Pliocene, and called this framework of NW central Anatolia. In this paper, the ‘Ankara Orogenic Phase’. This suggestion was we first introduce the overall regional tectonic frame- based on the south-vergent thrusting of the east– work and then present seismological and InSAR west-trending Izmir–Ankara suture zone over investigations of the 6 June 2000 Orta–Ç ankırı earth- Neogene sedimentary units towards the east and quake. Finally, its implications on the internal west in the western margin of the Ç ankırı basin. deformation of the Anatolian plate will be discussed. Koçyig˘it et al. (1995) also claimed that the sedimentary units were deposited in thrust-related basins. After the Late Pliocene, intracontinental Neotectonic framework of NW central convergence gave way to an extensional regime as Anatolia indicated by the NE–SW-trending normal faults with related horizontal Pliocene deposits SW This section generally focuses on the less well- Ç ankırı (Koçyig˘it et al. 1995). known reactivated section of the Izmir–Ankara However, Seyitog˘lu et al. (1997) demonstrated suture zone between Ankara and Ç ankırı rather that south-vergent thrusting of the suture zone than relatively well-known structures such as the does not exist NW of Ankara and argued that inter- North Anatolia Fault (Ambraseys 1970; Ambraseys continental convergence must have been ceased & Jackson 1998; McKenzie 1969, 1970; Nowroozi before Miocene times because Miocene to Pliocene 1972; S¸engo¨r 1979; Stein et al. 1997; Taymaz et al. geochemical evolution of the Galatia volcanic 2002a, b, 2004) and its splay, the Kırıkkale–Erbaa series (Wilson et al. 1997; Gu¨rsoy et al. 1999) Fault (Figs 3 and 4; Polat 1988). shows lithospheric thinning rather than thickening The remnant of the Neo-Tethyan ocean known in the region. Alternatively, an extensional regime as the Izmir–Ankara–Erzincan suture trends as a result of orogenic collapse has been proposed approximately east–west to the west of Ankara.

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