Source characteristics of the 6 June 2000 Orta-Çankiri (central Turkey) earthquake: A synthesis of seismological,geological and geodetic (InSAR) observations, and internal deformation of the Anatolian plate

This paper is concerned with the seismotectonics of the North Anatolian Fault in the vicinity of the Orta-Çankiri region, and consists of a study of a moderate-sized (Mw = 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 Çankiri, 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-Çankiri, t _o = 02:41:53.2, Mw = 6. 0) earthquake. We compared the shapes and amplitudes of long-period P- and SH-waveforms recorded by GDSN stations in the distance range 30-90°, for which signal amplitudes were large enough, with synthetic waveforms. The best-fitting fault-plane solution of the Orta-Çankiri 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 2° ± 5°, dip 46° ± 5°, rake -29° ± 5°; Nodal Plane 2: strike 113°, dip 70°, rake -132°; principal axes: P = 338° (48°), T = 232° (14°), B = 131° (39°); focal depth 8 ± 2 km (though this does not include uncertainty related to velocity structure), and seismic moment M _o = (140-185) × 10 ¹⁶ 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-Çankiri 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 = 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-Çankiri earthquake could be localized clockwise rotations as a result of shear of the lower cmst and lithosphere.