The 10 June 2012 Fethiye Mw 6.0 aftershock sequence and its relation to the 24–25 April 1957 Ms 6.9–7.1 earthquakes in SW Anatolia, Turkey
Introduction
The Fethiye earthquake (EQ) occurred at 12:44:16.9 GMT on 2012 June 10. The mainshock was a moderate size (Mw = 6.0) event at a depth of 30 km. The Fethiye EQ is located along the Fethiye–Burdur Fault Zone (FBFZ) of the western Anatolia extensional system (Fig. 1). The mainshock is revealed by a strike-slip motion with a reverse component. Fault rupture zone of an earthquake of this size extends typically from 8 to 20 km length. This region is one of the most seismically active parts of the western Anatolia extensional tectonic regime. The western Anatolia is characterized by very uniform (in magnitude and orientation) plate velocity vectors from Global Positioning System (GPS) indicating SW motion at about 30–40 mm/yr (Fig. 1, McClusky et al., 2000, Reilinger et al., 2006, Reilinger et al., 2010). It is dominated by a series of graben and horst structures bounded by normal or oblique faults (Taymaz et al., 1991, Koçyiğit et al., 2000, Bozkurt, 2001). The main tectonic structures of the eastern Mediterranean region are the Aegean and Cyprus arcs that compose convergent plate boundaries where the African plate (AF) to the south is subducting beneath the Anatolian (AT) and Aegean Sea plates (AS) to the north (inset in Fig. 1). Subduction in the eastern Aegean arc has traditionally been interpreted as occurring along the Pliny and Strabo trenches. Compressional motion is transformed into strike-slip on the Pliny and Strabo trenches (McClusky et al., 2003, Gürer et al., 2004, Nyst and Thatcher, 2004, ten Veen et al., 2004). A line with a projection of the Pliny trench along the Rhodes Island and southwest Turkey is the interpreted as the Rhodes Transform Fault (RTF) and Fethiye–Burdur Fault Zone (FBFZ) (see Fig. 1; McClusky et al., 2003, Gürer et al., 2004, Nyst and Thatcher, 2004, Reilinger et al., 2010, Çevikbilen and Taymaz, 2012). Bathymetric trends associated with the Pliny trench in the Rhodes basin link with N70°E striking faults in the Turkish continental slope. This basin is the continuation of the Pliny trench. This interpretation, strongly supported by marine geophysical data (ten Veen et al., 2004), precludes an alternative interpretation in which the Pliny trench is related to the hypothetical FBFZ, a correlation previously suggested by several authors (Taymaz and Price, 1992, Barka et al., 1997, Temiz et al., 1997, ten Veen et al., 2004). The FBFZ and RTF are evaluated as a wide left lateral fault zone with a large component of extension and well-defined seismicity (Taymaz et al., 1991, Taymaz and Price, 1992, McClusky et al., 2000, McClusky et al., 2003, Gürer et al., 2004, Nyst and Thatcher, 2004, Reilinger et al., 2010, Çevikbilen and Taymaz, 2012). Magnetotelluric studies in southwest Anatolia revealed that the depth of the crust/upper mantle boundary varies between 30 and 50 km (Gürer et al., 2004). On the basis of S receiver functions, the lithosphere-asthenosphere boundary of the subducting AF is at 100 km depth beneath the southwestern part of Anatolia and dips beneath the volcanic arc to a depth of about 225 km and therefore implies a thickness of 60–65 km for the subducted African lithosphere (Sodoudi et al., 2006).
In this study, a stress tensor inversion of earthquake focal mechanism data is performed to obtain a more accurate picture of the Fethiye EQ stress field. For this purpose, seismic waveforms at local and regional distances are used to calculate source parameters of 23 events (3.7 ⩽ Mw ⩽ 6.0) of the Fethiye Mw 6.0 EQ seismic sequence using the waveform inversion method (Nakano et al., 2008). This provides additional information on the stress field that may improve kinematic models for the FBFZ and thus develop the understanding of the local and regional tectonics. Furthermore, the Coulomb stress analysis is applied to determine the expanded spatial distribution of the Fethiye EQ seismic sequence. Determination of accurate source parameters, especially source locations, using data from the local and regional seismic network are crucial for investigations of the seismotectonics in and around Turkey.
Section snippets
Data and waveform inversion method
Fig. 1 displays the distribution of the Kandilli Observatory and Earthquake Research Institute (KOERI), Disaster and Emergency Management Presidency Earthquake Department (AFAD) and the National Observatory of Athens (NOA) broadband seismic stations used in this study. We use seismic records obtained from 8 KOERI stations, 3 AFAD stations and 2 NOA stations in this study (see Fig. 1).
Centroid moment tensor (CMT) solutions of earthquakes along the FBFZ are computed using the waveform inversion
Stress tensor inversion
The stress tensor has six unknown, either three principal stresses and orientations, or three normal and three shear stress components (e.g., Zang and Stephansson, 2010). Four of the unknown are resolved by the inversion of the stress tensor, the fifth unknown is calculated by the assumption that slip occurs in the direction of maximum shear stress (Wallace–Bott hypothesis), and the sixth unknown is usually resolved using the assumption that the stress tensor is homogeneous and constant in the
Results
The waveform inversion of earthquakes is carried out with local magnitudes (Ml) ⩾ 3.5 determined by KOERI. CMT solutions are computed for 23 earthquakes in the period between 2012 June 10 and November 13 (Fig. 2a and Table 2). The moment magnitudes (Mw) range from 3.7 to 6.0. Fig. 2a shows 23 fault plane solutions in lower hemisphere equal-area/angle projection in map view. The along strike dimension of the activated zone is approximately ∼30 km and its width is ∼20 km, in accordance with the
Discussion and conclusions
We study the spatio-temporal source characteristics of the 2012 June 10 Mw 6.0 Fethiye EQ seismic sequence, which occurred 30 km SW of Fethiye Gulf in the eastern Mediterranean. The distribution of epicenters and the focal mechanisms clearly indicate the activation of N76°E trending left lateral strike-slip fault with reverse component. The stress tensor inversion results reveal a predominant strike-slip stress regime with a NNW–SSE oriented maximum horizontal compressive stress (SH). The entire
Acknowledgements
The author thanks all members of National Earthquake Monitoring Center (NEMC) at Kandilli Observatory and Earthquake Research Institute (KOERI), Disaster and Emergency Management Presidency Earthquake Department (AFAD) and the National Observatory of Athens (NOA) for providing the continuous seismological data used in this study. The author is also grateful to Dr. Masaru Nakano for providing the waveform inversion code. The author would like to thank Bor-ming Jahn (Editor-in-Chief), Dapeng Zhao
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