The 10 June 2012 Fethiye M_w 6.0 aftershock sequence and its relation to the 24–25 April 1957 M_s 6.9–7.1 earthquakes in SW Anatolia, Turkey

Highlights

• CMTs for 23 events are computed by applying a waveform inversion method.

• The geometry of focal mechanisms reveals a strike-slip faulting regime.

• It is performed to obtain a more accurate picture of the Fethiye Gulf stress field.

• The Coulomb stress change is computed to identify the expanded stress distribution.

• High-resolution hypocenter relocation of the Fethiye seismic sequence is examined.

Abstract

The 10 June 2012 M_w 6.0 aftershock sequence in southwestern Anatolia is examined. Centroid moment tensors for 23 earthquakes with moment magnitudes (M_w) between 3.7 and 6.0 are determined by applying a waveform inversion method. The mainshock is a shallow focus strike-slip with reverse component event at a depth of 30 km. The seismic moment (M_o) of the mainshock is estimated as 1.28 × 10¹⁸ Nm and rupture duration of the Fethiye mainshock is 38 s. The focal mechanisms of the aftershocks are mainly strike-slip faulting with a reverse component. The geometry of the focal mechanisms reveals a strike-slip faulting regime with NE–SW trending direction of T-axis in the entire activated region. A stress tensor inversion of focal mechanism data is performed to obtain a more accurate picture of the Fethiye earthquake stress field. The stress tensor inversion results indicate a predominant strike-slip stress regime with a NW–SE oriented maximum horizontal compressive stress (S_H). According to variance of the stress tensor inversion, to first order, the Fethiye earthquake area is characterized by a homogeneous interplate stress field. The Coulomb stress change associated with the mainshock and the largest aftershock are also investigated to evaluate any significant enhancement of stresses along the Gulf of Fethiye and surrounding region. Positive lobes with stress more than 0.4 bars are obtained, indicating that these values are large enough to increase the Coulomb stress failure towards NNW–SSE and E–W directions.

Introduction

The Fethiye earthquake (EQ) occurred at 12:44:16.9 GMT on 2012 June 10. The mainshock was a moderate size (M_w = 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 ⩽ M_w ⩽ 6.0) of the Fethiye M_w 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.