Quantifying Complex Anisotropy beneath NW Segment of North Anatolian Fault Zone (Turkey) obtained from Joint Interpretation of Receiver Functions and SKS Phases

Abstract:

Accurate knowledge of seismic anisotropy can provide key constraints on the depth extent of deformation zones in tectonically complex regions such as the northwestern part of the North Anatolian Fault Zone (NAFZ). In order to examine the distribution and nature of seismic anisotropy at crustal scales, we adopt a harmonic decomposition analysis over P-receiver functions (PRFs). PRFs were extracted from teleseismic earthquakes (30°≤ Δ ≤ 90°) recorded at the DANA Seismic Network. From depth-converted PRFs, we obtained k=0, k=1 and k=2 harmonic coefficients. Lateral variation of the k=0 harmonic (representative of the isotropic response of the crust) suggests an increase in crustal thicknesses from the Sakarya Zone to the Istanbul Zone. The energy distribution of anisotropic harmonic coefficients (k=1 and k=2) is a proxy for the presence of anisotropy and/or dipping velocity contrasts at depth. Our results suggest that the dominant source of anisotropy can be related to the dipping axis of symmetry at all depth ranges. The high-energy distribution is localized near the northern and southern branch of NAFZ in the upper crust (0-20 km), except for the northern area of the Istanbul Zone. The orientation of anisotropy is usually NNW-SSE and nearly perpendicular to both branches of the NAFZ. Moreover, we observe that regions located away from the segments of NAFZ exhibit stress-induced anisotropic orientations. Seismic anisotropy at about 20-35 km is more diffusive and can be explained by a presumable upwelling of hot material and widening deformation zone in the lower crust. Overall, we do not observe any clear variation in the k=2 harmonics except for the upper crust. The relatively high-energy variation from k=2 harmonic can be related to the deformation induced by the motion on the NAFZ at upper crustal depths.

Furthermore, we model the actual geometry of anisotropic structures at several selected stations with relatively high-quality observations by means of PRFs inversion with a neighborhood algorithm. Finally, SKS splitting measurements were used as a complementary tool to quantify seismic anisotropy at greater depths. Combining results from different datasets contributed to elucidate the type of deformation and the degree of coherency in the transmission of that deformation at various depth scales.