Constraining crustal and uppermost mantle structure beneath station TBZ (Trabzon, Turkey) by receiver function and dispersion analyses

Abstract

SUMMARY We jointly invert teleseismic radial-component receiver functions and regional Rayleigh and Love surface-wave group velocities for 1-D shear-wave velocity structure beneath station TBZ located on the northern side of the eastern Pontides. An influence factor is employed to control the relative influence of receiver function and surface-wave dispersion on the resultant velocity–depth profile. Radial- and transverse-component receiver functions at station TBZ exhibit an azimuthal amplitude and polarity pattern consistent with 2-D receiver structure that has a general dip direction towards approximately south. The radial-component receiver functions are least affected by the dipping structures along the strike direction and thereby we prefer teleseismic events sampling along-strike structures to alleviate the deflecting effect of dipping interfaces on the 1-D solution. The 1-D inversion effectively reveals the two-layer nature of the crust which is perturbed by high- and low-velocity layers, and serves as a provisional model for the 2-D forward modelling. Minor-to-moderate changes to the 1-D model, such as changing depth to and velocity contrast across an interface, are needed to achieve the results with the 2-D modelling. Dipping interfaces and seismic anisotropy are included in the 2-D modelling to fit both radial- and transverse-component receiver functions. The upper crust is characterized by a shear velocity of ∼3.5 km s−1 and cut through by a 4 km thick high-velocity (i.e. ∼3.8 km s−1) layer. Overlying the upper crust, the sedimentary cover (i.e. the top 5 km) has velocities within the range ∼2.0–3.5 km s−1. A mid-crustal velocity discontinuity between the upper granitic crust and the lower basaltic crust is identified at ∼16-km depth. This boundary is analogous to the mid-crustal discontinuity found under the Black Sea basin across which the shear velocity jumps from 3.5 to 4.1 km s−1. A relatively thick (i.e. ∼12 km) low-velocity layer in the lower crust with a velocity reversal from 4.1 to 3.7 km s−1 is needed to better explain reverberations off this depth range. We infer a 2-D Moho discontinuity placed at ∼35-km depth beneath the station. The proposed dip angle for the Moho is rather steep (i.e. ∼25°), although coincident with regional gravity studies. The associated Sn velocity (i.e. ∼4.4 km s−1) is rather low, indicating disturbed upper-mantle structure beneath the region. Initial amplitudes of transverse-component receiver functions are rather energetic, for which we propose substantial P and S velocity anisotropy (∼12 per cent) for the topmost depths (<5 km).