SUMMARY Ambient seismic noise tomography has, over the last two decades, developed into a well-established tool for imaging seismic properties of the Earth’s crust. Fundamental mode Rayleigh and Love wave phase velocity dispersion curves can be measured from ambient noise cross-correlation functions (CCF) either using a high-frequency approximation theory, or by fitting the spectrum of the CCF to a Bessel function. Here, we advance the latter approach and present an automated algorithm that fits the phase of the Hankel function to the phase of the causal symmetric part of the CCF in order to determine phase velocity curves as continuous functions of frequency. Synthetic tests verify the reliability of the proposed method in the presence of low signal-to-noise ratio (SNR). Moreover, usage of the phase allows for robust phase velocity measurements at longer periods than when using the zero crossings of the Bessel function only and is, therefore, particularly useful at short inter-station distances. In the frequency domain, acceptable bandwidths of smooth phase velocity curves are obtained in an automated procedure using a set of fine-tuned quality criteria. We apply the method to 2.5 yr of continuous waveform data recorded by 58 temporary and permanent broad-band seismic stations in northern Oman. We obtain 1072 and 670 phase velocity curves for Rayleigh and Love waves, respectively, in the period range of 2–40 s. The data are inverted for isotropic and azimuthally anisotropic period-dependent phase velocity maps. Synthetic reconstruction tests show that the phase velocity maps have a lateral resolution of ∼30 km. The results suggest distinctly different middle to lower crustal architecture between the northern and eastern Oman Mountains. Azimuthal anisotropy shows contrasting fast propagation orientations in the shallow and deep crust, which we attribute to stress-induced and structural anisotropy in the upper crust and to lattice-preferred orientation in the lower crust.

The Pannonian region is a back-arc basin located within the arcuate Alpine-Carpathian mountain chain in central Europe. Beneath the basin both the crust and the lithosphere have smaller thickness than the continental average. During the last few decades several studies have been born to explain the formation of the Pannonian Basin but several key questions remain unanswered. In this study we construct a new high-resolution 3D P-wave velocity model of the crust and uppermost mantle in the Pannonian Basin which may help us to understand better the structure and evolution of the region. For the 3D P-wave velocity structure estimation over 32 thousand traveltime picks have been derived from the ISC bulletin and the local Hungarian National Seismological Bulletin, and altogether we used more than 3200 seismic events (local, near-regional and regional) and more than 150 seismic stations from the time period between 2004 and 2014. For the 3D velocity field inversion we used the FMTOMO software package which uses the so called Fast Marching Method for calculating the traveltime estimations, and the subspace inversion method to recover the model parameters. We also performed several checkerboard tests both to select the appropriate regularization parameters and to help the interpretation of the resulting P-wave velocity model. On the resulting tomographic image the seismic velocity anomalies well resolve the effects of deep sedimentary basins and also Moho topography and the associated updomings of the asthenosphere below the Pannonian Basin. Different major tetonic units and fault zones separating those seem to show characteristic velocity anomalies. Subrecent volcanic activity or associated melt and fluid percolation, heat transfer in the upper mantle and crust may also have an impact on the propagation of seismic waves.

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