Crustal and uppermost mantle S-wave velocity below the East European Craton in northern Poland from the inversion of ambient-noise records
The P-wave velocities (Vp) within the East European Craton in Poland are well known through several seismic experiments which permitted to build a high-resolution 3D model down to 60 km depth. However, these seismic data do not provide sufficient information about the S-wave velocities (Vs). For this reason, this paper presents the values of lithospheric Vs and P-wave-to-S-wave velocity ratios (Vp/Vs) calculated from the ambient noise recorded during 2014 at “13 BB star” seismic array (13 stations, 78 midpoints) located in northern Poland. The 3D Vp model in the area of the array consists of six sedimentary layers having total thickness within 3–7 km and Vp in the range 1.85.3 km/s, a three-layer crystalline crust of total thickness ~ 40 km and Vp within 6.15–7.15 km/s, and the uppermost mantle, where Vp is about 8.25 km/s. The Vs and Vp/Vs values are calculated by the inversion of the surface-wave dispersion curves extracted from the noise cross correlation between all the station pairs. Due to the strong velocity differences among the layers, several modes are recognized in the 0.021 Hz frequency band: therefore, multimodal Monte Carlo inversions are applied. The calculated Vs and Vp/Vs values in the sedimentary cover range within 0.992.66 km/s and 1.751.97 as expected. In the upper crust, the Vs value (3.48 ± 0.10 km/s) is very low compared to the starting value of 3.75 ± 0.10 km/s. Consequently, the Vp/Vs value is very large (1.81 ± 0.03). To explain that the calculated values are compared with the ones for other old cratonic areas.