Noise directivity and group velocity tomography in a region with small velocity contrasts: the northern Baltic shield

• Published in: Geophysical Journal International
• Main Authors: Poli, P., Pedersen, H. A., Campillo;, M., the POLENET/LAPNET Working Group
• Format: Text
• Language: English
• Published: Oxford University Press 2013
• Subjects: Seismology; Northern Finland; Lapland
• Online Access: http://gji.oxfordjournals.org/cgi/content/short/192/1/413; https://doi.org/10.1093/gji/ggs034

Ambient noise tomography (ANT) is widely used to image strong velocity variations within the upper crust. Using careful processing, we obtained a 3-D model of shear velocities in the upper crust beneath northern Finland, where the lateral velocity variations are less than 3 per cent. As part of the tomography, the noise field is analysed. It is strongly heterogeneous but the signal-to-noise ratio is sufficient to obtain stable dispersion curves for all profile azimuths. Our results show that the directions of dominant noise sources of Rayleigh and Love waves are the same, but the amplitude distribution with azimuth is different for the two types of waves. More intriguingly, the high frequency Love waves are dominated by a mixture of higher modes rather than the fundamental mode. The reconstructed 3-D model shows the Lapland Granulite Belt as a high velocity body with a limit at surface in excellent agreement with geological observations at surface. Following this interface at depth, our results are compatible with previous studies suggesting an Archean north oriented subduction.