Small-scale heterogeneities below the Grafenberg array, Germany from seismic wavefield fluctuations of Hindu Kush events

Abstract

Small-scale elastic heterogeneities (<5 km) are found in the upper lithosphere underneath the Grafenberg array, southeast Germany. The results are based on the analysis of broadband recordings of 17 intermediate-depth (201-272 km) events from the Hindu Kush region. The wavefront of the first P arrival and the following 40 s coda are separated into coherent and incoherent (scattered) parts in the frequency range from 0.05 to 5 Hz. The frequency-dependent intensities of the mean and fluctuation wavefields are used to describe the scattering characteristics of the lithosphere underneath the receivers. It is possible to discriminate a weak-fluctuation regime of the wavefield in the frequency range below approximately 1.5-2.5 Hz and a strong-fluctuation regime starting at 2.0-2.5 Hz and continuing to higher frequencies. In order to explain the observed wavefield fluctuations, an approach with seismic scattering at random media-type structures is proposed. The preferred model contains heterogeneities with 3-7 per cent perturbations in seismic velocity and correlation lengths of 0.6-4.8 km in the crust. This is compatible with models from active seismic experiments. Scattering in the lithospheric mantle is not required, but cannot be excluded at weak velocity contrasts (<3 per cent).