ALBERT

Description: An energy-flux model (EFM) and a teleseismic fluctuation wavefield method (TFWM) have been applied to interpret the teleseismic P coda observed at three temporary and two permanent networks in northern and central Europe. The aim is to determine the small-scale random structure of the lithosphere below the receivers. Various subsets of these data have been exploited previously with one of the two methods. The main objectives here are: To compare the performance of both methods with synthetic data sets. To map the random structure of the lithosphere in terms of inverse scattering Q (Q−1s), correlation length a, RMS velocity fluctuation σ, thickness L of the scattering layer and autocorrelation function (ACF) using a combination of both methods. With TFWM, the product aσ2 can be reliably determined if L is known. L can be roughly estimated with EFM. Although EFM can, in principle, resolve a and σ separately, a is recovered with a rather large uncertainty. TFWM does not distinguish much between the ACF type, whereas with EFM determination of the ACF type is sometimes possible. By combining the results of both methods we determine improved random medium parameters of the lithosphere for eleven subregions in northern and central Europe. In the Baltic shield, Rhenohercynian belt, Ardenne and Brabant mountains, eastern Rhenish massif, Eifel, Hunsrück mountains, Lorraine, Frankonian Jura and massif Central scattering predominantly occurs in the crust. For the frequency range from 0.5 to 5 Hz correlation lengths of 1–7 km and rms velocity fluctuations of 3-7 per cent are obtained. For the Rhenohercynian belt (RH) and the N-German basin RMS velocity fluctuations and correlation lengths could not be resolved. The data from the N-German basin (NB) cannot be explained by scattering within the crust only. Smallest scattering Q was found in the N German basin (Qs≈ 100 at 1 Hz) and largest scattering Q in the Baltic shield (Qs≈ 450 at 2-3 Hz). For the Frankonian Jura only a Gaussian ACF can fit the Q−1s values. The data from the eastern Rhenish massif also indicate a random medium with Gaussian ACF. For all other subregions we cannot distinguish between exponential or Gaussian ACF for the random medium structure.

Description: Data from the passive Teleseismic experiment TOR across the Trans-European Suture Zone (TESZ), which took place in 1996-1997, are used to study the random structure of the lithosphere NE of the TESZ (the Baltic Shield beneath southern Sweden) and beneath the north German/Danish Basin. P coda waves from deep Teleseismic events give information on the small-scale heterogeneity of the near-receiver lithosphere on scales of one wavelength. Using an energy flux model, the time and frequency behaviour of the P coda has been interpreted in terms of scattering (Qs) and anelastic attenuation (Qi). Estimates Qs were obtained for the frequency range 0.5-3 Hz for the north German/Danish Basin and 0.5-7 Hz for the Baltic Shield. The two areas show a different scattering behaviour. In Denmark/northern Germany the scattering (Qs ˜ 125-200) is stronger than in southern Sweden (Qs ˜ 275-500) and the peak of Qs-1 occurs at higher frequencies NE of the TESZ. Anelastic attenuation is negligible for the Baltic Shield whereas for the north German/Danish Basin anelastic attenuation is present but still much weaker than scattering attenuation. With a modified energy flux model for depth-dependent scattering, and assuming an exponential autocorrelation function, the depth variation of correlation length a and rms velocity fluctuation e can in principle be resolved. For the Baltic Shield the scattering is mostly confined to the crust with a ˜ 1 jn and e ˜ 4 per cent. The subcrustal lithosphere only shows weak fluctuations. The data from the north German/Danish Basin are a = 5-10 km and e ˜ 8 per cent. For the subcrustal lithosphere, correlation lengths of 10-20 km and rms velocity fluctuations of 6-8 per cent are found. Additional tests confirmed that the thick sedimentary cover in this area has no significant effect on these results for the deeper structure. Correlation lengths for the sediments are smaller than 5 km and rms velocity fluctuations e are 7-8 per cent.