THE NORTH-SEA LG-BLOCKAGE PUZZLE

The North Sea Lg blockage for wave paths across crustal graben structu res is a well-established observational fact. Analysis of such observa tions implies that Lg blockage takes place in graben areas associated with sedimentary basin formation and crustal thinning. These intriguin g observations have triggered many theoretical studies aimed at highli ghting specific Lg loss mechanisms, albeit so far with only moderate s uccess. Our approach to this problem is to simulate seismic wavefield propagation through the crustal waveguide using 2-D finite-difference techniques. The graben structures are known in detail from oil explora tion works in the North Sea, which has enabled us to use realistic cru stal models in our Lg synthetics. In the most extreme model tested, th e crystalline crust thickness beneath the graben amounted to only 5 km , while the overlying sedimentary pile is nearly 10 km thick. At the b ase of the crust in the graben area the Moho is elevated nearly 10 km. This model has similarities to the oceanic crustal waveguide, where. total Lg blockage is claimed for path lengths exceeding 100 km. The sy nthetic wavefields are displayed in terms of snapshots, semblance velo city analysis and time-space rms amplitudes. The dominant structural L g loss mechanisms are the delay of the Lg waves in the thick sediments , Lg-to-Rg conversions (scattering) by lateral heterogeneities in the sediments, and S-wave leakage out of the crustal waveguide and into th e upper mantle. A fraction of these upper-mantle S waves return to the crust and appear as Sn coda. Observationally, strong Sn phases of lon g duration are often associated with weak Lg phases and vice versa. Ou r synthetics produced Lg amplitude decay amounting at most to 6-10 dB, while observational data imply blockage amounting to 15-20 dB. The la tter is equivalent to a Pn-Lg magnitude difference of nearly one magni tude unit. The main outcome of this study is therefore that Lg-wave pr opagation is very robust and that a dominant blockage effect associate d with intrinsic attenuation, that is Q values of the order of 100 at 2 Hz for a path length of minimum 100 km, is necessary to conform to o bservations.