Mapping of crustal heterogeneity in the North Sea basin via the propagation of Lg-waves

Abstract

Summary. The propagation of Lg-waves within the continental area in and around the North Sea basin shows strong dependence on the path between source and receiver. Paths lying within the British Isles and Norway show very clear Lg phases, but paths which cross the graben zone lying in the middle of the North Sea basin have very weak Lg. Over 150 paths have now been studied across the region and the character of the Lg-wave has been described by comparison with the size of the Sn phase. For shallow events this gives a stable measure of the efficiency of Lg propagation. The regions which appear to block the transmission of Lg are quite localized in the middle of the North Sea, in the region with graben structures, and extend on into The Netherlands. A weak zone of poor transmission appears to be associated with the Oslo graben. With the relatively dense areal coverage provided by paths crossing the basin with a wide range of azimuths, it is possible to attempt to invert for the pattern of crustal heterogeneity which gives rise to the observed character of the Lg propagation. An interative scheme has been devised to find the propagation properties in cells of a 1° X 1° grid, and a well-defined map of the heterogeneity is obtained. The strongest heterogeneities correlate very well with the major tectonic features of the Viking and Central Grabens running north–south through the basin. Where the path coverage is greatest, the position of the heterogeneity can be tightly constrained and lies within a rather narrow zone around 100 km wide. Despite the number of paths employed it is not possible to obtain this level of resolution over the whole region. Seen in the light of theoretical studies of Lg-wave transmission, the heterogeneity map gives an indication of the strength of horizontal gradients in seismic properties in the crust. These are most likely to be associated with crustal thinning beneath the graben structures.