Numerical modelling of Lg wave propagation across the North Sea Central Graben

Abstract

SUMMARY In an effort to study the effect of strong crustal lateral variations on the propagation characteristics of Lg waves, we perform a numerical modelling of these waves at 1 Hz in a graben structure. We use a coupled mode method and a simplified 2-D model of the North Sea Central Graben. We show that at 1 Hz the energy reflected by the structure is negligible, and that on the average only 20 per cent of the Lg incoming energy is converted to waves propagating into the mantle. The energy remaining in the Lg wavetrain is subject to a strong redistribution with depth, which has an effect at the surface of the model which is very dependent on source type and depth, multiplying the surface amplitude by a factor of 0.6 for shallow sources and 0.9-1.4 for mid-crustal sources, on the average. The phase of the wavetrain is shown to be stable enough with period to avoid a destructive interference of the signal. Crossing the Graben perpendicularly or at an angle leads to equivalent results. Lg wave propagation appears to be surprisingly robust in crustal structures with strong lateral variations. The large-scale geometric features of the Graben prove insufficient to explain the strong attenuation of the Lg waves which has been observed in the North Sea area.