On the nature of regional seismic phases-II. On the influence of structural barriers

The blockage of the Lg wave by crustal barriers such as continental margins and graben structures has long been recognized as providing a very useful tool for mapping large-scale lateral crustal variations along the propagation path. Numerical simulation of Lg-wave propagation in complex anelastic media using the pseudospectral method provides insight into the nature of the propagation process using both snapshots of the wavefield and synthetic seismograms. A variety of 2-D structures have been investigated, including the influence of sediments, crustal thickness and attenuation. Thick sedimentary basins covering a graben structure can have a major influence, since they remove Lg energy by generating P conversion and scattering—the principal mechanisms for strong Lg attenuation across a graben. The reduction of the Lg energy is reinforced by anelastic attenuation in the sediments as well as the influence of the gradually thinning crustal waveguide associated with an elevated Moho. The extinction of Lg in a sequence of explosions fired across the central graben of the North Sea can be simulated by numerical calculations for the structure derived from refraction experiments.