FINITE-DIFFERENCE MODELING OF LG BLOCKAGE

A P-SV finite difference method is used to calculate seismic wave propagation in the crust and upper mantle. The anelastic effect is excluded from the calculation and therefore any wavefield changes can be attributed to either geometrical or structural effects. Snapshots of displacement distributions within, and synthetic seismograms at the free surface of different models revealed that the inclusion of a water column over an appropriate section of the crust is necessary to explain the severe Lg attenuation widely observed in many parts of the world, in particular for an oceanic propagation path. They clearly show that the effect of a water column over the crust is an important factor in blocking Lg propagation. A propagation path of more than 100 km over an oceanic crust can completely block the transmission of Lg waves originating in a neighbouring continental crust. The energy of the Lg wavetrain is redistributed at continental margins. Most of the incoming energy is either converted to waves which propagate into the mantle or is backscattered to form waves which propagate in the continental crust in the opposite direction. Very little energy is presented at the expected arrival times at the other side of the oceanic crust, which is consistent with the Lg-blocking phenomena. Simplified 2-D models of the North Sea Central Graben are used to simulate Lg propagation in a laterally varying continental path. The inclusion of a water column over the crust enhances the attenuation of Lg. The deeper the water column, the greater the attenuation. The undulation of sea bottom can further enhance the attenuation of Lg.