Lg-Wave Modelling For the North Sea

Theoretical seismograms for inhomogeneous 2-D media are computed via summation of surface wave modes and use of transmission coefficient matrices, in an attempt to explain the observed Lg-wave barrier within the continental structure of the North Sea. Where other scientists have been able to treat Lg-wave propagation in the North Sea for one frequency, many surface wave modes and a complicated structural model of the North Sea grabens, our method takes into account many frequencies, and is consequently limited to a few significant modes, and a relatively simple structural model. The Lg-wave barrier could be due to one or several of the following reasons: (1) energy pumping down, (2) spreading of the energy over longer duration, (3) geometrical scattering, (4) reflection, or (5) anelastic absorption. Our computations explain the observations through (1), (2) and (5).