Finite element modelling of surface wave transmission across regions of subduction

SUMMARY propagation of Rayleigh waves across the regions of subduction of Japan and of the North Island of New Zealand is examined in the frequency domain by the finite element method. Study of the Japan region of subduction ESE of the island of Honshu has shown that, for perfectly elastic 2D models, although Love waves decrease in velocity across the region, Rayleigh waves of periods of 35-60s increase slightly in velocity. The material properties of the predominantly crustal material being subducted appear to slow the Love waves, whereas the Rayleigh waves of periods of from 35 to 60 s tend to retain their oceanic velocities, which are higher than the velocities of Rayleigh waves of those periods for the island of Honshu. Amplitude calculations have shown that, at periods of 60 s down to 10 s, Love waves are increasingly forward scattered into higher Love modes, whereas, at a period of 20s, 98 per cent of the energy of Rayleigh waves is transmitted as the Rayleigh fundamental mode. This result, together with that of Drake for the continental margin at Berkeley, California, confirms the practice of using Rayleigh waves at a period of approximately 20 s measured at seismographic stations near continental margins and near regions of subduction to estimate the magnitude of teleseisms. Study of the New Zealand region of subduction SE of Lake Taupo on the North Island of New Zealand has shown similar results to those for the Japan region of subduction. Although Love waves decrease in velocity across the region, Rayleigh waves of periods of 60 s down to 20 s increase slightly in velocity. Amplitude calculations have shown that at periods of 60 s down to 15 s, Love waves are increasingly forward scattered into higher Love modes. On the other hand, at a period of 20 s, 96 per cent of the energy of Rayleigh waves is transmitted as the Rayleigh fundamental mode. At a period of 10 s, in contrast to the results for the Japan region of subduction and for the continental margin at Berkeley, 73 per cent of the energy of Love waves is transmitted as the Love fundamental mode and 97 per cent of the energy of Rayleigh waves is transmitted as the Rayleigh fundamental mode. Amplitude and phase changes of Love and Rayleigh waves at continental margins and in regions of subduction need to be allowed for when waveform inversions of waves which have traversed these regions are made.

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