ANOMALOUS SURFACE-WAVES ASSOCIATED WITH DEEP EARTHQUAKES, GENERATED AT AN OCEAN RIDGE

Broad-band seismograms observed at regional distances from deep earthq uakes display large-amplitude Rayleigh and Love waves with a predomina nt period of about 20 s, although deep earthquakes in laterally homoge neous structures do not generate such surface waves. Since the propaga tion direction of the surface waves inferred from particle motion is a pproximately radial and the group velocity of the surface waves observ ed at each station is similar, it can be assumed that the surface wave s have travelled close to the great-circle paths. Back projection allo ws us to determine the region in which these surface waves are generat ed. Many of the zones from which the surface waves appear to originate are located close to the Norfolk Ridge, where crustal structure has s trong lateral heterogeneity. This suggests that the surface waves are generated by the interaction of body waves from the deep earthquakes w ith this strongly heterogeneous crustal structure. In order to investi gate the effect of such heterogeneity in crustal structure on seismic wavefields, we have modelled the P-SV wavefield using a pseudospectral method. The results show that surface waves such as those observed ca n be generated by ridge structures and indicate that sedimentary layer thickness plays a very important role. We suggest that such unexpecte d surface waves from deep earthquakes may prove useful for delineating heterogeneity in shallow structure.