Anomalous surface waves associated with deep earthquakes, generated at an ocean ridge

Abstract

SUMMARY Broad-band seismograms observed at regional distances from deep earthquakes display large-amplitude Rayleigh and Love waves with a predominant period of about 20 s, although deep earthquakes in laterally homogeneous structures do not generate such surface waves. Since the propagation direction of the surface waves inferred from particle motion is approximately radial and the group velocity of the surface waves observed at each station is similar, it can be assumed that the surface waves have travelled close to the great-circle paths. Back projection allows us to determine the region in which these surface waves are generated. Manyof the zones from which the surface waves appear to originate are located close to the Norfolk Ridge, where crustal structure has strong lateral heterogeneity. This suggests that the surface waves are generated by the interaction of body waves from the deep earthquakes with this strongly heterogeneous crustal structure. In order to investigate the eiect of such heterogeneity in crustal structure on seismic wave¢elds, we have modelled the P^SV wave¢eld using a pseudospectral method. The results show that surface waves such as those observed can be generated by ridge structures and indicate that sedimentary layer thickness plays a very important role.We suggest that such unexpected surface waves from deep earthquakes may prove useful for delineating heterogeneity in shallow structure.