An anomalous spatial pattern of shear-wave splitting observed in Ocean Bottom Seismic data above a subducting seamount in the Nankai Trough


*Also at: Edinburgh Anisotropy Project, British Geological Survey, Edinburgh, EH9 3LA, UK.


In the summer of 2000, an ocean bottom seismometer (OBS) array was deployed along the western end of the Nankai Trough, off Cape Muroto. The array crosses an area where a buried subducting seamount was identified under the Nankai accretionary prism sediments during a multichannel seismic (MCS) reflection survey. In this paper we report the first results of shear-wave splitting (SWS) analysis of the three-component OBS data. One interesting result is a peculiar anomaly in the behaviour of shear-wave polarizations observed near the subducting seamount. We attempt to explain the observed spatial pattern in shear-wave polarizations by complex structure of stress field in accretionary prism sediments around seamount. We conclude, that the spatial pattern of SWS consistent with expected stress perturbations around seamounts, but there are several other reasons that can result in a similar type of seismic anisotropy. Finally, we have a somewhat broader look at the spatial pattern of shear-wave polarizations observed with another OBS array near the active front of Nankai Trough.