A QUANTITATIVE STUDY OF SEISMIC ANISOTROPY IN THE YAMATO BASIN, THE SOUTHEASTERN JAPAN SEA, FROM REFRACTION DATA COLLECTED BY AN OCEAN BOTTOM SEISMOGRAPHIC ARRAY

The anisotropy of the uppermost mantle beneath the Yamato Basin in the southeastern Japan Sea was studied quantitatively on the basis of data acquired during the DELP-85 Wakashio Cruise. Detailed seismic experiments were conducted along two seismic profiles, one of which was parallel to the strike of the oblong basin (Line A) and the other perpendicular to it (Line B). The inversion of the traveltime data of PmP (reflected from the Moho) provides a reliable constraint on the Moho depth. This inverted Moho and a previously reported 2-D model were used to isolate the effect of the lateral variations on the value of anisotropy. We obtained azimuthally good coverage of Pn (refracted in the uppermost mantle) data by combining shot points and observation points located on different lines. From these Pn data, the uppermost mantle velocities for each azimuth were determined by iterative ray tracing. A study of these results shows that the azimuthal anisotropy of Pn velocity is 9.1 per cent. The maximum velocity occurs at about 138-degrees clockwise from north, which is perpendicular to the strike of the Yamato Basin (Line A). The magnitude of the anisotropy, 9.1 per cent, is nearly the largest value ever observed in the ocean. This high anisotropy may be due to the very fast spreading rate. The azimuthal dependence of the P- and S-wave velocity in the lower crust is not significant.