Seismic Evidence of Anisotropy in the Yamato Basin Crust

Abstract

A seismic anisotropy was found in the upper crust of the northern Yamato Basin, Japan Sea, during the downhole experiment in 1989. An ocean broadband downhole seismometer (OBDS) and nine ocean bottom seismometers (OBSs) were installed during Ocean Drilling Program (ODP) Leg 128 in and near Hole 794D. Traveltimes of P-wave recorded by the OBDS, and nearest three OBSs, which were corrected due to offset ranges from a shot to the receiver and thicknesses of the sedimentary layer, were fitted by an anisotropic velocity model: the azimuthal dependence of the velocity of P-wave propagation is expressed in terms of sinusoidal functions up to the sixth-order terms. Both the anisotropic terms, the second- and the fourth-order terms, and the laterally heterogeneous term, the first-order term, were significant in the model. The direction of P-wave propagation was measured by the OBS array data and found to deviate from the sagittal plate. The traveltimes and the direction of propagation both indicate that P-wave propagates faster east-west than north-south by 4%-7%. The anisotropy in the basement layer is interpreted to be due to stress induced opening of cracks.