The seismic anisotropy beneath Southern Sakhalin: Evidence from the S-wave parameters of deep-focus earthquakes

The measurements of the parameters of split shear (S) waves from local deep-focus earthquakes recorded in 2005–2007 by a network of 12 seismic stations in Southern Sakhalin are presented. The results revealed the heterogeneous distribution of the anisotropic properties beneath Southern Sakhalin. The azimuths of the fast S-wave polarization beneath the stations in the central part of the peninsula are oriented along the NNW and NNE-NE directions normal to and along the Kuril Trench. Beneath the stations located along the western and eastern coasts, the azimuths of the fast S-wave polarization change their direction from NNW in the northern area to E-SE in the southern area. The highest anisotropy degree (up to 0.9–1.5%) is recorded beneath the central part of Southern Sakhalin. The maximum values of the discrepancy in the arrival time of the split S-waves are observed when the azimuth of the fast S-wave is oriented along the NNE beneath the active fault zones. The analysis of the variations of the S-wave lag time shows their weak depth dependence. The highest anisotropy is assumed in the upper layers of the medium (down to a depth of about 250 km). The results obtained for the dominating wave frequency of 1–5 Hz represent mainly the medium-scale anisotropy of the top of the studied region.