We determine upper mantle anisotropy just beneath active basins by comparing ScS and sScS waveform splitting. We have measured sScS-ScS shear-wave splitting with data from the SPANET and GSN seismic networks. The observed sScS-ScS shear-wave splitting suggests that there is a large lateral variation of anisotropy beneath the Lau back-arc basin. ScS- and sScS-waves which sample the southern part of the Lau basin show small splitting. In contrast, the northern part of the Lau basin, which is characterized to be tectonically active, is observed to have large seismic anisotropy with a WNW-ESE direction. The cause of anisotropy may be related to the mantle dynamics of the active basin. ¿ 2001 American Geophysical Union