EBSD-based seismic anisotropy study of 14 naturally deformed epidote and lawsonite blueschists sourced from 8 exhumed subduction terranes. Blueschists show strong seismic anisotropy (up to 20% in AVp), largely controlled by glaucophane modal abundance and fabric (CPO) strength. Seismic anisotropy developed largely by ductile deformation of glaucophane in the blueschists through climb-accommodated dislocation creep. The distinctive seismic properties of mafic blueschists show potential for mapping the extent and deformation of blueschists along the subduction interface from the base of the seismogenic zone to the sub-arc depths (where metamorphic reactions transform the blueschists to eclogite).