The Yamasaki fault zone in southwestern Japan currently has a high potential for producing a large damaging earthquake. We carried out a seismic tomographic study to determine detailed crustal structures for the region. The velocity model clearly images a low-velocity and high Vp/V s (high Poisson's ratio) anomaly in the lower crust beneath the Yamasaki fault zone at a depth of ∼ 15-20 km. This anomaly may be associated with the existence of partially-melted minerals. The existence of this anomaly below the fault zone may contribute to changing the long-term stress concentration in the seismogenic zone. Copyright © The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS).