Serpentinized peridotite is detected seismologically by mapping Poisson's r
atio beneath the Kanto district, central Japan, because serpentinite has a
higher Poisson's ratio than other rocks in the crust and upper mantle. We d
etermine three-dimensional P and S wave velocity models using travel time t
omography and then construct a three-dimensional map of Poisson's ratio. Th
e models show low velocity anomalies near the upper boundary of the slab in
a depth range from 20 to 70 km. In the low velocity region, there is an ar
ea with Poisson's ratio greater than 0.3 at depths of 20-45 km, which we at
tribute to serpentinized peridotite. The rest of the low velocity region is
interpreted as descending oceanic crust with a Poisson's ratio of about 0.
25. The high Poisson's ratio area is associated with low seismicity and wea
k interplate coupling, which is consistent with the ductility of serpentini
te.