We show there are large temporal P-wave velocity changes in the crust of the Kanto-Tokai region in Japan from repeated observations of explosions. We find that the variation is the response of the crustal rocks to two components of stress change, the tidal stress and the secular stress accumulation. The inferred annual rate of fractional velocity change (Δv/v/year) caused by the stress accumulation in the upper crust is of the order of 10-3/year. The results indicate that the build-up of stress in the crust can be monitored by precise observations of seismic wave velocities. ¿ 2001 American Geophysical Union