Seismic velocity reductions caused by the 2003 Tokachi-Oki earthquake

We use four repeating earthquake sequences located near Hokkaido to identify velocity changes caused by the M_W 8.0 2003 Tokachi-Oki earthquake. Using a moving window cross-correlation technique, we identify delays in the arrival time of seismic waves that accumulate linearly with time into the seismogram. This behavior is indicative of multiple scattering within a medium where the seismic velocity has been reduced. For all of our earthquake-receiver geometries, we find evidence of significant velocity reductions close to the receiver. The correlation of the size of the velocity reductions with both strong shaking and site characteristics suggests that these velocity reductions are caused by damage to near-surface materials created by nonlinear strong ground motion. For earthquake/receiver geometries where the seismic waves cross the Tokachi-Oki rupture zone, we identify particularly large reductions in velocity as a result of the earthquake. For these geometries, we believe that the rupture zone of the Tokachi-Oki earthquake or the shallow crust above it represents a second region where seismic velocities are reduced as a result of the main shock.