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Seismological evidence on characteristic time of crack healing in the shallow crust

A continuous observation of shear wave splitting for 17 years reveals a unique temporal variation in seismic anisotropy in the shallow crust induced by a larger earthquake (MW5.7) beneath the Tokai region, Japan. The delay time between the fast and slow wavelets coseismically increased and then decreased back to the pre‐event value. The duration of the decreasing stage is about two years. The decrease may indicate crack healing in the upper 10 km of the crust. We approximate the temporal variation in the delay time as a function of logarithm of time, which is concordant with healing phenomena of cracks reported by laboratory experiments. The observation indicates that healing of cracks in crustal rocks is complete in approximately two years.