A new Measuring Technique of Time Delay of Shear Waves and its Application in Northeastern Japan

In comparison with the measurement of shear wave polarization, measuring the time delay between fast and slow shear waves is more inaccurate. What causes such a problem is the difficulty of identifying accurately the first motion of slow shear wave. Among the various methods for its measurement, the majority neglect the non-orthogonal relation between P and S wave in anisotropy. On the three-component seismograms by aperture network or local observation, the late arriving waves following P wave has an obvious interference upon fast shear wave, while the first motion in slow shear wave is affected simultaneously by the late arriving waves in P wave and fast shear wave. In anisotropic or heterogeneous medium, polarization relations between P and S waves, between fast and slow shear waves are both non-orthogonal. These lead to the difficulty mentioned above. Based on the result of synthetic simulation in polarization vector of seismic body wave, the paper presents a theoretical method which eliminates the mutual interference from each seismic phase upon the time delay between fast and slow waves in shear wave on three-component seismograms. By applying the method in analyzing the hi-net data in north-eastern Japan, the research shows an evident increase in the results of time delay between fast and slow shear waves. It also improves the method of calculating shear wave windows.