Polarization anomalies of Love waves observed in and around Japan

Polarization anomalies of Love waves, sometimes called quasi-Love waves, are likely to be caused by lateral variations of azimuthal anisotropy in the upper mantle. Polarization anomalies of Love waves from the 9 October 1995 earthquake near the coast of Jalisco, Mexico, have been recorded by the Pacific21, IRIS, and Japan Meteorological Agency networks of broadband seismometers in and around Japan. The Love-to-Rayleigh conversion areas for the clearly detected quasi-Love waves are located by using the group velocities of the observed surface waves. In the cases of the stations in the northern part of Japan and Philippine, the locations of the Love-to-Rayleigh conversions mostly concentrate near the trenches, and suggest that the properties of the azimuthal anisotropies in the upper mantle have lateral variations across the trenches. We infer that the lateral variations of the azimuthal anisotropies may reflect the changes of the mantle flow due to the subducting slabs. The Love-to-Rayleigh conversion areas for the other stations mainly concentrate near the Emperor seamounts and the Mid-Pacific mountains. Several results of surface wave tomographic studies show that the azimuthal anisotropies in these regions are much weaker than those in the central part of the Pacific along the paths. The lateral variation in azimuthal anisotropy may cause the Love-to-Rayleigh conversions.