POLARIZATION ANOMALIES OF SURFACE-WAVES RECORDED BY A BROAD-BAND SEISMOMETER NETWORK IN HOKKAIDO, JAPAN

Polarization anomalies of surface waves suggest the existence of later al variations of isotropic and azimuthally anisotropic velocity struct ures in the upper mantle. We investigate the polarization anomalies of fundamental-mode Rayleigh and Love waves (37 earthquakes, 128 paths) at periods of 5-30s as recorded by a local four-station network of bro adband seismometers in Hokkaido, Japan. The network has been operated by the Research Center for Earthquake Prediction of Hokkaido Universit y since December 1988. Rayleigh waves coming from many back-azimuthal ranges show three types of particle motion anomalies, which are usuall y called inclined, tilted, and sloping motions. The Rayleigh anomalies observed in the data for the Vanuatu region are mainly caused by the azimuthally anisotropic structure beneath the northwestern Pacific, be cause the effects of the lateral heterogeneities on the inclined motio ns are considered to be negligible. The Love waves coming from the ear thquakes located near Oregon and California, USA, show anomalous waves in the vertical and radial components. It was expected that the waves were higher-mode Rayleigh waves. We calculate synthetic waveforms wit h normal modes for an oceanic spherically symmetric Earth model for th e August 17, 1991, earthquake off the coast of northern California, wh ich shows significant anomalous Love waves. A comparison of the synthe tic and observed waveforms suggests that the anomalous waves are not h igher-mode Rayleigh waves and require the Love to Rayleigh conversion. The conversion locations concentrate in and around the Kuril trench r egion. The Love wave anomalies may be caused by lateral variation in t he isotropic or anisotropic structures beneath the Kuril trench region .