Surface Waves Observed in the Western Coast Plain of the Taiwan Island During the 1999 Chi-Chi, Taiwan, Earthquake and its Aftershocks

During the 1999 Chi-Chi, Taiwan, earthquake and its consequent aftershocks, significant surface waves were observed in the western coast plain of the Taiwan island. Ground-motion records from the 1999 Chi-Chi mainshock and its five aftershocks with magnitude larger than M_w 6.0 are analyzed in this study. The ground-motions in the western coast plain, which can be classified as NEHRP E site, were dominated by the surface wave in the period range larger than 1.0 second. The distance between the sources and the instruments in the western coast plain are about 20--45 km. We study the attenuation of the surface waves with distance to source. The Rayleigh waves attenuate faster than the Love waves in the coast plain. The amplitude of horizontal Rayleigh wave is about 1.5 time of that of the corresponding Love wave on average in the near-source area. The strong-motions observed in the coast plain are a mixture of P, S, Love, and Rayleigh waves. S-wave was overlapped by the surface waves in a relative wide period range (0.5--1.5 sec). However, the body wave and the surface wave were separated quite well in some seismograms triggered by one aftershock (Sep. 20, 1999, 18:03:41.16, M_w 6.2) of the Chi-Chi mainshock. Based on a basic structural model of the studied area, we use 3D finite-difference modeling to study the special mechanism separating the body wave and surface wave. We find it is really difficult to generate the near-source seismograms with the body wave and surface wave being separated perfectly. Depth of the source and the azimuth of instrument to source are the two key elements that control the separating of the body wave and surface wave.