Site Resonance Observed at Yan-Liau Station in Hualien, Taiwan

An earthquake which registered magnitude 6.9 on the Richter scale occurred on 13 December 1990 in the south of Hualien. A temporary array which consisted of 15 triaxial digital accelerographs had been deployed in the epicentral area so as to monitor aftershocks. Roughly 600 earthquakes triggered this array during the deployment of three months and 162 of them were detected at Yan-Liau, one station of this temporary array. Most of its accelerograms have exhibited evident resonance phenomenon. Polarization analyses of 30 well-recorded accelerograms have been conducted in this paper so as to examine the resonance effect. The predominant frequencies of P waves have been indicated by the results to have a wide range of distributions which span from 8 to 18 Hz, while most of the S waves and their following resonance waves fell in a narrow frequency band between 7 and 8 Hz. Although resonance phenomenon is significant at Yan-Liau, the resonance waves do not polarize in a preferred orientation and no particular connections have been found among the polarization directions of the P, direct S and resonance waves.