Long-Period Ground Motion Observations along Two Linear Profiles from the 26 December 2006 Pingtung Offshore Earthquakes


Strong ground motions generated by the 26 December 2006 Pingtung offshore twin earthquakes were analyzed to investigate the characteristics of ground motions along two linear profiles: one lines up the twin epicenters to the Taipei basin across the western side of the Central Range which is the backbone of Taiwan, and the other from the twin epicenters to the Lanyang basin along the Longitudinal Valley (LV). The attenuation relationship of the horizontal peak ground acceleration (PGA) as a function of distance derived by Lin and Lee (2008) fits the observed horizontal PGAdata reasonably well along the two linear profiles for both events. However, the horizontal PGA and PGV of long-period (> 1 sec) ground motions are anomalously high in an area with a length of 40 km extending northwards from the entrance into the LV and then decrease with increasing distance, suggesting a funneling effect of wavefield into a major fault valley. The anomalously high PGA and PGV of the long-period ground motions may also be the amplification of the thick soil and soft rocks in the fault zone. In addition, the long-period ground motions are amplified by a factor from 2 to 10 in the Taipei basin and Lanyang basin at the northern end of Taiwan which are about 350 km away from the source region. The long-period ground motions after the S-waves observed in the two basin areas are mainly composed of surface waves on the basis of the Rayleigh-type elliptical particle motions.

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