dc.description.abstract | The Longitudinal valley is the convergent boundary of Eurasian Plate and Philippine Sea Plate. There are numerous earthquakes and the underground active tectonics are very complicated. In this study, the strong ground-motion data obtained by the Taiwan Strong-Motion Instrumentation Program (TSMIP) are used to investigate site effect and the trap wave. All the earthquakes from 2005 to 2009 reported by the Central Weather Bureau (CWB) have been collected and analyzed in this study. After data analysis, the dominant frequency and site effects in this area are discussed and hope to find the effect of the structure of Longtudinal valley on the seismic wave propagation.
Nakamura (1989) proposed that it can simply use the Horizontal-to-Vertical (H/V) spectral ratio of surface station to get the site effect in the station location easily. After basic data processing, the H/V ratios of every station are calculated, and then picked out the dominant frequency for each station. The results show that the dominant frequencies are very low (< 2Hz) in the vicinity of Hualian and Taitung cities, and the value of H/V ratio is in the contour high area. It shows the site amplification effect is high in this area. The dominant frequency is low in the northern part of the valley, and H/V ratio is about 3~8, the site effect seems also strong. The dominant frequency is high in the middle part of the valley, it shows the alluvium thickness is the thinnest, mainly is the rock site response. For the southern part of the valley, the stations near the mountain area have higher dominant frequency and lower frequency for the stations near the center of the valley. Finally, compared the results with the topography, logging, microtremor measurements, and the geoelectrical survey, it shows that the alluvium is thicker in the alluvium plain areas located in the north and south end of the valley and it is thinner within the valley. The thickness of the alluvium from thick to thin within the valley are from northern, southern, to middle part of the valley.
To discuss the effect of Longitudinal valley on the seismic wave propagation direction, three events of the 2003 Chenkung, 2006 and 2008 Taitung offshore earthquakes are selected in this study. We analyze the particle motions for direct P wave and the waves after S wave. We find that the particle motion of the wave after S wave displays a northeast-southwest movement. That seems like the wave after S wave is propagating along the valley. And we also found a NW-SE movement and an ENE-WSW movement. That reveals seismic wave propagated through the valley and trap inside. And the trap wave propagated not only along the valley but also reflected within the valley.
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