台灣東部花東縱谷場址分析及陷波的可能性之探討

The site effect and the possibility of trap wave in the Longitudinal valley of Eastern Taiwan

林貴梅(Kuei-Mei Lin)

指導教授 ： 溫國樑

國立中央大學/地球科學學院/地球物理研究所/碩士(2011年)

摘要

台灣位處於板塊活動邊界上，地震相當頻繁，且其地下構造亦相當複雜。本研究主要利用中央氣象局所設置的TSMIP強震站所收取到的資料，針對花東縱谷進行場址分析與陷波之研究。地震資料為從2005年到2009年的加速度地動紀錄。本研究的目的是希望藉由強震儀所收到的加速度資料，經過挑選與分析，探討分析該區域各處共振主頻及場址效應，進一步期望能找尋到花東縱谷是否會對震波傳遞的路徑造成改變。
　　場址分析部分，我們利用1989年Nakamura所提出的單站頻譜比法來進行場址分析的工作，此方法簡單使用地表測站水平及垂直向的頻譜比關係，用以表示測站位置之場址效應。將研究區域內各個強震站所接收到的紀錄中，挑選品質較好的地震資料以進行分析，並進行共振主頻挑選、特定頻率H/V Ratio值之分析與討論。結果顯示在花蓮市和台東市區附近主頻值都較低，約2Hz以下，而其H/V Ratio值顯示為高區，表示該區域場址放大效應強烈；縱谷北段主頻值大致偏低，H/V Ratio值約3~8間，場址效應反應也較為強烈；縱谷中段部分主頻值較高，沖積層厚度屬全區域最薄，主要反應良好岩盤站反應：縱谷南段裡，測站靠山區則主頻值較高，靠近縱谷中央主頻則偏低。最後將結果與地形地貌、鑽井資料、以及前人的微地動研究或地電探測成果做比較與討論。整體研究區域的地表沖積層厚度為縱谷南北兩端的平原區較厚、縱谷中則略薄，而在縱谷間又以縱谷北段較厚，中段最薄，南段居中。
　　陷波分析部分為探討花東縱谷對於地震震波傳遞路徑之影響，包含震波震幅放大效應以及質點運動分析。選取2003成功地震、2006年和2008年台東外海的地震，共三筆資料做分析。由S波10~15秒後之質點運動分析結果顯示：在縱谷中有三種方向的運動模式較為顯著，其一平行縱谷方向，大致上呈東北－西南走向，另一為西北－東南走向，還有一東北東－西南西的運動方向。且這些方向的運動在三筆事件中都可以被找到，暗示地震震央位置對於該運動的方向影響不大。推測陷波在縱谷間除了平行縱谷方向傳遞，也會利用反射的行為在縱谷間傳遞，因此我們可以找到類似反射方向的運動模式。

關鍵字

陷波 ； 質點運動 ； 單站頻譜比法 ； 花東縱谷 ； 場址分析

並列摘要

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.

並列關鍵字

he Longitudinal valley ； site effect ； H/V ratio ； trapped wave ； particle motion

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