本研究區域為蘭陽平原，想藉由Nakamura（1989）所提出單站頻譜比法分析微地動資料，來探討當地場址特性，再進一步和中央氣象局強震站所收到地震資料做比較，結果可作為將來建築物耐震設計及土地使用規劃之參考。此外，也以地震儀陣列量測微地動資料，利用頻率-波數法來求得表面波頻散曲線，再逆推得淺層沈積層S波速度構造。蘭陽平原微地動測量，由各頻率放大倍率圖發現「彎月形」高區在頻率0.3Hz出現，0.5Hz時幾乎分佈在整個蘭陽平原，0.6Hz時逐漸縮小， 1.5Hz時不見，取而代之的是山邊高區，以及宜蘭－羅東連線東側的線性帶狀高區貫穿蘭陽平原，且越高頻有逐漸往海邊移動的情形。宜蘭市及羅東鎮市區，在頻率1.0Hz以下是位於高區，以上則位於低區。主頻分佈圖結果顯示宜蘭－羅東連線東側線性帶狀高區，貫穿蘭陽平原，主頻在1Hz?2Hz，連接了蘭陽溪出海口北邊的高區，及冬山鄉向北凸出的高區，這兩高區主頻在2Hz左右。宜蘭市及羅東鎮市區主頻在0.3Hz?0.5Hz，礁溪鄉市區主頻在1Hz左右。蘭陽平原微地動陣列測量發現，在兩速度界面間地層比較特別，此層S波波速由海邊往內陸從變化不大，轉變成有明顯低速層，再變為逐漸增加。南北向剖面發現，由海邊逐漸往內陸移動時，在蘭陽溪出海口北側，速度界面二深度，從深0.81 km逐漸變淺到0.58 km，再逐漸變深到0.71 km，和微地動主頻圖所看到現象相同。最後，希望能整合以上兩種結果，得到蘭陽平原較詳盡速度構造，以解釋蘭陽平原所觀測到之場址效應，同時此結果亦可作為未來二維、三維理論地動模擬之基礎。 The purpose of this study is trying to understand the site effect and to compute near surface S wave velocity structure of the Lan-Yang plain. We used very dense microtremor measurements to study the site effect, and used portable seismic array to estimate the near surface sediment's S wave velocity structure. In this study, we performed 354 microtremor measurements, and analyzed site effect by applying single station spectral ratio method (Nakamura, 1989). After that, we conducted 9 portable seismic arrays. The analysis procedures can be categorized as (1) Frequency- Wavenumber analysis to get the dispersion curve, (2) to search the good velocity models using genetic algorithm, and (3) to refine the SURF and GA searched models using traditional surface wave inversion technique. From the predominant frequency of microtremor measurement, we can figure out the alluvium thickness become thicker toward the sea. But there were some higher region (thinner alluvium) beside the east of I-lan and Lotung cities, and north of Lan-Yang River cutlet. The portable seismic arrays also showed similar results. This research result was slightly different from previous study.
