| 摘要: | 2006 年12 月26 號20 點26 分及20 點34 分，在屏東恆春外海相繼發生兩起芮氏規模7.0 的地震，根據中央氣象局定位結果，地震震央分別位於墾丁地震站西南方38.4公里以及墾丁地震站西方33.1 公里。這次恆春地震也是近幾十年間以來，高屏恆春地區所發生的最大地震，並且前後兩個主震對恆春地區造成些許災情，其中於恆春南灣地區造成土壤液化災情，顯示恆春地區可能有土壤非線性反應的發生。而高屏地區(除了恆春地區)相對台灣其他地區鮮少有大地震發生，但由於這次地震的發生，提供了高屏地區詳細且寶貴的強地動資料，以供我們探討高屏地區之場址效應，尤其是強震與弱震間場址效應的差異。 本研究選取了台灣強地動觀測計畫(TSMIP)中，1993 年到2004 年高屏地區代號為KAU 之測站所收到的弱震紀錄，以及兩筆恆春強震紀錄，利用單站頻譜比法進行資料的處理與分析。首先總共選取了131 筆弱震記錄分析各測站弱震之平均場址效應，並探討共振主頻分布以及某特定頻率於各測站的放大行為，得知高屏地區的共振主頻大部分位於0.5?3Hz 之間。此後利用各測站前後兩筆恆春地震之單站頻譜比與弱震平均場址相互比對，探討強、弱震間場址效應的變化行為。由研究發現強震場址壓抑放大行為主要發生於恆春東部丘陵河谷平原與高屏沿岸地區的測站，而恆春南灣液化區非線性反應頻帶位於1?3Hz 之間，應該是由於地動加速度不夠大，造成非線性頻帶較為集中且強的結果。另外，由於液化均為局部回填土造成，因此未造成較大頻帶之非線性現象。 除了一些強、弱震頻譜比間變化較明顯的測站之外，其餘測站並不容易判斷是否有非線性反應發生。所以本研究利用定量的方法來描述強震與弱震之偏差，我們將強、弱震各頻率的頻譜比值相減，在將得到的頻譜比差值除上弱震之標準偏差值，就可以得到 強弱震頻譜比的相對變化率。依此方法我們分析1?10Hz 或1?20Hz 與3?20Hz 三個頻帶的強弱震頻譜比變化率強度，發現各測站非線性反應主要發生在較高頻帶的部份，於是我們計算3?20Hz 內非線性程度最大的頻帶之平均變化率，其結果較能突顯各地區非線性的程度，更具有實際參考價值。 至於工程上所評估之液化潛能高區是否也較容易於真實地震頻譜上有非線性反應的行為發生，本研究參考國家地震工程研究中心所列出之高屏地區之液化潛能區，與有收到恆春地震資料的各測站進行非線性定量分析之結果進行比對，發現液化潛能區主要位於高屏沿海地帶，與本研究發生非線性區域相符。而屏東內陸平原區因為地質組成相對堅硬，並且地動加速度不高，故並沒有明顯非線性反應。 由潛能圖上可以發現在河口附近之液化潛能都較高，主要是因為在此地區SPT-N 值都偏低，且含有大量的沉泥質砂及砂質沉泥，當承受較強的地震動時，極容易產生土壤非線性反應。這可能也是恆春地區之土壤非線性反應主要集中在恆春東部丘陵河谷沖積扇的主因。因為設置於牡丹、滿州的測站都位於該地區的河口或是河川曲流附近河川泥質沉積物較為豐富並且鬆軟。至於恆春西部平原區，除了南灣附近地區之外，反而沒有顯著非線性反應行為，推測是因為此區主要以石灰岩地形為主，其上覆蓋砂岩質的四溝層及現代沖積層，但是上部軟弱砂泥質沖積層不厚，導致在恆春沖積平原非線性反應不強，且靠近沿海地區主要為石灰岩與砂岩組成，地形也較高使得沖積層因而更薄，造成沿海等地如車城、貓鼻頭與鵝鑾鼻主要為放大反應。 On December 26, 2006 at 20:26 and 20:34, two successive earthquakes with the same magnitude 7.0 on the Richter scale occurred in the southeast Taiwan Strait near HengChun peninsula, with one epicenter approximately 38.4 km southwest of HEN station, and another is located in 33.1km west of NEN station.The Central News Agency is reporting that it is the strongest earthquake to hit HengChun in one hundred years. The earthquake not only caused casualties and building damages, but also damaged several undersea cables, disrupting telecommunication services in various parts of Asia. Besides, in Nanwan area of HengChun suffered some disasters of soil liquefaction, it implies that the nonlinear soil response might occur during earthquake. The data we use include weak motion records observed by KAU stations during the period from 1993 to 2004 and 2006 HengChun earthquakes, which were collected by the Taiwan Strong Motion Instrumentation Program (TSMIP) of Central Weather Bureau (CWB).We use the Horizontal-to-Vertica(H/V) Spectral Ratio Method to conduct the analysis in this study because a good rock referent site is hard to find in PingTung alluvial plain. From the analysis of weak motion of H/V Fourier spectrum, we find the magnification near the seacoast, the site effect exists from 0.5~2Hz, and that above 2Hz the contour high area move toward the inland. Follow the result of above we can infer the thickness of accretion in PingTung plain is getting deeper from the direction of northeast toward southwest. The aim of this study is to research whether the nonlinear soil response occurred or not during the HengChun earthquake. Two characters of nonlinear soil response from earthquake spectrum between weak and strong motions :(1)the dominant frequency of weak motion is larger than strong one; (2) the soil amplification factor have the same result as former. From our result, we find most of the nonlinear soil response usually occurred in soft alluvial plain near shore area of Kau-Ping plain.And the nonlinear soil response in the western part of HengChun are not obviously, on the contrary, we can see apparent nonlinear response in the eastern valley of hill-zone. The liquefaction site, Nanwan area, besides frequency band 1~3Hz, we can’t see the phenomenon of deamplification from the spectra between weak and strong motions in most frequency, because the phenomena of liquefaction was caused by the local refill soil of ground surface. |
