龜山島位於台灣東北方海域距離宜蘭外海約十公里處。為一個正在活動的火山島。前人研究指出其最近一次活動的年代，約在全新世，距今約七千年前。而在該區域所觀察到的硫磺噴氣孔和熱湧泉現象更反映出活躍的火山活動跡象。為了監測和研究台灣東北部海域火山活動，中央研究院地球所於2008年1月在宜蘭沿岸及龜山島上設置了一個由12部寬頻及4部短周期地震儀所組成的地震觀測網。從為期一年的觀測資料中，我們總共挑選出了425個地震並建立了一個地震目錄。初步的頻譜分析結果顯示大部分地震的頻率皆高於10 Hz，但也有少量的地震具有低於10 Hz的頻率。這次研究中，我們對觀測到的地震P波及S波的走時差進行逆推，得到了一個區域性的一維速度模型。利用這個一維速度模型及NONLINLOC地震定位軟體，我們將地震目錄中的地震進行震央的重新定位工作。結果顯示一位在龜山島的東北方地震群集，且地震深度多位於10公里以上。本研究也針對資料品質較好且芮氏規模大於3.5的地震進行地震矩張量的逆推(moment tensor inversion)並解算這些地震的震源機制。由對規模最大(芮氏規模4.4)的地震事件分析後發現，此地震擁有高百分比的補償線性向量偶極(compensated-linear-vector-dipole)且具有低於10 Hz的頻率，屬於低頻地震。綜合這次研究中觀察到的結果以及前人的研究，我們建立了一個可能的龜山島火山系統的模型。其中包含了一條流體補充的通道及一個深度約在10公里以下的岩漿庫。而地震目錄中規模最大的地震，成因可能與流體的注入有關。根據此地區地震的震源機制我們推測，此區域除了受到沖繩海槽的擴張應力影響外，還受到了一個小區域應力場的影響。最後，針對龜山島的噴發特性我們整理出可能產生的災害，像是火山灰及山崩所造成的海嘯。這些都有可能對宜蘭地區造成直接或間接的影響，值得更深入的研究。Kueishantao island is located at northeastern Taiwan, near the I-Lan coast. According to some geophysical and geochemical studies, Kueishantao island is an active volcano since the latest eruption occurred in the Holocene (7 ka). In January 2008 sixteen seismic stations were deployed along the I-Lan coast and on Kueishantao island. These instruments were equipped with both short-period and broadband sensors. The dominant seismicity around this area is high frequency (>10 Hz) earthquakes during the one year observation. Low frequency events (< 10 Hz) were also present during the same period, even though they occurred rarely. A minimum 1D velocity model was computed by inverting P- and S- wave travel times from these events. All these events were then located by calculating a non-linear, probabilistic locations using the newly derived model. The location results show a seismicity cluster northeast of Kueishantao island above the depth of 10 km. Events with magnitude greater than ML 3.5 were selected for moment tensor inversion. Four events were eventually included in the inversion process. The result yielded a high percentage in compensated-linear-vector-dipole (CLVD) component for two events including the largest (ML 4.4) event in December 17, while the other two showed high percentage of double-couple (DC) component. In general, the observations in Kueishantao indicate the possible existence of a fluid channel and magma chamber beneath the island. The event that occurred in December 17 could be interpreted as the result of fluid-injection. The mechanisms for events that occurred in this area are related to the regional and local stress field. The possible hazards of a future eruption at Kueishantao, such as ash fall and tsunami generated by a landslide should be considered by future studies.
