南沖繩海槽南端火成活動相關地震研究

Abstract

台灣東北外海的南沖繩海槽為琉球弧溝系統中的弧後擴張盆地，是由菲律賓海板塊向下隱沒至歐亞大陸板塊的產物之一，為火山及地震活動頻繁的地區。但由於此研究區域位於海域，較難進行實際觀察和資料收集工作，對海槽內所發生的火山活動狀況及分布的了解非常缺乏。因此，本研究希望藉由中央氣象局所提供的陸上地震觀測站資料，利用波形比對和頻率分析的方法來了解南沖繩海槽內火成活動相關地震的分布和活動情況，以評估其對台灣可能造成的影響。 前人研究指出，與火成活動相關地震波形通常具有不明顯的P和S波，並且在頻譜分布上具有較一般地質構造活動地震低頻的分布特性。基於這樣的原則，本研究運用人工檢視的方法，共分析了從1991年至2013年底，於台灣東北外海所記錄到的2263個地震的波形資料。為了增加訊號的可信度，本研究進行了路徑效應的分析，因為所有地震訊號均為陸上觀測站所記錄，為避免是由震源和測站間的路徑效應所產生的低頻訊號，我們在數個被認定為火成活動相關地震的震源附近也辨識出了標準的構造地震訊號，且此兩訊號均為同一測站所接收，藉此證明了被挑選出的火山訊號並非因為路徑效應影響所產生。最後，共挑選出了67個可能的火成活動相關地震訊號。 這些事件幾乎都坐落於南沖繩海槽中心軸部，或是在龜山島東南方。在海槽中心的事件深度大多淺於20公里，並有越向海槽中心越深的趨勢。而靠近龜山島的事件則都幾乎淺於15公里。這些地震的規模則多在3.5以下。在藉由與已知地質構造比較後，推測本研究所選出的台灣東北外海火山相關地震與沖繩海槽的擴張有關，擴張作用在海槽中心最強，因而越向海槽中心有越深的火成活動分布，龜山島附近的火成活動較淺則被認為是南沖繩海槽向西擴張初期的現象。而本研究在討論的過程中亦發現仍有無法克服的困難存在，即地震的定位與地震接收範圍的限制，因而導致在南沖繩海槽中依然有部分區域的地震未能完整的呈現，須仰賴更進一步且精準的地震資料進行觀測方能克服。

Located offshore northeastern Taiwan, the Southern Okinawa Trough (SOT) is a back-arc basin, which belongs to the Ryukyu subduction system. Originated from the subduction of Philippine Sea Plate to the Eurasia Plate, the SOT is characterized by a very high density of active volcanic and earthquake activities and has a very high geohazard potential for northern Taiwan area. However, as the SOT is in the offshore area, it is difficult to carry out oceanographic observations to study the tectonic structures in the area. Therefore, by using the onshore seismic waveform and frequency analysis which collected by the onshore stations data collected from Central Weather Bureau(CWB) , this study aims to understand the activity of the volcano related earthquakes in the SOT, and assess the potential impact to Taiwan area. Previous studies shown that, the waveforms of the volcano-related earthquakes are usually characterized by unclear P and S wave arrival, and relatively lower frequency with regard to most of the tectonic earthquakes. Based on these criteria, we manually analyzed the waveform of 2263 earthquakes collected from 1991 to the end of 2013 to select the possible volcanic events. To insure that low frequency waveform content resulted from the source effect, instead of the ray path, we also identify several tectonic earthquakes from the same source area and recorded by the same station as the low frequency events, In total, 67 possible volcano related seismic earthquakes were identified. Most of these events are located in the central axis of the SOT and in the southeastern Kueishan Island. The depths of the events in the SOT are mostly shallower than 20 km, and become deeper when the hypocenters get closer to the central axis of SOT, However, all the events near Kueishan Island are shallower than 15 km. The magnitude of most volcanic events magnitude is lower than 3.5. By comparing the position of volcanic earthquakes and the available geophysical data in the area, we suggest that the major volcanic activities in northeast Taiwan offshore area are related to the extension of SOT. However, we found that some fundamental problems still need to be solved when using onshore station to observe offshore seismic events, such as the location accuracy and the capability for the signal detection.