中央山脈南段地震活動特性之探討

The study of Earthquake activity characteristic at the South section of Central Range

陳若豪(Jo-Hao Chen)

指導教授 ： 顏宏元 ； 張建興

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

從中央氣象局地震觀測網(CWBSN)發布的地震目錄中，得知在中央山脈內，有一地震活動較低的地區，大致位於緯度23.5度上下。然而台灣地底下側向變化劇烈，地體構造相當複雜，為了能夠對此區地震活動有更進一步的了解，本研究使用7個高解析度的地震儀，希望能夠更了解此區域的地震活動分佈。本研究將研究區域定為中央山脈南段東經120.65-121.55度，北緯23.0-23.9度。在不易得知地殼速度構造之情況下，勢將引起地震定位的誤差。本研究前半段將人工震源的資料(TAIGER)帶入三種不同的速度模型做比較，分別為中央氣象局一維速度模型(CWB1D)、Wu et al.(2007)發表的台灣三維速度模型(Wu3D07)以及謝獻祥(2009)發表的密度轉速度構造(Hsieh3D09)，試圖提升地震定位的精確程度。本研究使用CWB1D模型和Hsieh3D09模型做為地震定位的速度模型。本研究後半段則使用中央研究院地球科學研究所於2008年在花蓮佳心架設的寬頻地震站，以及佳心站周圍6個中央氣象局地震觀測網的地震站，並提高記錄器的解析度(24位元)。對此區2010年7月的地震資料重新搜索，一共挑出354個CWBSN沒有記錄的地震，其中至少4個地震站有地震記錄或是超過6個波相才會被選取，並且ERZ和ERH小於5公里。由一維定位的結果，地震大都集中於利稻站、富里站附近，以及花東縱谷。和同時間CWBSN記錄的地震分布大致相符，但本研究玉里以北地區地震活動度較高，且呈一條帶狀的地震群。使用一維定位和三維定位的結果相比，兩個結果的地震分布大致相符，但三維定位的各項誤差量較小，且有系統性的偏移。此外使用佳心站也大幅提高本研究地震定位的數量及品質。加入佳心站後，本研究地震數量由161個提升至354個。而本研究有32個只有佳心站收到的地震事件，其P波、S波非常完整。從測站幾何來看，可能是來自中央山脈的地震。此外，王郁如(2009)所做的三維Q值模型指出中央山脈無震區屬於高衰減值地區，本研究結果於該區還是有地震發生。最後，
利稻站附近的地震剛好位於磁力異常區(Yen et al,.2009)，前人研究也曾於南橫發現火成岩出露，顯示該區地底可能為高溫區，此區地震深度大約集中於5~20公里，顯示該區地震的發生可能與高溫的活動有關。

關鍵字

無震帶 ； 地震活動 ； 中央山脈南段 ； 佳心站

並列摘要

From the earthquake catalog of CWBSN, we infer that there exists a low seismicity zone beneath Central Range, around 23.5°N. However, the lateral variation of tectonic beneath Taiwan is very large. To know more about the seismic activity in this area, we use 7 high-resolution seismographs in this study to monitor the distribution of seismicity here. The study area is set in the southern part of Central Range, 120.65°E-121.55°E, 23.0°N-23.9°N. On the condition that unknowing the real velocity of seismic wave in the crust part, there must be some errors during locating an earthquake. For the first part in this study, we use the seismograms of artificial sources(TAIGER) and applying them into three kinds of velocity model, 1-dimension velocity model from CWB(CWB1D), Taiwan 3-dimension velocity model(Wu3D09) from Wu et al.(2007) and Hsieh(2009) 3-dimension velocity model transferred from density model(Hsieh3D07). To improve the accuracy of locating earthquake, we compare the results of three kinds of model and finally choose the CWB1D and Hsieh3D07 model as our velocity model while we’re locating earthquakes. And the other part in this study, we use a broadband seismograph, build by Institute of Earth Scienceof Sinica in 2008 in Haulien(Chiahsin佳心) and the other 6 stations, build by CWB, around Jiashin station, to improve the resolution of recorder(24-bit). To re-find earthquakes during July, 2010, we totally pick 354 earthquakes up, which are not recorded by CWB. In the condition that while an earthquake is recorded by at least 4 stations or there are more than 6 phases, and the ERZ and ERH are less than 5 km, we pick the earthquake up as our new earthquake catalog. From the result of 1-dimension velocity model, most of earthquakes center on Lidau(利稻), Fuli(富里) stations and northern part of Longitudinal Valley. Comparing the distribution of earthquakes during the same time from records of CWB, the pattern is similar. But in this study, it shows that the seismicity in north part of Yuli(玉里) station is more higher and as a strip-shaped like. In the other way, using the new earthquake catalog, comparing the results of 1-dimension and 3-dimension velocity models, it shows that there are similar to each other. But the misfit of 3-dimension velocity model in every aspect is much smaller and has a systematic offset. On the other hand, using the data from Jiashin station is more effective in locating earthquakes for quality and quantity aspects. If we remove the data of Jiashin station, we only have 161 earthquakes, less than half of original earthquakes; besides, in this study, we have 32 complete P-wave and S-wave phases, only detected by Jiashin station. From geometry of stations, these earthquakes may come from the Central Range. Furthermore, discussion with prior researchers, Wang (2009) indicated that the aseismic zone in Central Range belongs to high attenuation zone by using 3-dimension Q-value model. This study shows that this area should have earthquakes. In addition, earthquakes around Lidau station are just located on the area of magnetic anomaly (Yen et al., 2009). Previous research also showed that there had outcrop of igneous rock in Southern Cross-Island Highway. It means that beneath the surface, it may have a high-temperature zone. Depths of earthquakes in this area usually around in 5-20 km, infer that in this area, the occurrence of earthquakes may have related with the high-temperature activities.

並列關鍵字

Earthquake activity ； aseismic zone ； Chiahsin

參考文獻

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