台灣東北地區非火山的微震訊號分析與研究

The Analysis of Non-volcanic Tremor in Northeastern Taiwan

黃敬棠(Ching-Tang Huang)

指導教授 ： 陳朝輝

國立中正大學/理學院/地球與環境科學系地震研究所/碩士(2013年)

摘要

本研究使用波形吻合比對法，藉由2011年3月11日發生於宮城外海規模9.0的地震事件之主震及其餘震的表面波訊號偵測位於台灣中央山脈北段地區被觸發的非火山地區的微震訊號（Non-volcanic tremor, NVT)。我們使用CWBSN及台灣寬頻地震觀測網（BATS)位於台灣東北部地區的測站觀測並挑選清楚的低頻地震(Low-frequency earthquakes, LFEs）當作樣本事件，與311大地震發生後的一小時內的資料進行波形交對比（cross-correlation），以找出其它無法以人工方式挑選的LFEs。經由雙差分重新定位，共計偵測到84個LFEs，其深度範圍介於13-32公里之間，即莫荷面與一般地震活躍區域之間，其分布區域亦位於歐亞大陸板塊與菲律賓海板塊碰撞帶。NVT大多數係由一系列的LFEs所組成，通常發生的位置通常會在隱沒帶及板塊邊界等構造型態。對於NVT的生成受很多因素影響，例如：孕震區所收到的表面波振幅、主震震波入射角度及震波頻率等皆有可能造成NVT被觸發。為了了解LFEs附近環境的地下速度構造，我們利用初始設定的一維速度模型以及中央氣象局即時觀測網(CWBSN)分布於台灣東北部地區的測站之走時資料進行逆推反演而得到台灣東北部地區的三維速度構造，從結果可發現LFEs分布於Vp/Vs梯度變化較大的區域；另外，利用dlnVs/dlnVp參數分析LFEs附近環境的地下速度構造亦可得到較高異常值，推測其位於下部地殼因脫水作用而導致該區域相對於周圍環境有較高的含水成份，易促使LFEs的生成。

關鍵字

非火山的微震

並列摘要

We adopted a matched filter technique to detect non-volcanic tremor (NVT) signals beneath the Northern Central Range in Taiwan during the 2011 Mw9.0 Tohoku earthquakes’ mainshock and aftershocks. We selected stations in the Broadband Array in Taiwan for Seismology (BATS) and Central Weather Bureau Seismic Network (CWBSN) in the northeastern Taiwan, and picked up clear low-frequency earthquakes (LFEs) as the sample events. In this study, the sample events compared with the waveforms that after one hour of the Tohoku earthquake by using the cross-correlation technique to search another LFEs . After initial located and using a double-difference algorithm, we get 84 LFEs and the depth range from 13 to 32 km is among the active seismic zone and the Moho interface. In addition, the LFEs also distributed at the collision zone where is between the Eurasian Plate and the Philippine Sea Plate. Due to the NVT mostly consists of many LFEs, and usually observed at subduction zones and other major plate boundary faults. For the conditions of the NVT's generation, we conclude several factors, such as: the surface wave's amplitude, seismic waveforms’ frequency and incidence angle, these factors may trigger the NVT. In order to understand the structure around the LFEs, we also invert the 3D tomography structure by using the initial 1D velocity structure and the travel time data from the CWBSN in the northeastern Taiwan. From the analysis of Vp/Vs profiles, we can find that the LFEs distributed at a larger gradient area. Otherwise, by using “dlnVs/dlnVp” factor can help us to analyze the zone around the LFEs, and the results exhibit higher dlnVs/dlnVp ratio nearby the LFEs. Therefore, we can speculate that the region with the higher fluid composition in the lower crust maybe indicated to dehydration and increase the potential to trigger the LFEs.

並列關鍵字

Non-volcanic tremor

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