臺灣非火山長微震之活動特徵及可能之孕震構造和機制

Characteristics and possible mechanisms of non-volcanic ambient tremor in Taiwan

戴心如(Tai Hsin-Ju)

指導教授 ： 陳卉瑄

國立臺灣師範大學/理學院/地球科學系/碩士(2016年)

https://doi.org/10.6345/NTNU202203747

摘要

非火山自發型長微震(ambient tremor)常見於隱沒帶孕震區深部，是種介於一般地震的快速破裂與無震滑移之間的能量釋放形式，這種活動所輻射出的地震波非常微弱而持續時間長、，釋放的地震矩能量可對等於規模六的地震事件，對於了解隱沒帶應力傳遞提供了重要資訊。
本研究首先比較不同偵測手段的長微震事件本質之差異，以進行目錄整合、並提供最佳偵測之標準化流程。利用(1)多測站波形具到時一致的能量脈衝；(2)初步定位結果收斂；(3)無體波特徵及；(4)持續時間大於60秒為條件，本研究設計了半自動化的長微震偵測系統，偵測出前人目錄四倍以上事件量。藉由分析新的2007-2012長微震目錄，我們發現長微震事件比過去研究發現之群聚現象更向北方延展，對應到一個東南北-西南走向、往東南傾的可能構造，深度約在20-40 公里間。長微震事件被發現多發生於潮位升高時，並且與理論潮汐的應力模型相比，亦在潮汐引發剪應力增加時處有最高相關性，指示長微震的發生可能受潮汐應力觸發。我們並利用長微震和潮汐的相關性，尋找長微震對應的構造面，最佳影響面及錯動型態為(strike,dip,rake)=(60°,40°,90°)。同時我們也發現，長微震具有的年週期活躍特性，與其他連續資料相比，對應到高潮位、高降雨、高地下水位、低氣壓環境，說明水氣的季節性變化與深部的長微震有可能有相同機制，或是具有因果關係。在空間特徵上，台灣的長微震位於中央山脈南段低速區域，其向上延伸有一低電阻、高含水的無震區；長微震本身亦處於高Vp/Vs與高地熱梯度區域，暗示區域可能因高度變質脫水作用而富含流體。

關鍵字

非火山長微震 ； 慢地震

並列摘要

In the seismogenic zone, ordinary earthquakes experience rapid slip on a fault with a few ~ tens of second durations. While below the seismogenic zone, slow rupture propagation and/or low slip rate can be also taking place with a wide range of duration from minutes to days. This class of slow-slip events, characterized by noise-like, long-lasted signals with consistent arrival at various stations is called tremors. It is believed to play an important role in the assessment of regional seismic hazard and plate boundary processes. Unlike subduction zones and transform faults where the ambient tremors occur on a known fault plane, the tremors in Taiwan are located in a place where no active faults have been identified. To better understand the possible generation mechanism, carefully examining different detection schemes for a more complete tremor catalog is necessary.
In this study we adopt the identification scheme similar to Ide et al. (2015) but applied slightly different techniques: (1) Higher waveform cross-correlation coefficient (>0.6) (2) careful visual inspection for excluding local earthquakes and short-lasted event (duration < 60 s) (3) Signal to noise ratio higher than 1.2 and lower than 30 (4) No spatio-temporal clustering technique used. We also develop an approach to systematically determine the duration of tremor events. As a result, 1893 tremor events with duration ranging from 60 s to 2216 s are found during the period of 2007-2012. They are mainly located underneath southern Central Range, forming a NS striking and SE-dipping ellipsoid structure at a depth of 15-45 km. The up-dip extension of this tremor structure reaches an aseismic zone under the western flank of Central Range at shallow depth, where is an area characterized by high heat flow, low Vp and Vs anomaly. Power spectrum analysis of tremors reveals several sharp peaks that are consistent with that from tidal data, indicating strong tidal modulation. The most significant annual periodicity is also found in ground water and strainmeter data, indicating an influence of Sun on hydro-geological activity. Tremor activity exists a clear tidal modulation of the annual, semidiurnal, and diurnal constituents, ~67% tremors happened while the observation of tide level higher than average, ~83% tremors happened while tide level rise. This phenomenon indicates solid earth and ocean tide may influence tremor activity. With this tidal-tremor correlation, we found a best response structure of tidal induced shear stress: (strike,dip,rake)=(60°,40°,90°), which is similar with both ellipsoid structure and the mechanism of very-low frequency event in previous study.

並列關鍵字

slow earthquake

參考文獻

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