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科技部與中央研究院聯合新聞稿:全球率先以衛星資料探索地殼岩石強度之時空變化

在科技部及中央研究院長期的支持下,許雅儒博士研究團隊透過1999年九二一集集大地震震後長達十年以上由全球衛星觀測定位系統(GPS)量測的地表位移時間序列,解析臺灣造山帶下部地殼變形特性。為全球首次成功的以衛星資料研究下部地殼岩石的變形特性,顯示應力和應變速率呈現非線性關係,有別於過去所認為的線性關係;此結果暗示著岩石的強弱在地質時間尺度上有橫跨好幾個數量級的變化,而大地震過後短期內大量及快速的變形對臺灣造山運動亦有顯著的貢獻。研究成果於今年2月27日刊登在美國科學發展協會下的頂尖學術期刊《科學前緣》(Science Advances)。

 

九二一集集大地震是臺灣歷史上最嚴重的自然災害之一,在一夜之間奪走數千條生命,更造成後續高達數千億元的經濟損失。因其對社會的衝擊甚鉅,長期以來一直是地球科學界投入大量人力與物力的研究焦點,期望對地震孕震過程有更多的了解,並在未來達到地震防災與減災的目的。

 

大地震過後,包含中央研究院在內的許多研究機構如交通部中央氣象局、經濟部中央地質調查所、以及內政部國土測繪中心開始在全臺灣布設密集且連續的全球導航衛星觀測網,長期監測地殼變形行為,使集集地震成為世界上被最完整紀錄的地震之一。大量且長期的觀測資料提供了高度的時間與空間解析度,讓本項研究不僅是國內首次應用衛星觀測資料來分析下部地殼的塑性變形行為,更是全球率先以衛星資料直接探測造山帶岩石圈的流變特性。

 

岩石圈強度及其隨時間的演化在地震災害風險評估中是一個重要的指標參數。然而,岩石圈的強度受到如應力、岩石組成、結晶粒徑、含水量、岩壓與溫度等諸多物理條件的控制,使得了解岩石在自然環境下的受力變形行為一直是一大挑戰。為了克服此一障礙,許博士團隊和臺灣大學、中央大學、新加坡南洋理工大學及美國南加州大學合作,開發新的演算方法並利用大地測量資料直接計算地震過後地殼的應變分佈,深入剖析臺灣造山帶下部地殼的變形特質。

 

在過去傳統的震後變形研究,岩石圈內應力與應變速率之間的關係通常被認為呈線性相關。然而,包括在美國加州、蘇門答臘、日本以及智利,隨著越趨完整的觀測資料,越來越多的證據顯示應力與應變速率之間的比值隨時間並非一恆定值。許博士團隊發現,集集地震後十數年的觀測資料無法由單一的線性變形來解釋。取而代之的是,集集地震的震後變形很有可能涉及地殼的暫態蠕變行為,以及後續的非線性變形。這樣的結果顯示,地殼內部的塑性變形是由礦物晶體的錯位攀移所控制的運動,這種變形容易在地殼中產生許多小裂縫,並使得礦物結晶朝著特定方向排列,而這樣的變形特徵和地震學觀測資料不謀而合。

 

除此之外,結合實驗室岩石試驗測量到的岩石蠕變參數,許博士團隊估算由臺灣西部平原到中央山脈的地溫梯度,大致由20°C/km向東遞增為30°C/km,與過去利用地質與地球物理方法所推算的數值相去不遠。這項驚人的發現說明,雖然實驗室環境與大自然之間在時間與空間尺度上存在巨大的落差,岩石的流變特性似乎並沒有因為尺度差異的影響而產生大幅度的偏差。此項研究表明大地測量資料有可能彌合實驗室以及岩石圈尺度之間的差異,用來研究地殼流變學和大地構造的演化過程,進而估算合適的物理參數做為未來地球動力學建立模型的參考依據,為地震孕震過程及臺灣造山運動提供新的見解與思維。

 

發表資訊:

Tang, C.-H., Y.-J. Hsu, S. Barbot, J. D. P. Moore, W.-L. Chang, (2019), Lower-crustal rheology and thermal gradient in the Taiwan orogenic belt illuminated by the 1999 Chi-Chi earthquake, Sci. Adv., 5, eaav3287.

 

研究成果聯絡人:

許雅儒 研究員

中央研究院 地球科學研究所

電話:(02) 2783-9910 #1415

Email:yaru@earth.sinica.edu.tw

 

聯絡人(1):科技部自然科學及永續研究發展司陳慧真博士

電話:(02)2737-7445

E-mail:huichen@most.gov.tw

 

聯絡人(2):中央研究院秘書處媒體小組郭姵君科員

電話:(02) 2789-8821

E-mail:deartree@gate.sinica.edu.tw

 

The world’s advanced discovery of using satellite data to explore the spatio-temporal evolution of crustal rheology

 

Under the long-term support by the Ministry of Science and Technology and Academia Sinica, Dr. Ya-Ju Hsu and her research group investigated the lower-crustal rheology beneath the Taiwan orogenic belt using the Global Positioning System (GPS) observations more than a decade following the 1999 Chi-Chi earthquake. Their result manifested the non-linear relationship between stress and strain rate in the lower crust, against the linear relationship often considered previously. The short-term modulation of lower-crustal strength during the seismic cycle throughout the geological time may largely facilitate the mountain building. This work was published in Science Advances, the well-known multidisciplinary open-access scientific journal under the American Association for the Advancement of Science (AAAS), on 27 February 2019.

 

The strength of the lithosphere and its temporal evolution play a critical role in seismic hazard and risk assessment. However, the uncertainties are controlled by the situ physical conditions, such as stress, rock composition, grain size, water content, confining pressure, and temperature. Therefore, to understand how rocks deform under stress in their natural settings is always challenging. To overcome this issue, Dr. Hsu and her group cooperated with National Taiwan University (NTU), National Central University (NCU), Earth Observatory of Singapore (EOS) and University of Southern California (USC) to develop a new algorithm that probes the deformation in the deep crust directly using geodetic data. The new approach allowed them to explore the rheology of the lower crust with the strong constraint provided from numerous and dense GPS data.

 

Dr. Hsu and her group found that a uniform linear rheology cannot explain the GPS data following the Chi-Chi earthquake. In contrast, they suggested that transient creep followed by a non-linear steady-state creep has dominated the behavior of accelerated viscoelastic flow in the lower crust after the Chi-Chi earthquake. This indicates that the ductile deformation involves the movement of dislocations through the crystal lattice of the material, which is in accordance with the presence of seismic anisotropy.

 

Incorporating the laboratory data and associated uncertainties, inferred thermal gradients suggest an eastward increase from about 20°C/km in the Coastal Plain to 30°C/km in the Central Range of Taiwan. Although the inferred thermal gradients are subject to uncertainties, they are consistent with a number of independent studies. The new model reconciles geodetic observations, seismic anisotropy, and heterogeneous thermal gradients in the Taiwan orogenic belt. The rheological parameters may be integrated in geodynamic models of the Taiwan orogeny to incorporate the short-term effect of the seismic cycle.

 

Reference

Tang, C.-H., Y.-J. Hsu, S. Barbot, J. D. P. Moore, W.-L. Chang, (2019), Lower-crustal rheology and thermal gradient in the Taiwan orogenic belt illuminated by the 1999 Chi-Chi earthquake, Sci. Adv., 5, eaav3287.

 

Link

http://advances.sciencemag.org/content/5/2/eaav3287

 

Media Contact:

Dr. Ya-Ju Hsu, Institute of Earth Sciences, Academia Sinica

(Tel) +886-2-2783-9910 ext,415,  yaru@earth.sinica.edu.tw    

 

Ms. Pei-Chun Kuo, Media Team, Secretariat, Central Administrative Office, Academia Sinica

(Tel) +886-2-2789-8821, deartree@gate.sinica.edu.tw

 

Dr. Hui-Chen Chen, Program Manager, Department of Natural Sciences and Sustainable Development, Ministry of Science and Technology

(Tel) +886-2-2737-7445,  huichen@most.gov.tw

 

圖:建於南投七彩湖的GPS衛星觀測站(蘇宣翰先生提供)

Figure: The GPS station at Chi-Tsai Hu (the rainbow lake), Nantou. Credit: Mr. Hsuan-Han Su

更新日期 : 2019/03/28