由大地測量探究地震周期間岩石圈之流變性質

Probing Rheological Properties of the Lithosphere over Seismic Cycles from Geodesy

唐啓賢(Chi-Hsien Tang)

指導教授 ： 許雅儒 ； 吳逸民

國立臺灣大學/理學院/地質科學研究所/博士(2021年)

https://doi.org/10.6342/NTU202101877

摘要

在地震周期之間，岩石圈內部的應力與應變隨著時間的積累與消散和岩石圈的流變性質息息相關。雖然藉由岩石力學潛變試驗可以了解在實驗室條件之下的岩石流變性質，現地自然條件之下的岩石流變性質仍然需要透過其他方法來加以釐清。有別於實驗室岩石潛變試驗，大地測量學觀測如全球導航衛星系統（Global Navigation Satellite System, GNSS）可以長時間追蹤由大地震同震應力擾動所引起的暫態地殼變形，藉以反推岩石在現地條件下的流變性質。為了模擬大地震後的震後變形，本研究設計了新穎的運動學反演模型，由地表觀測位移同時反算同震斷層面上的震後滑移以及岩石圈中的非彈性應變張量。新方法有別於常見的正演模型，不需要事先決定岩石圈的流變機制或參數，而是利用觀測資料來直接探索岩石圈的流變性質。本研究中採納新方法研究兩個震矩規模大於7的地震所引起的震後變形，分別是1999年臺灣集集地震震後14年的震後變形，以及2010年墨西哥El Mayor Cucapah地震震後8年的震後變形。兩次地震的震後變形皆可以由同震滑移面之延伸上的震後滑移以及下部地殼內的黏彈性變形所解釋。下部地殼的強度在主震過後被立即弱化，並在隨後數年內逐漸增強，有效黏滯係數的變化範圍大約為10^18 Pa s至10^20 Pa s之間。這些模型顯示瞬態潛變（transient creep）、穩態位錯潛變（steady-state dislocation creep）、以及下部地殼流變性質的橫向變化對於震後變形的模式皆扮演了一定的角色。本研究顯示透過大地測量學觀測資料及運動學反演，探索現地岩石圈流變特性之潛力。
在測站空間包覆性良好的狀態之下，大地測量學己經可以對陸地斷層系統及岩石圈流變特性有較佳之約制，但是對於大尺度的隱沒帶構造，例如台灣東北部的琉球及南部的馬尼拉隱沒帶，若沒有海床變形的資料仍然無法窺知全貌。因此，本研究亦分析了中央研究院地球科學研究所在臺灣東北外海蒐集的海底大地測量資料，將臺灣的地殼變形觀測自陸域延伸至海域，為後續隱沒帶相關研究奠立基石。海底大地測量結合動態GNSS和音響測距（GNSS-A）來求取海床相對陸域GNSS參考站之位移。本研究制定了一種非線性反演方法分析2012-2020年於臺灣東北外海的兩個海床測站所蒐集到的觀測資料。兩個海床測站分別為位於沖繩海槽內的OILN，以及位於南澳海盆內的OHUA。結果顯示，OILN相對於澎湖白沙的變形速率為52.3 ± 7.0 mm/yr，方位角為168 ± 7°。OHUA相對於澎湖白沙的變形速率則為45.2 ± 12.2 mm/yr，方位角為219 ± 16°。此外，本研究亦發現兩個測站的海水聲速構造皆具有顯著的週期性振盪，其週期大約為12小時，很可能是由臺灣鄰近海域中盛行的半日內潮所控制。這些分析表明GNSS-A觀測資料不僅能研究海底地殼的變形，亦具備探索海洋物理相關現象的潛力。

關鍵字

地震周期 ； 大地測量 ； 流變學 ； 全球導航衛星系統 ； 海底大地測量 ； 音響測距 ； 內潮

並列摘要

Seismic and aseismic slip on the fault zones and viscoelastic flow in the lithosphere control the states of stress and strain over seismic cycles. The rheology of rocks governs inelastic deformation across a broad range of spatial and temporal scales. Laboratory experiments have provided key insights into the rheological behavior of rocks, but the rheology of rocks in their natural settings is still poorly resolved. This deficiency highlights the need to explore rheology in tectonic settings across multiple spatio-temporal scales. To model the distributed inelastic strain in the lithosphere, this study designs a novel kinematic inversion that directly relates surface displacements to off-fault inelastic strain, as opposed to a conventional forward modeling approach with prescribed rheological properties. The new approach is implemented to investigate the 14 years of postseismic deformation following the 1999 Chi-Chi, Taiwan, and the 8 years of postseismic deformation following the 2010 El Mayor-Cucapah, Mexico, earthquakes. The postseismic deformation of both earthquakes observed by the Global Navigation Satellite System (GNSS) is contributed from afterslip on the downdip extensions of coseismic rupture planes and the viscoelastic flow in the lower crust. Both case studies show that the effective viscosity of lower crust dropped to ~10^18 Pa s immediately after the mainshock and gradually increased to ~10^20 Pa s in a few years. The models illustrate the role of transient creep, steady-state dislocation creep, and lateral variations of rheological properties in the lower crust over seismic cycles, showing the potential of geodetic inversions for exploring the rheology of the lithosphere under tectonic settings.
Despite successful applications of this new approach, land-based geodetic observations are insufficient for probing fault slip behaviors and rock rheology in the mantle and subduction zones surrounding Taiwan. Seafloor geodetic measurements play an important role in expanding the geodetic network in Taiwan. A seafloor geodetic technique that combines kinematic GNSS and acoustic ranging (GNSS-A) is applied to track 3D deformation on the seafloor. In this study, a nonlinear inversion scheme is designed to analyze the GNSS-A data collected from 2012-2020 at the two seafloor sites, OILN and OHUA, maintained by the Institute of Earth Sciences, Academia Sinica. The two seafloor sites are located in the Okinawa Trough and the Nanao Basin, respectively. The estimated secular velocities at OILN and OHUA are 52.3 ± 7.0 mm/yr towards 168 ± 7° azimuth and 45.2 ± 12.2 mm/yr towards 219 ± 16° azimuth, respectively, with respect to the Chinese continental margin. This study also reports the fluctuations of sound speed structure in the ocean with a period of ~12 hours, which is likely associated with the semidiurnal internal tide prevalent in the ocean. These analyses show that the GNSS-A measurements not only provide constraints on seafloor crustal deformation but also have the potential for studying physical oceanography.

並列關鍵字

Seismic cycle ； Geodesy ； Rheology ； GNSS ； Seafloor geodesy ； Acoustic ranging ； Internal tides

參考文獻

Alford, M. H., Peacock, T., MacKinnon, J. A., Nash, J. D., Buijsman, M. C., Centurioni, L. R., Chao, S.-Y., Chang, M.-H., Farmer, D. M., Fringer, O. B., Fu, K.-H., Gallacher, P. C., Graber, H. C., Helfrich, K. R., Jachec, S. M., Jackson, C. R., Klymak, J. M., Ko, D. S., Jan, S., … (David) Tang, T.-Y. (2015). The formation and fate of internal waves in the South China Sea. Nature, 521, 65–69.

Barbot, S. (2018). Deformation of a half‐Space from anelastic strain confined in a tetrahedral volume. Bulletin of the Seismological Society of America, 108, 2687–2712.

Barbot, S., Fialko, Y., Bock, Y. (2009). Postseismic deformation due to the Mw 6.0 2004 Parkfield earthquake: Stress-driven creep on a fault with spatially variable rate-and-state friction parameters. Journal of Geophysical Research: Solid Earth, 114, B07405.

Barbot, S., Moore, J. D. P., Lambert, V. (2017). Displacement and stress associated with distributed anelastic deformation in a half‐space. Bulletin of the Seismological Society of America, 107, 821–855.

Blackwell, D., Richards, M. (2004). Geothermal Map of North America (scale 1:6,500,000). American Association of Petroleum Geologists.