| 摘要: | 摘要
印度與歐亞大陸板塊的碰撞約始於五千萬年前，造就喜馬拉雅東部延伸區域的複雜逃脫運動。此逃脫構造活動引起南中國地塊、中南半島地塊和思茅地塊等微型大陸之間的相對運動，促成在越南北部的多階段地質活動與複雜地殼構造。然而，北越的複雜地質構造仍不明確，相關構造爭論—例如紅河剪切帶是否是中南半島與南中國地塊的板塊邊界—仍在持續中。在本論文中，我採用多種方法來改進從地殼到上地幔的震波速度模型，目的是透過更清楚的震波成像來為越南北部提供更精細的構造解釋。
本論文主要由兩個主題構成。在第一個主題中，我旨在以體波層析成像技術結合多測站網資料研究區域尺度的北越速度構造。跟隨黃等人（2013）結合地殼直達波與地幔首波來更好地約制地震位置與增進下部地殼與莫荷面構造的階段性反演程序，我進一步改進北越區域的P波速度模型，並建立了第一個S波速度模型。我們新的速度模型顯示與地表地質和區域地質特徵有良好的相關性，並揭示出松馬縫合帶比紅河剪切帶更有可能是中南半島和南中國地塊間的板塊構造。而紅河剪切帶似乎是一個地殼尺度的斷層構造，而奠邊府斷層則可能是一個岩石圈尺度的斷層構造。
在第二個主題中，透過利用東北東-西南西走向的密集線性地震陣列，我聚焦在進一步提升對地殼與斷層構造的影像解析力。我利用馬爾科夫鏈蒙特卡羅（MCMC）聯合反演方法結合以波束成型技術求得的瑞利波相速度、以環境噪聲交叉相關技術得到的瑞利波橢圓率、與遠震資料的接收函數來建立每個測站下方的高精度剪力波速度剖面。這三種資料具有不同且互補的深度敏感算核，能幫助制約極淺層到莫荷面不同深度的速度變化。最終合併得到的高解析度二維震波影像清楚解析出淺部的盆地構造，包括松紅與安州盆地，以及地殼在紅河剪切帶下方自32公里減薄到29公里左右的莫荷面變化。結果同時也發現在幾個主要的斷層帶下方有垂直低速帶的分佈，可能反應了此區長期剪動量的應變累積。本論文研究得到的高解析速度模型，不只對北越長期的構造演化與複雜的斷層構造提供了新的見解，同時也能提升對該區未來地震災害潛勢評估的能力。
;Abstract
The collision between the Indian and Eurasian continents commenced approximately 50 million years ago and results in pronounced extrusion movements in the eastern Himalayan syntaxis region. The extrusion tectonics leads to relative movements between microcontinents such as the South China block, Indochina block, and Simao subterrane and creates multi-phase tectonic activities and complex geological structures in Northern Vietnam. However, these complex structures remain poorly constrained, and debates persist—for instance, whether the Red River Shear Zone represents the actual plate boundary between the Indochina and South China blocks. In this dissertation, I have conducted multiple approaches to enhance the resolution of seismic velocity models from the crust to upper mantle, with the goal of improving earthquake location accuracy and enabling refined tectonic interpretations.
The dissertation mainly consists of two projects. In the first project, I aim to investigate detailed regional velocity structures in Northern Vietnam by using body-wave tomographic inversion method with an integrated dataset from multiple seismic networks. The method follows Huang et al. (2013) to employ crustal and mantle-lid head waves to better constrain the earthquake location and improve resolution for lower crustal and Moho structures in a step-wise procedure. Through that, I improve the P-wave velocity model and construct the first S-wave velocity model for the region. Our results demonstrate a strong correlation with surface geology and regional tectonic features, suggesting that the Song Ma suture is a more suitable candidate for the plate boundary compared to the Red River Shear Zone. The Red River Shear Zone, on the other hand, likely represents a crustal structure, whereas the Dien Bien Phu Fault likely represents a lithospheric structure.
In the second project, I focus on illuminating further details of crustal and cross-fault structures by utilizing a dense NE-SW trending linear array across the study region. I integrate Rayleigh wave phase velocity derived from the beamforming technique, Rayleigh wave ellipticity obtained through the ambient noise cross-correlation method, and receiver functions from teleseismic events into a Markov Chain Monte Carlo joint inversion scheme to construct station-wise S-wave velocity profiles. The simultaneous use of three datasets shows complementary sensitivity and achieve greater resolution near surface and across the Moho. The obtained high-resolution 2-D imaging reveals distinct basin structures, including the Song Hong and An Chau basins, as well as crustal thinning across the RRSZ from approximately 32 km to 29 km. Moreover, it suggests the presence of vertical low-velocity zones within the crust beneath major faults, potentially reflecting long-term strain accumulation associated with large-scale shear faulting. Our derived velocity models not only provide new insights into the detailed geological structures associated with long-term tectonic evolution and major fault zones, but also offer a better assessment of seismic hazards in Northern Vietnam. |
