越南北部區域岩石圈及上部地函之地震波走時層析成像

Abstract

經過越南北部地區的紅河剪切帶由西藏高原東邊向東南延伸至東京灣, 全長超過1000公里。而紅河剪切帶則認為與西藏高原有著密切的關係, 在過去的研究中, 則有數種不同模型來解釋西藏高原的形成及之後的地殼變形和紅河剪切帶的關係, 包括剛性陸塊脫逸、歐亞板塊岩石圈增厚及較塑性的下部地殼流動模型。而本研究則利用在越南北部的地震測站所得到的資料, 做橫跨紅河剪切帶之區域性地震波走時層析成像的研究, 以期能更加瞭解剪切帶的地體構造特性。 本研究使用了中央研究院地球科學研究所於2005年10月開始在越南擺設的寬頻地震網資料。總共使用24個測站且使用從2005年12月至2008年6月間, 規模大於5.5、震央的大圓距介於三十度至九十度的遠震資料。而使用P波進行壓力波波速,S波進行剪力波波速的震波層析成像。所使用的地震P波總共577筆,S波總共472筆, 並且使用高頻與低頻資料, 運用有限頻寬理論以及多重尺度參數化的方式進行震波層析成像。 在P波結果中可見到紅河剪切帶於深度60公里以上呈現一狹長型的快速構造, 但在S波的結果中則無法觀察到。而在兩者的結果之中都可觀察到在紅河剪切帶與Song Ma斷層之間、東京灣西南部、廣東/廣西區域皆有慢速構造存在, 而深度分布則分別為0至60、20至300、100至350公里。另外還挑選了共有的地震做了Vp/Vs 比值的研究, 且可見到幾乎是以紅河剪切帶為分界的結果。 從S波及Vp/Vs比值的結果來看, 紅河剪切帶兩側的確表現出相當不同的性質, 在剪切帶西側的下部地殼甚至於最上部地函(約100公里深) 都呈現較慢速構造。表示在紅河剪切帶開始錯移的時候, 有伴隨著較高溫的岩石圈地函一起脫逸, 且此較高溫地函可能與新生代晚期越南北部的岩漿活動有關。

The Red River shear zone (RRSZ) that runs through Northern Vietnam has been considered closely linked with the Tibet resulting from the northward indention of India into Asia. In previous research, several tectonic models has been proposed to explain the formation of Tibet and the crustal deformation after collision in relations with RRSZ, including rigid block extrusion, distributed crustal thickening and ductile lower crust flow. Since October 2005, Institute of Earth Science, Academia Sinica has deployed a regional broadband array with station spacing of about 40 km in Northern Vietnam for earthquake and seismic structure studies. We collect data from teleseismic earthquakes with magnitude bigger than 5.5 between December 2005 and June 2008. Using this new dataset, we report results of 3-D, tomographic inversion of P-wave and S-wave relative travel time residuals, as measured by inter-station cross-correlation of waveforms at both high- and low-frequencies. We employed physically realistic 3-D sensitivity kernels for finite-frequency traveltime data and data-adaptive, multi-scale parameterization in the tomographic inversion. Though less distinct in the S velocity model, an elongated fast anomaly about 60 km wide that strikes parallel to the RRSZ and subvertically extends to the depth of 60 km clearly emerges in the P wave model. In addition to a prominent low velocity zone revealed in the crust between the RRSZ and Song-Ma suture. The other resulting salient features are two pronounced, larger-scale low-velocity anomalies distributed at depths greater than 100 km. One is located around 17o−18oN to the south of the Tonkin Gulf, and the other is situated near the northeast border of our study region in Guangdong and Guangxi Provinces, Southeast China. Further we use common event of P and S wave to construct Vp/Vs ratio result, and indicates that a clearly different pattern across RRSZ. In summary, our tomographic results reveal the noticeable difference between both sides of the RRSZ. The low S wavespeeds and Vp/Vs ratio widely-dispersed in the lower crust and uppermost mantle(to depth 100km) to the west of the RRSZ. Our result indicates there has been extrusion of relatively hot lithospheric mantle along with the RRSZ, and such hot mantle may have relations with Late Cenozoic volcanism in Northern Vietnam.