本文選題：核幔邊界 + 下地幔底部�。� 參考：《中國(guó)科學(xué)院廣州地球化學(xué)研究所》2017年博士論文

【摘要】：作為地球內(nèi)部結(jié)構(gòu)最復(fù)雜、動(dòng)力學(xué)過(guò)程最活躍的區(qū)域之一,核幔邊界及其附近結(jié)構(gòu)一直是地球科學(xué)研究的熱點(diǎn)。其中,核幔邊界是地球內(nèi)部物質(zhì)成分變化最劇烈的界面,是固態(tài)的硅質(zhì)地幔與液態(tài)的鐵質(zhì)外核相接觸的分界面。而其下地幔底部作為一種“熱-化學(xué)”邊界層,可能是地幔對(duì)流過(guò)程中俯沖板塊的最后歸宿以及地幔柱上涌的起點(diǎn),同時(shí)也是地核和地幔物質(zhì)與能量交換的重要場(chǎng)所。雖然全球尺度的層析成像模型已經(jīng)揭示出該區(qū)域大尺度的不均一性速度結(jié)構(gòu),但是由于分辨率不足的原因,無(wú)法得到一些小尺度結(jié)構(gòu)的圖像。而小尺度結(jié)構(gòu)的地震學(xué)特性對(duì)理解下地幔的動(dòng)力學(xué)演化、物質(zhì)成分組成具有十分重要的指示作用。因此,在本論文中我們選取了PKP前驅(qū)波以及PKKP衍射波作為研究手段,對(duì)產(chǎn)生這些地震信號(hào)的下地幔底部的異常結(jié)構(gòu)體進(jìn)行了研究,并且探討其背后的地球動(dòng)力學(xué)意義。首先,本文選取了發(fā)生在勘察加半島和阿留申群島的地震,利用南極臺(tái)網(wǎng)記錄到的PKP前驅(qū)波信息,通過(guò)偏移成像以及對(duì)前驅(qū)波振幅的擬合,發(fā)現(xiàn)產(chǎn)生這些前驅(qū)波的強(qiáng)散射體結(jié)構(gòu)位于太平洋異常北部的高速異常體中,大體分布在30-40°N之間,深度在核幔邊界附近400公里范圍內(nèi)。同時(shí)這些散射體的結(jié)構(gòu)存在著橫向不均一性:中部區(qū)域(160-180°E)的散射體呈現(xiàn)出1.0~1.2%的P波速度擾動(dòng),而兩側(cè)區(qū)域(140-160°E,180-200°E)的速度擾動(dòng)只有0.5%。利用ScS-S相對(duì)走時(shí)殘差的方法對(duì)同樣的區(qū)域進(jìn)行分析,得到了剪切波速度結(jié)構(gòu):中間區(qū)域的S波速度異常為2.0~3.0%,而兩側(cè)的速度異常為1.0~1.5%。這些散射體結(jié)構(gòu)可能代表了古俯沖板塊在核幔邊界處的殘余,而這種散射強(qiáng)度的橫向不均一性變化可能有兩種原因?qū)е?(1)不同的俯沖板塊來(lái)源;(2)同一板塊的內(nèi)部組分在分離過(guò)程中產(chǎn)生的不同物質(zhì)成分。其次,本文綜合分析了澳大利亞ASAR以及WRA臺(tái)陣所記錄的發(fā)生在南美洲西海岸的地震產(chǎn)生的PKP前驅(qū)波的信息。利用臺(tái)陣處理方法分析了相關(guān)性較好的前驅(qū)波的慢度信息,該結(jié)果表明在澳大利亞?wèn)|南部以及太平洋東部的下地幔都存在著散射體結(jié)構(gòu)。而利用AxiSEM方法進(jìn)行波形模擬的結(jié)果進(jìn)一步表明,在太平洋異常的東部邊界則存在著小尺度的、塊狀的超低速區(qū)異常結(jié)構(gòu)。這些超低速區(qū)P波的速度擾動(dòng)在-6~-10%,厚度變化在20~40公里、寬度大約為30公里。然后利用我們開(kāi)發(fā)的偏移成像方法得到了太平洋東部下方核幔邊界處的散射區(qū)域分布圖像,散射強(qiáng)度最大的散射體中心位于(-89°E,-10°N),該結(jié)果也與波形模擬的結(jié)果相吻合。對(duì)于產(chǎn)生PKP前驅(qū)波的超低速區(qū)的來(lái)源,則可能是部分熔融或者化學(xué)成分異常所導(dǎo)致的。最后,本文選取了發(fā)生在太平洋西岸的地震事件,利用美國(guó)USArray記錄到的PKKPab衍射波,采用拉東變換的方法得到了PKKPab衍射波的射線(xiàn)參數(shù)信息,并將其轉(zhuǎn)換為下地幔底部采樣區(qū)域的速度擾動(dòng)值。研究結(jié)果表明在非洲LLSVP的西北邊界以及蘇門(mén)答臘島的南部,其下方核幔邊界處的速度擾動(dòng)為-4%~-8.5%,可能暗示了低速區(qū)/超低速區(qū)的存在。此外,在北大西洋下方,衍射波的速度變化可能暗示了離LLSVP邊界的遠(yuǎn)近對(duì)于超低速區(qū)速度變化的影響。通過(guò)對(duì)小尺度散射體以及超低速區(qū)的研究,本文認(rèn)為這兩者可能存在著成因上的聯(lián)系。具體來(lái)說(shuō),板塊在俯沖到核幔邊界的過(guò)程中,分離的洋殼會(huì)形成小尺度的散射體分布在下地幔,正是這些小尺度的散射體產(chǎn)生了本文觀測(cè)到的隨機(jī)擾動(dòng)的PKP前驅(qū)波。同時(shí)這些洋殼物質(zhì)可能會(huì)導(dǎo)致超低速區(qū)的形成。
[Abstract]:As one of the most complex and dynamic regions of the earth's internal structure, the boundary of the nuclear mantle and its adjacent structures have always been the hot spots in the earth science research. Among them, the boundary of the core mantle is the most intense interface of the earth's internal material components, and the interface between the solid silicon mantle and the liquid iron outer core. The bottom as a "thermal chemical" boundary layer may be the final destination of the subducted plate and the starting point of the upwelling of the mantle plume during the convection of the mantle, and it is also an important place for the exchange of material and energy of the earth's core and mantle. Because of the lack of resolution, some small scale structures can not be obtained. The seismological characteristics of the small scale structure are very important to the understanding of the dynamic evolution of the mantle and the composition of the material components. Therefore, in this paper, we selected the PKP precursor wave and the PKKP diffraction wave as the research means. The anomalous structures at the bottom of the lower mantle at the bottom of these seismic signals are studied and the Geodynamic Significance behind them is discussed. First, this paper selects the earthquakes occurring in the Kamchatka Peninsula and Aleutian Islands, using the PKP precursors recorded by the Antarctic network, and through the migration imaging and the fitting of the amplitude of the precursors. The strong scatterers of these precursors are located in the anomalous body of abnormal high speed in the north of the Pacific Ocean, which are generally distributed between 30-40 N and 400 km near the boundary of the nuclear mantle. At the same time, the structure of these scatterers has a transverse heterogeneity: the scatterer of the middle region (160-180 E) shows the P wave velocity disturbance of 1.0~1.2%. The velocity disturbances in the two sides (140-160 E, 180-200 E) are only analyzed by 0.5%. using ScS-S relative travel time residuals, and the shear wave velocity structure is obtained. The S wave velocity anomaly in the middle region is 2.0~3.0%, and the velocity anomaly on both sides is 1.0~1.5%., which may represent the ancient subduction plate in the region. The remnants of the core mantle boundary, and the transverse heterogeneity of this scattering intensity may have two reasons: (1) different subduction plate sources; (2) the internal components of the same plate in the separation process of the different material components. Secondly, this paper synthetically analyses the occurrence of Australia ASAR and WRA array in South America. The information of the PKP precursors produced by the earthquake in the west coast. By using the array processing method, the slowness information of the better precursors is analyzed. The results show that there is a scatterer structure in the lower mantle in southeastern Australia and the east of the Pacific. The results of the waveform simulation using the AxiSEM method further indicate that in Taiping. The eastern boundary of the oceanic anomaly has a small scale, massive ultra low velocity region abnormal structure. The velocity of P waves in these ultra low velocity regions is disturbed at -6~-10%, the thickness varies at 20~40 km, and the width is about 30 km. Then we use the migration imaging method we developed to get the scattered region distribution image at the eastern part of the Pacific Ocean. The center of the scatterer with the largest scattering intensity is located at (-89 E, -10 N). The result is also consistent with the result of the waveform simulation. The source of the ultra low velocity region of the PKP precursor may be caused by the partial melting or the abnormal chemical composition. Finally, this paper selects the earthquake events in the West Bank of the Pacific Ocean, using the American USArray record. The PKKPab diffraction wave recorded is obtained by using the method of Radon transform to obtain the ray parameters of the PKKPab diffraction wave and transform it into the velocity perturbation value in the bottom sampling area of the lower mantle. The results show that the velocity perturbation at the boundary of the nuclear mantle beneath the north-west boundary of the African LLSVP and the south of Sumatra island is -4%~-8.5%. It implies the existence of the low velocity region / ultra low velocity region. In addition, the velocity variation of the diffraction wave below the North Atlantic may imply the effect of the distance from the LLSVP boundary to the velocity variation in the ultra low velocity region. By the study of the small scale scatterers and the ultra low velocity regions, this paper considers that the two may have a genetic link. During the process of subduction to the boundary of the nuclear mantle, the separated oceanic crust will form a small scale scatterer in the lower mantle. It is these small scatterers that produce the PKP precursor of random perturbations observed in this paper.
【學(xué)位授予單位】：中國(guó)科學(xué)院廣州地球化學(xué)研究所
【學(xué)位級(jí)別】：博士
【學(xué)位授予年份】：2017
【分類(lèi)號(hào)】：P315.2
，

本文編號(hào)：1977567