区域地壳速度结构的天然地震成像研究

【作者】 孙安辉；

【导师】 陈棋福； 赵大鹏；

【作者基本信息】 中国地震局地震预测研究所 ， 固体地球物理学， 2007， 硕士

【摘要】 地震波之所以可以反映地球内部的结构，是因为地震波经由地下介质传播的过程中，传播路径、振动方向、波的性质、波形和能量都由于地下介质的作用而发生变化，分析比较这些变化，便可以从中提取到地球内部结构的信息。综合研究地震波速度以及其他地球物理信息，人们可了解地球内部的分层结构以及不同深度处的密度、温度、力学参数和物质的组成。随着记录地震波的仪器和方法，以及分析地震波的理论的不断改进和完善，地球内部地震波速度的测定也越来越精确。本文的研究目的是如何利用记录到的地震波的资料来获得地球内部的区域地壳速度结构。众所周知，数据是一切结构反演研究的起点，只有准确可靠的数据才能保障反演得到优良的结果。在三维层析成像反演中，离不开地震到时、震源位置、初始一维速度模型等数据。研究表明，良好的一维初始速度结构和精确的地震发生时间和空间位置会改善三维层析成像研究的结果。因此本论文的研究从新疆乌鲁木齐地区的地壳一维速度结构研究入手，然后对地震频发的日本西南地区下的地壳及上地幔顶部进行了较高精度的三维成像研究，并探讨了速度结构异常与构造及大地震之间的联系。乌鲁木齐市是我国西北的政治、经济和文化的中心，位于新疆中部，地处天山北麓、准噶尔盆地南缘，该地区位于全球重要的地震活动带上，乌鲁木齐市所处的北天山地区连续多年被划入全国地震重点危险区和值得注意的地区。自新疆乌鲁木齐区域数字遥测地震台网建立以来，大大改善了该区域的监测能力和定位精度，为该地区提供了完整的地震目录和原始地震波形资料。已有研究者获得了该区域若干条二维剖面的速度结构，但一直缺乏公认的适用于该地区的地壳一维速度模型。由于地震精定位数据和一维地壳速度模型会直接影响层析成像结果的置信度。本文根据布设在乌鲁木齐活断层探测区内的流动宽频带地震台阵和区域地震台网的走时数据，应用HYPOSAT、HYPOINVERSE和HYPODD三种震源精确定位方法于不同的一维速度模型，对新疆乌鲁木齐地区(E85°30′-88°30′，N43°00′-44°40′)2004年8月—2005年8月发生的地震进行了重新定位。通过与中国地震局地球物理勘探中心测定的深反射剖面结果作比较，探讨改进了乌鲁木齐探测区范围内的地壳一维P波速度结构，为后续定位和成像等相关研究提供了可靠的基础。结果表明，该区域改进后的一维速度模型结合HYPODD精定位方法获得了与剖面结果较为一致的结果。同时小震活动的精定位结果较清晰地反映了乌鲁木齐市周边的构造活动的强弱特征，展示了该区域在统一的构造变形作用下在15-18km深处可能存在着滑脱构造，地震活动明显受天山山前断裂和准葛尔盆地南缘断裂的逆冲推复构造所控制。震源深度优势分布集中在22km以上，与当地存在深度为10km以上低速沉积层的地质构造情况相一致，说明中下地壳的塑韧性，缺乏地震活动性，除了一些与已知断层相关的地震事件外，还发现一些有待证实的活断层。地震层析成像技术是获得地球内部三维结构的最重要的方法之一，自从Aki和Lee(1976)首先提出以来，其方法和应用都得到了长足的发展，产生了巨大而深远的影响。相对于基于波动方程的波形层析成像方法而言，基于射线理论的走时层析成像方法具有一定的局限性，但其仅利用走时信息，因此目标函数中局部极值个数较少，所用的数据量大大减少，非线性程度相对较弱，计算量也相应降低，使得计算和求解相对稳定。且走时数据的信噪比较高，无论是柱面波还是球面波走时的规律都相同。走时层析成像研究发展较早，技术方法已比较成熟，是目前世界上地震学者们广泛采用的地震层析成像方法。西南日本位于亚欧板块和菲律宾板块及太平洋板块的交汇地区，地质构造复杂，大地震灾害频繁。为了监测日本列岛及其周边的地震活动并研究地下结构，日本开始构建气象厅台网、大学联合台网、Hi-net台网等三大主要台网，目前已有2200多个地震台比较均匀地覆盖了日本全国，为地震科学研究提供了大量高质量的地震数据资料。同时，西南日本已有公认的一维地壳速度结构。这些都是高精度的三维层析成像的充分条件。因此，本文选用Hi-net台网记录的2002-2004年间发生的具有较精确定位结果的836个地震事件，共23，895条P波和21，969条S波的到时资料，进行三维地震层析成像研究。通过反演得到了日本西南地区水平分辨率约33km，垂直分辨率4-15km的三维地壳P波和S波速度结构，并进一步获得泊松比分布。研究表明，在大山火山下存在显著的低速异常，表明该火山可能是一个潜在的活火山。速度异常图像清楚地显示了菲律宾板块俯冲至西南日本地区下，其上方存在的低速高泊松异常表明可能有流体的存在，分析得出可能来自板块俯冲过程中的脱水作用。为充分利用数字地震波形记录信息，有效提高地壳结构成像的精度，以更好的理解速度异常与地质构造和大地震的关系，同时弥补常规近震直达波成像在下地壳的分辨率常常欠佳的局限性。本论文选取该地区有板块俯冲、火山、大断层、大地震等复杂地质构造及现象的南北向约60km宽的条带区域，根据莫霍面反射波PmP的射线路径的特性，参考理论地震图的精确计算拾取了29个台站记录的60个事件的P波初至和后续的反射波(PmP)的到时，并采用这些数据，获得了该地区更加精细的地壳三维速度结构。速度图像呈现的不均匀性的分辨率在水平方向约为25km，深度上为4-10km。结果显示：大山火山下存在明显的低速异常，伴随着大量的地震发生，表面该火山可能是潜在的活火山。菲律宾俯冲板块的上方有显著的低速异常，可能与板块俯冲过程中产生的脱水现象有关。在中央构造线(MTL)下有一条大约15km宽度的低速异常，并与菲律宾俯冲板块上的低速带相连。南部德岛大地震发生在高速体和低速体的过渡地带，可能与板块脱水产生的流体上流有关，西部鸟取地区的大地震发生大山火山下的周围区域，可能与岩浆的上涌有联系。结果表明，PmP波数据大大改善了下地壳成像结果的分辨率，进一步提高了对该地区的认识。地震层析成像技术为我们打开了探索地球内部构造的窗口。本论文的新疆乌鲁木齐地区的地壳一维速度结构确定，以及对地震频发的日本西南地区下的地壳及上地幔顶部进行的较高精度的三维成像反演，展示了系统地认识和研究地球内部构造的有效途径，对速度结构异常与构造及大地震之间的关系进行探讨，为将来研究其他多震灾地区的三维速度结构打下了良好的基础。更多还原

【Abstract】 Why can we determine the inner structure of earth through seismic wave? Because some aspects of seismic wave will vary with medium when passing by, such as travel path, vibration direction, property, waveform and energy, and so on, from which we can extract information of structure. Together with the analysis of seismic wave velocity and other geophysical results, we get to know the stratum structure of the earth and density, temperature, physical parameter, composition of different depth. With development of recorder of earthquake and theory of seismic wave, now we can determine the seismic wave velocity more and more precisely.The aim of the thesis is to obtain a fine three-dimensional (3-D) velocity structure of the regional crust. As known, data are the basis of inversion for structure, and precise data are vital for us to achieve reliable result of the 3D seismic tomography, such as travel time, location, and reference 1D model. Former studys indicate that good 1D reference model, precise origin time and location of earthquake can help us improve the reliability of the result of seismic tomography. Hence we started with studying on 1D crustal velocity model of Urumchi area in Xinjiang municipality, then conducted a fine 3D structure of crust and uppermost mantle under Southwest Japan, and discussed the relationship between velocity anomalies, tectonics and large earthquake.Urumchi city is political, economic and cultural center of the Northwest China, which is located between the Tianshan Mountain and Junggar basin in the middle of Xinjiang municipality. The region is the most important seismic activity zone in the world, and the nearby region, North Tianshan Mountain, has been marked as national seismic hazard area for several years. Since the establishment of the regional digital seismic network, we efficiently improved the ability of the earthquake monitoring, the precision of earthquake locating, and prepared the standard catalog and waveform data for study. Though some researchers have obtained the 2D structure of several sections in this area, we have not determined an authoritative 1D regional crustal velocity model yet.The precise earthquake location data and 1D crustal velocity model can directly improve the reliability of seismic tomography, the earthquakes occurred in Urumchi area (E85°30’-88°30’,N43°00’-44°40’) from August 2004 to August 2005 are relocated by applying three different precise locating method (HYPOSAT, HYPOINVERSE and HYPODD) with different 1D velocity models, the earthquakes were recorded by portable broadband seismic array and regional seismic network of the active fault exploration zone in Urumchi area. By comparing with the deep reflection profile of the geophysical exploration center of CEA, we try to improve the 1D crustal velocity model for locating experiment and seismic tomography in the future. We found that the location result from the HYPODD method based on the modified 1D crustal velocity model is well consistent with the deep reflection profile. The precise locations of the small earthquakes clearly reflect the tectonic activity around Urumchi city and there may be a decollement at the depth of 15 to 18 km under the process of the tectonic distortion. The seismicity was greatly affected by the thrust block in the rapture zone between the Tianshan Mountain and the Junggar basin, the earthquakes dominantly occurred above 22km depth, which agree with the tectonic evidence that low velocity sediment is more than 10km thick and show the plasticity, toughness and aseismicity below upper crust. In addition to several known fault, we reveal some new active faults to be further confirmed in the future.Seismic tomography is one of the most important techniques to invest the structure of the earth. Since the pioneering work of Aki and Lee (1976), there have been significant advances in the theory and applications of seismic tomography in the last decade. It’s no doubt that the seismic tomography has played a great role in earth science and has a profound impact. Though comparing with the waveform seismic tomography based on the wave theory, the travel time tomography based on ray theory has some limitation due to the theory itself. However, the latter only use arrivel time, which can greatyly decrease the number of extremes in target functions and the data size, weaken nonlinear, reduce the computation size, and make the solving process stable. Moreover, the travel time data has good signal-to-noise performance, whether cylinder wave or spherical wave has the same rule for travel time likewise. Now, the travel time tomography has been used by many seismologist all over the world for its great development.Southwest Japan is located among the Eurasia Plate and the Philippine Plate and the Pacific Plate, where the tectonic structure is very complicated with many destructive earthquakes. In order to monitor the seismicity of the Japan Island and investigate the structure underground, about 2200 seismic stations were installed evenly distributing over the whole Japan belonging to three seismic networks (i.e. Japan Meteorological Agency (JMA), Japan university seismic network and High-sensitivity Seismic Network (Hi-net)), These networks have recorded plenty of high quality data. Together with the relative accurate 1D crustal velocity model in Japan, we are able to determine a high resolution 3D seismic tomography for this area.We used 23,895 P and 21,969 S wave high-quality arrival times from 836 local earthquakes during 2002 to 2004 recorded by the dense Hi-net. Detailed three-dimensional seismic velocity (Vp and Vs) structures of the crust under Southwest Japan are determined with a horizontal resolution of about 33 km and a depth resolution of 4-15km. We also estimated the Poisson’s ratio (a) structure according to the inverted Vp and Vs data. Our results revealed a significant low-velocity anomaly beneath the Daisen volcano, which suggest that Daisen is potentially an active volcano. The subducting Philippine Sea plate is clearly imaged beneath Southwest Japan. Low-velocity and high-Poisson’s ratio anomalies are visible above the Philippine Sea slab, which may indicate the existence of fluids released by the slab dehydration. In addition, we discussed the important role of the fluids played in the generating and triggering process of the large crustal earthquakes.In order to better understand the relationship between velocity anomalies and tectonics and large earthquake, and improve the resolution of the tomographic image, especially that of the lower crust where is limited in usual crustal tomography with only using direct arrivels. So, we marked off a 268-km long and 60-km width region over Southwest Japan, in which one of the Quaternary volcanoes (Daisen) is located, and the Median Tectonic Line (MTL) runs through this region, and the Philippine Sea plate (PHS) is subducting under the area. The western Tottori earthquake and southern Tokusima earthquake occurred in the northern and southern part of the study area respectively. Crustal reflected waves from the Moho discontinuity (PmP) are identified clearly by comparing the observed waveforms with synthetic seismograms computed using the technique of Wang (1999). We determined a fine three-dimensional (3-D) P-wave velocity structure of the crust under this north-south belt region by using the first arriving P-waves and the reflected P-waves of 60 events recorded by 29 stations. Lateral and vertical heterogeneities are imaged up to about 25 km and 4-10 km of the study region, respectively. Our results revealed significant low-velocity anomalies beneath Daisen volcano accompanying with swarm of earthquake, which suggest that Daisen is potentially an active volcano again. In the lower crust, we detected low-velocity anomalies overlying the subducting PHS, which may indicate the existence of fluids released by the slab dehydration. Beneath the Median Tectonic Line, we found an about 15 km wide vertical low-velocity belt joint with the low velocity part over the subducting PHS. The southern Tokusima earthquake occurred between the low and the high velocity zones, suggesting that it may be caused by the dehydrated fluid flew up through the crack of the active fault system. The western Tottori earthquake and other nearby strong earthquakes occurred around the Daisen volcano may be related to the upwelling magma. The detailed resolution analyses show that the PmP reflected waves can significantly improve the resolution of the lower crust tomographic images, and broaden our knowledge of the structure.Seismic tomography opens a window for us to probe into the inner structure of the Earth. This thesis studied the approaches to systematically recognize structure of the earth by amending the 1D crustal velocity model for Urumchi area and conducting a detailed 3D seismic tomography of crust and uppermost mantle under Southwest Japan. Then we discussed the relationship between velocity anomalies and tectonics and large earthquakes, and establish the basis for future studies on the 3D velocity structures of other regions with large earthquake hazard.更多还原