利用界面转换波研究南海海底沉积物特征

SEDIMENT THICKNESS AT SEAFLOOR OF SOUTH CHINA SEA BASIN REVEALED BY TELESEISMIC P-TO-S CONVERTED PHASES

杨港

11849177

硕士

力学

杨挺

2020-05-20

2020-06-08

哈尔滨工业大学

深圳

海底沉积物不仅记录了全球气候变化历史，而且能反映了区域的构造演化。作为边缘海，南海的海陆交互作用比开放大洋更为剧烈，因此其海底沉积物对气候变化和构造演化更为敏感。此外，相对较厚的南海沉积层所导致的地震波走时延迟也会影响深部结构成像的精度。尽管人工地震剖面方法和IODP在局部对于南海沉积层进行深入研究，但依然存在广度不足的问题。南海大规模的被动源海底地震仪（OBS）布设为进行区域性二维海底沉积层特性分析提供了一种新手段。本文即是利用天然地震信号中的P波在沉积基底界面转化为S波的这一性质来获得南海整个海盆内沉积层厚度分布和横向变化，在此基础上，结合沉积物物质来源、火成岩分布、对南海构造演化给出沉积层的响应分析。当天然地震激发的直达P波从深部传播至沉积层-玄武岩地壳界面处时，部分能量会转换成径向分量上的SV波，而台站接收到的垂直方向上的初至P波和径向上的（ ）之间的时间延迟取决于沉积层厚度，因此我们通过垂直分量的初至波P和径向分量上转换波 之间的时间延迟来反演沉积层厚度。在本研究中，我们收集了不同时期在南海的各个海盆布设的共32个被动源OBS台站数据，选取了全球范围内震级 5.5、震中距在24 到96 之间的远震三分量地震波形数据，经预处理后，剔除了其中6个无效记录台站，最终使用了26个OBS台站的329个三分量地震记录。在数据处理上，因为被动源OBS记录中有更高的噪声水平，对有效信号的识别干扰较大，为了提高信噪比和数据的利用率，我们应用了几个不同于陆地地震数据处理的方法，首先，因为OBS地震计的水平方位未知，为了获得精准的径向分量，我们应用Rayleigh波的极化特性了确定OBS的水平方位；其次，有些OBS地震计会存在一定角度的倾斜，通过求取水平分量到垂直分量之间的转换函数,可以求得倾斜角度和方向，据此对于原始的三分量记录做校正。在处理后波形上拾取到时，并获得了遍布南海海盆的329个时差 后，基于已有的南海海盆中 和 之间的经验关系，可以将两个震相之间的时差转换为沉积层厚度，得到整个南海海盆中沉积层厚度分布图。结果表明，海盆内沉积物厚度存在明显的空间变化：在整体上，沉积层沿洋中脊呈近似对称分布，反映了岩石圈年龄是沉积层厚度的决定因素；此外，在某些站点，以及临近海山和扩张后火山活动区域，沉积物厚度存在局部变浅的现象，反映了多期裂谷作用和火山作用等后扩张构造事件在局部改变了该海盆的沉积物的分布，说明扩张后海盆的构造事件也是决定南海海盆沉积层分布的一个控制因素。

Seafloor sediments not only record the history of global climate change, but also reflect the regional tectonic evolution. As a marginal sea, the sea-land interaction in the South China Sea (SCS) is more intense than in open oceans, its sediments, therefore, are more sensitive to climate change and tectonic evolution. In addition, the seismic wave travel time delay caused by the relatively thick sedimentary layer of the South China Sea will also affect the accuracy of deep structure seismic imaging.Although the active-source seismic profiling and IODP have conducted extensive studies, on local scales, on the sedimentary layers of the SCS, there is still a lack of problem of insufficient breadth. The large-scale passive source ocean bottom seismograph (OBS) deployment in the SCS provides a new approach for regional 2D seafloor sediment characteristics analysis. In this paper, the converted P-to-S waves at the sedimentary base from earthquakes are used to obtain the thickness and lateral change of the sediment layer in the entire SCS basin, through which the sediment responses to the tectonic events are presented. When the direct P-waves of earthquakes propagate from the deep to the sediment-basaltic crust interface, part of the energy will be converted into Sp waves in the radial component. The time delay between them depends on the thickness of the sedimentary layer, so we derive the thickness from the time delay between the first arrival wave P of the vertical component and the converted wave of the radial component. In this study, we collected data from a total of 32 passive source OBS stations deployed in various basins in the South China Sea, and selected the three components of global teleseismic events with magnitude greater than 5.5 and epicenter distance from 24 to 96 . After preprocessing the seismic waveform data, 6 invalid stations were eliminated, and finally 329 three-component seismic records from 26 OBS stations were used. Because the passive source OBS records have much higher noise, identifying phases is more difficult. We applied several different methods to improve the signal-to-noise ratio and data utilization. First, to obtain accurate radial components, we use the Rayleigh wave polarization characteristics to determine the horizontal orientation of the OBS; second, some OBS seismometers have tilt due to imperfect levelling. By calculating the transfer function between the horizontal component and the vertical component, the tilt angle and direction can be obtained, and the original three-component records are corrected accordingly. 329 P-to-S time delays are obtained by hand-picking on the processed waveform. Based on the existing empirical relationship of P and S velocities in the SCS, the time delays were converted into the thickness. The results show that there are obvious spatial changes in the thickness of the sediments in the basin: overall, the sediment layers are approximately symmetrically distributed along the mid-ocean ridge, reflecting that the lithosphere age controls the sediment thickness; in addition, at sites close to the seamounts and the post-spreading volcanic activity areas, the thicknesses are evidently decreased, suggesting the multi-phase rift, volcanism and other post-spreading tectonic events can locally change the distribution of sediments. This indicate that such post-spreading tectonic events are also factors affecting the distribution of sediments in the SCS.

OBS 噪声 沉积层厚度 延迟时间 转换 S 波 南海

OBS noise sediment thickness delay time converted S wave SCS

中文

联合培养

南方科技大学

杨港. 利用界面转换波研究南海海底沉积物特征[D]. 深圳. 哈尔滨工业大学,2020.