利用接收函数方法研究Cascadia海域地区地壳结构特征

Research on the Crustal Structure of Cascadia Sea Area by Receiver Function Method

杜浩然

11749042

硕士

物理学

杨挺

2019-05-27

2019-05-29

哈尔滨工业大学

深圳

位于太平洋东北部的Cascadia海域地区包括了Juan de Fuca洋中脊及其邻近的转换断层、Juan de Fuca板块和Cascadia俯冲带等活跃的地质构造带。作为中速扩张洋中脊的代表，Juan de Fuca洋中脊及其生成的板块在全球海洋岩石圈演化中占据独特地位；而Cascadia俯冲带可能发生的地震又对北美大陆西部的广大区域及在该区内密集分布的人口构成了严重的威胁，因此对于这一海域地区的岩石圈结构进行研究有着重要的地质构造和现实意义。本论文使用美国“Cascadia Initiative”项目布放在Cascadia海域地区的被动源海底地震仪（Ocean Bottom Seismometer，以下简称OBS）台阵所记录的天然地震数据，应用接收函数方法对这一海域地区的地壳结构特征进行了初步研究。由于OBS地震计难以实现与海底完美耦合，以及海洋活动导致的各类噪声，OBS数据，尤其是水平分量的数据质量相对不高，为了获得有效的接收函数，对OBS地震波形数据进行预处理是非常必要的。首先，我们对于OBS水平方位进行了验证。确定OBS水平方位的常规方法是利用Rayleigh波的极化特性，我们应用一种不同的方法，即利用Rayleigh波和Love波在径向分量和切向分量上能量分布的不同特征，对每个OBS的水平方位进行了再验证。其次，某些OBS的接收函数中呈现出与方位角相关的P波负极性表明其地震计可能存在倾斜的状况，我们对其进行了倾斜校正。此外，我们还去除了与水压有关的海底屈服噪声（compliance noise），来提高低频信号的质量。这些预处理工作有效地改善了OBS数据质量，由此我们得到了质量较高的接收函数。我们对于横跨Juan de Fuca板块两个剖面上的15个OBS所记录的、震级≥5.5、震中距在30°到90°之间的经筛选的214个全球地震进行了接收函数分析，通过叠加其接收函数得到了Moho面Ps转换波的时差，并根据这个时差利用H-к叠加搜索方法对该区域的地壳结构特征进行了探讨。结果表明，Cascadia海域内由Juan de Fuca洋中脊处至Cascadia俯冲带，两个剖面上接收函数结果中P波与Ps转换波的时差值从0.7s均匀变化到1.8s，这一特征说明海底地壳厚度随着年龄逐渐增大，地壳上层的沉积物随着年龄的增加而固结成岩是导致这一观测的可能原因。为了进一步考虑该海域内沉积层对接收函数可能产生的影响，我们利用直达P波与其在沉积层-地壳上界面处的PdS转换波之间的时差来对沉积层厚度进行约束。结果表明，该区域内的沉积层在靠近洋中脊处的厚度较小，由其造成的波形到时差在0.3s左右，对接收函数的影响可以忽略；而在靠近俯冲带处，沉积层厚度明显增大，波形到时差均值在1.5s以上，对接收函数结果产生了不容忽视的影响。因 此，基于传统接收函数方法难以准确地约束靠近俯冲带的Juan de Fuca板块的地壳厚度。

The Cascadia sea area, which is located at the northeastern part of the Pacific, includes many active tectonic units, such as mid-ocean-ridge, fracture zone, oceanic plate, and the subduction zone. As a typical intermediate spreading center, the Juan de Fuca Ridge and the plate generated from it have unique characteristics in global oceanic lithosphere evolutions. In addition, the possible earthquakes in the Cascadia subduction zone pose a serious risk to the densely populated area in western North America. Therefore, studying the lithospheric structure in this sea area has geological and societal significances. This paper uses the passive-source seismic data recorded by the Ocean Bottom Seismometer (OBS) array in the Cascadia sea area, which was supported by the US “Cascadia Initiative” program and employs the receiver function method to investigate the crustal structure beneath this region.Due to the fact that OBS cannot achieve perfect coupling with the seabed and various types of noise caused by ocean activities, the quality of OBS data is relatively low, particularly for the horizontal components. In order to obtain valid receiver function, it is critical to improve the quality of OBS waveform by applying a few preprocessing techniques. First, we verified the horizontal orientation of the OBS. The conventional method determining the orientation of OBS is to utilize the polarization of Rayleigh waves. We use a different approach, which is based on the relative changes of Rayleigh wave and Love wave amplitudes on the radial and tangential components, respectively, to revalidate the horizontal orientation for each OBS. Secondly, the back-azimuth dependent, negative polarities of the P signature on receiver functions at some OBSs indicate that the seismometers may be tilted. We corrected the tilts of seismometers. In addition, we improved the signal to noise ratio of low frequency contents by removing the compliance noise on waveforms. The quality of receiver functions had been noticeably improved after those procedures. We calculated receiver functions at 15 OBSs along two profiles traversing the Juan de Fuca plate. 214 earthquakes with magnitude greater than 5.5 and epicenter distance in the range of 30°to 90°were selected. By superimposing the receiver functions at each OBS, we obtained the average timing of the Moho converted phase Ps. We then applied the H-к method to reveal characteristics of crustal structure in the region. The results show that, from the Juan de Fuca Ridge to Cascadia subduction zone, the timings of Ps on the receiver functions at both profiles change systematically from 0.7 s to 1.8 s, suggesting the crustal thickness gradually increases. Consolidated sediments on the top of the crust are likely responsible for increasing crustal thickness as the plate aging.To further estimate the influences of the sedimentary layer on the receiver function in the region, we measured the time difference between the direct P wave on the vertical component and the slightly delated PdS on the radial component due to conversion occurred at the bottom of sediment. These measurements indicate the thickness of sedimentary layer is negligible near the Juan de Fuca Ridge with the time differences close to 0.3s. Their influences on the receiver function, therefore, can be neglected; However, for OBSs near the subduction zone, the thickness of sediment layer is significant with the average time difference greater than 1.5s. Such large time differences suggest the sediment has a huge impact on the receiver functions. Based on these observations, we conclude that it is difficult to use conventional receiver function analysis to constrain the crustal thickness due to the thick sediments near the Cascadia subduction zone.

接收函数 Cascadia海域 地壳厚度 沉积层

receiver function Cascadia sea area OBS crustal thickness sedimentary layer

中文

联合培养

南方科技大学

杜浩然. 利用接收函数方法研究Cascadia海域地区地壳结构特征[D]. 深圳. 哈尔滨工业大学,2019.