基于远震接收函数的宜昌地区地壳结构

通过湖北地震台网获得宜昌附近5个测震台站2020—2022年远震波形记录,采用时间域迭代反卷积方法,提取测震台站下方径向接收函数,并根据H-κ叠加方法进行反演,估算台站下方地壳厚度和平均波速比。结果显示,研究区地壳厚度呈EW向差异变化,数值为31.4—46.1 km,平均波速比为1.59—1.87。

Crustal P-wave velocity structure of the Longmenshan region and its tectonic implications for the 2008 Wenchuan earthquake

The P-wave velocity structure of the crust in the Longmenshan region has been imaged by seismic travel time tomography us ing local and regional first P-wave arrivals recorded from 2000 to 2008. The tomographic model provides a way to analyze the deep tectonics of the Longmenshan fault belt and the tectonic implications for the 2008 Ms8.0 Wenchuan earthquake. The P-wave velocity images indicate that the initial rupture site and focal depth of the Wenchuan earthquake, together with the di rection of rupture propagation, closely relate to the crustal structure of the Longmenshan region. The Pengguan massif to the west of the Longmenshan fault belt is characterized by high velocity anomalies, suggesting that the crust has a strong strain strength that can accumulate large stresses over a long period. The Ms8.0 Wenchuan earthquake is located at the southwestern end of the Pengguan massif and the western edge of the Sichuan Basin. The collision between the Pengguan massif and the Sichuan Basin becomes the primary reason for the occurrence of the Ms8.0 Wenchuan earthquake. To the north of Wenchuan, the occurrence and propagation of rupture benefit from low velocity anomalies along the Longmenshan fault belt; whereas to the south of Wenchuan, the brittle rupture can occur with more difficulty in relatively weak crust with low velocities. This may be one of the reasons for the absence of aftershocks to the south of Wenchuan, and the rupture induced by the Ms8.0 Wenchuan earthquake propagating from the north to the south along the Longmenshan fault belt. The deep geodynamics of the Ms8.0 Wenchuan earthquake may occur due to the discrepancy of crustal structures on the two sides of the Longmenshan fault belt. Ductile deformation and crustal flow can easily occur in the weak middle-lower crust beneath the Songpan-Garze orogenic belt. The eastward movement of the Tibetan Plateau is obstructed by the rigid lithosphere of the Sichuan Basin, and then the thick ening of the middle-lower crust and vertical deformation occur in the crust of the Longmenshan fault belt. In addition, the down-warping of the Moho and the basement thrusting onto the range front induced crustal deformation and strain accumula tion, which provided the potential energy to trigger the occurrence of the Ms8.0 Wenchuan earthquake.

Indian plate blocked by the thickened Eurasian crust in the middle of the continental collision zone of southern Tibet

The relationship of the crustal contact between the Indian and Eurasian plates is a key issue in understanding crustal thickening and the subduction of the Indian lithosphere beneath the Qinghai-Tibetan Plateau. Across the middle of the Yarlung-Zangbo Suture(YZS), we deployed an ~450-km-long SN-trending wide-angle reflection/refraction profile to observe the P-wave velocity(vP) structure beneath the northern Himalaya and the southern plateau. Our results show that, 1. the high vP(~7.1 km/s) indicates that the Indian lower crust extends no more than 50 km north of the YZS. 2. The lower crust beneath the southern part of the plateau features an extremely low vP(<6.7 ± 0.2 km/s). 3. Compared with the velocities of several typical crustal lithologies in different temperature regimes, the low vPin the lower crust can be explained by felsic-intermediate granulite, which has prevented the lower crust from further eclogitization. We propose that the dip angle of the Indian lithospheric slab beneath the YZS is partly controlled by the composition of the lower crust of the plateau. In the northern middle YZS, the crust of the southern plateau is too thick and blocks the northward advancement of the Indian lower crust, resulting in the subduction of the Indian lithospheric slab into the upper mantle. The lower crust in western and eastern Lhasa is dominated by a mafic composition, and it was delaminated after eclogitization before the Miocene. The void zone generated by delamination favors the flattening and underthrusting of the Indian lower crust.

南海北部海陆过渡带地壳平均速度及莫霍面深度分析

南海北部海陆过渡带是一条深地震探测的空白带,其地壳结构研究不仅可以揭示华南沿海地区地震活动规律和构造活动特征,而且可以将以往在海上和陆上使用深地震探测所获得的地壳结构模型进行有效的衔接。鉴于南海北部海陆过渡带地壳结构的重要性,2004年我们在香港外海域进行了一次海陆地震联测实验。本文以此次实验中广东和香港地区固定地震台网以及担杆岛流动台所获得的数据为基础,对各台站接收到的Pg和PmP震相进行了近似横向均一的二维结构模拟,获得了各台站对应的地壳平均速度和PmP反射点处的莫霍面深度值。其结果显示研究区地壳平均速度普遍为6.2km·s-1左右,比全球地壳平均速度6.45km·s-1要低,这可能与普遍发育于华南沿海地带的中地壳低速层有关。研究区东边的SHW台所获得的地壳平均速度为6.7km·s-1左右,要明显高于全球地壳平均速度,它可能是由于局部异常结构所造成的,其与地幔岩浆的侵入作用可能存在一定联系。研究区莫霍面深度在26—35km左右,变化趋势较为明显,从北西向南东方向逐渐变浅,属于地壳减薄型结构,推测其可能与南海北部边缘的形成演化有密切联系。研究表明南海北部海陆过渡带是一个明显的转换地带,它的深部地壳结构特征应该与南海的扩张演化有密切联系,这些认识推进了华南地区和南海北部陆缘地壳结构和构造属性的深入研究。

基于GPS速度场的中国大陆地壳运动特征研究

根据2007~2009年、2009~2011年、2011~2013年GPS速度场及前3期加权平均后的2007~2013年速度场,分析了中国大陆主要运动地块的活动特征。从整体来看,现今中国大陆地壳运动的驱动力仍来自于印度板块向北的挤压,地壳活动值得关注的区域是断裂、地块间GPS速度场存在大幅差异的区域,主要体现在青藏高原北部,天山地区,鄂尔多斯东、西缘以及川滇菱形地块。

接收函数、面波与重力联合约束地壳厚度与波速比

提出了一种利用接收函数、面波频散和重力数据联合约束地壳厚度、vP/vS和平均P波速度的改进方法,并基于两种地壳模型对改进后的方法进行了验证。结果显示,改进后的方法不仅可以提高地壳厚度和波速比的估计精度,还能更准确地估计地壳平均P波速度。在此基础上,将该方法应用于华南地区两个固定台站的地壳结构分析,相关结果也证实了该方法在确定地壳结构中的可行性。

接收函数H-k叠加方法在龙门山台阵的应用

龙门山地区的深部构造研究对于龙门山断裂带的深部驱动机制有重要的意义,但由于天然地震太站分布较少,难以获得水平分辨较高覆盖整个龙门山地区的深部构造图像.本文利用最新中德合作在龙门山断裂带两侧布置的天然地震阵列式台站数据,获取了各个台站的接收函数波形数据.通过对H-K叠加方法的研究和改进,并应用到各个台站的实际接收函数数据分析上,获得了断裂带两侧以及沿断裂带的地壳厚度和平均速度比的分布,通过进一步插值形成了覆盖整个龙门山地区的水平分辨20Km的Moho面三维形态.综合对应的速度比和深部物质赋存状态关系的研究,获得以下结论：四川盆地属于平均厚度35-40km冷地壳,松潘和夹金山等山区地壳则属于厚45-50Km的热地壳.而沿断裂带变化比较明显,南段类似高原地区有热而厚的地壳,中段为40-45 km的正常地壳,个别地形突起处深约50 km.北段则逐渐向盆地的冷而薄的地壳过渡,厚度从43-40 km逐渐变化.Moho面的分布特征表明了龙门山断裂带的中部和南部的差异,并推测其间存在一个明显的Moho面异常突起带,与该过渡带的余震的缺失存在一定的关联.