从长期分裂测量结果看中欧地下岩石圈-热成层系统地震各向异性的横向和纵向变化

IF 2.8 3区 地球科学 Q2 GEOCHEMISTRY & GEOPHYSICS
Yvonne Fröhlich, Michael Grund, Joachim R R Ritter
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引用次数: 0

摘要

摘要 德国西南部黑森林天文台(BFO)岩心折射剪切波(SKS、SKKS 和 PKS 相,合称 XKS)的剪切波分裂的背方位变化显示了地震各向异性的小尺度横向和部分纵向变化。然而,针对阿尔卑斯山前缘北部莱茵河上游地堑(URG)附近地区的现有各向异性研究和模型大多基于短期记录,因此存在反方位角覆盖范围有限以及在较大或整个反方位角范围内平均的问题。为了确定和划分该地区的横向约束各向异性机制,我们在六个相邻的(半)永久性宽带地震记录台站(台站间距 10-80 公里)进行了 XKS 分裂测量。我们对长期(部分为 20 年)记录进行人工分析,以获得足够的方位角覆盖范围,从而解决复杂的各向异性问题。在单事件和多事件分析中确定了分裂参数(快速极化方向 $\phi $,延迟时间 $\delta t$)。我们测试了具有水平或倾斜对称轴的一层和具有水平对称轴的两层(横向各向同性)的结构各向异性模型。为了考虑单个记录点周围的横向变化,我们对整个记录点和有限的方位角范围进行了建模比较。在此基础上,我们提出了具有各向异性空间变化的三维块体模型。根据延迟时间> 0.3秒以及不支持下地幔各向异性的SKS和SKKS阶段之间的缺失差异,将发现的各向异性置于岩石圈和星体层。分裂参数的空间分布以及横向和背方位变化证实了各向异性的横向和部分垂直变化。在 URG 的东侧,我们认为 Moldanubian 区(南侧)有两个各向异性层,Saxothuringian 区(北侧)有一个各向异性层。在莫尔达努比亚区,我们观察到 URG 东西两侧的快速极化方向发生了变化,这表明存在不同的纹理。在两块地体的交界处,模拟出了倾斜的各向异性,这可能与瓦利斯坎俯冲过程中的变形有关。关于在 URG 南部(BFO 西南部)观察到的大量空测量和不一致的分裂参数,对不同的假设进行了检验:小规模横向异质导致的地震波场散射、与凯泽施图尔火山群有关的垂直地幔流导致的垂直 a 轴,以及特定环境条件(高温、含水量)导致的橄榄石晶体的不同优先取向(不是 A 型,而是 C 型)。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
Lateral and vertical variations of seismic anisotropy in the lithosphere-asthenosphere system underneath Central Europe from long-term splitting measurements
Summary Backazimuthal variations in the shear wave splitting of core-refracted shear waves (SKS, SKKS, and PKS phases, jointly referred to as XKS) at the Black Forest Observatory (BFO, Southwest Germany) indicate small-scale lateral and partly vertical variations of the seismic anisotropy. However, existing anisotropy studies and models for the nearby Upper Rhine Graben (URG) area in the northern Alpine foreland are mostly based on short-term recordings and by this suffer from a limited backazimuthal coverage and averaging over a wide or the whole backazimuth range. To identify and delimit laterally confined anisotropy regimes in this region, we carry out XKS splitting measurements at six neighbouring (semi-)permanent broadband seismological recording stations (inter-station distance 10-80 km). We manually analyse long-term (partly > 20 yr) recordings to achieve a sufficient backazimuthal coverage to resolve complex anisotropy. The splitting parameters (fast polarization direction $\phi $, delay time $\delta t$) are determined in a single- and multi-event analysis. We test structural anisotropy models with one layer with horizontal or tilted symmetry axis and with two layers with horizontal symmetry axes (transverse isotropy). To account for lateral variations around a single recording site, modelling is compared for the whole and for limited backazimuth ranges. Based on this, we provide a 3-D block model with spatial variation of anisotropic properties. Based on delay times > 0.3 s and missing discrepancies between SKS and SKKS phases, which do not support lower mantle anisotropy, the found anisotropy is placed in the lithosphere and asthenosphere. The spatial distribution as well as the lateral and backazimuthal variations of the splitting parameters confirm lateral and partly vertical variations in anisotropy. On the east side of the URG, we suggest two anisotropic layers in the Moldanubian Zone (south) and one anisotropic layer in the Saxothuringian Zone (north). In the Moldanubian Zone, a change of the fast polarization directions is observed between the east and the west side of the URG, indicating different textures. At the boundary between the two terranes, an inclined anisotropy is modelled which may be related with deformation during Variscan subduction. Regarding the observation of numerous null measurements and inconsistent splitting parameters, especially (southwest of BFO) in the southern URG, different hypothesis are tested: scattering of the seismic wavefield due to small-scale lateral heterogeneities, a vertical a-axis due to a vertical mantle flow related to the Kaiserstuhl Volcanic Complex, as well as a different preferred orientation of the olivine crystals (not A-type, but C-type) due to specific ambient conditions (high temperature, water content).
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来源期刊
Geophysical Journal International
Geophysical Journal International 地学-地球化学与地球物理
CiteScore
5.40
自引率
10.70%
发文量
436
审稿时长
3.3 months
期刊介绍: Geophysical Journal International publishes top quality research papers, express letters, invited review papers and book reviews on all aspects of theoretical, computational, applied and observational geophysics.
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