Oceanic lithospheric S-wave velocities from the analysis of P-wave polarization at the ocean floor
Oceanic lithospheric S-wave velocities from the analysis of P-wave polarization at the ocean floor
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DOI：
10.1093/gji/ggw342
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发表时间：
2016-12
影响因子：
2.8
通讯作者：
K. Hannemann;F. Krüger;T. Dahm;D. Lange
中科院分区：
文献类型：
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作者：
K. Hannemann;F. Krüger;T. Dahm;D. Lange
我们对大洋岩石圈S波绝对速度的了解主要是基于全球面波层析成像、局部活动地震或利用海洋次重力波进行的柔度测量。层析成像结果较好地反映了S的实际波速结构，局部测量对弹性参数的范围或测量的几何形状有一定的限制。本文利用远震事件的P波偏振(视P波入射角)，研究了大洋地壳和单台下几十公里地幔的S波速结构。在这项研究中，我们提出了一个目前已知的新的海底视P波入射角与半空间S波速的关系。对海底台站不同周期范围内的夹角进行了分析，得到了视S波速剖面。这些剖面不仅与S波的波速有关，还与地下各层的厚度有关。因此，它们的解释产生了一套同样有效的模型。本文分析了中大西洋东部一组OBS数据集的视P波入射角。经过人工数据质量控制，我们能够通过三步定量模拟确定合理的S波速-深度模型，尽管层共振有时会影响估计的视S波速。大洋PREM模型用水柱、沉积物、地壳和代表上地幔的层组成的四层来代替上层，较好地解释了S的视波速分布。得到的沉积物厚度在0.3~0.9公里之间，S波速度在0.7公里S−1到1.4公里S−1之间，估计的地壳总厚度在4公里到10公里之间，S波速度在3.5公里S−1到4.3公里S−1之间。我们发现总地壳厚度从∼5公里到∼8公里在一个主要的板块边界-格洛里亚断裂方向向南略有增加。观察到的地壳增厚可能与断裂附近已知的主导挤压有关。此外，由此产生的地幔S波速度向断裂方向从约5.5公里的S−1下降到4.5公里的S−1。这种下降可能是由蛇纹岩作用引起的，并表明大洋转换断层影响了最上地幔的一大片区域。总之，该方法对海底地震台下局部S波速度结构的估计是有用的。它易于实现，主要包括两个步骤：(1)测量真实和合成数据在不同周期范围内的视P波入射角；(2)比较实际和合成数据确定的S波速，以估计S波速-深度模型。
Our knowledge of the absolute S wave velocities of the oceanic lithosphere is mainly based on global surface wave tomography, local active seismic or compliance measurements using oceanic infragravity waves. The results of tomography give a rather smooth picture of the actual S wave velocity structure and local measurements have limitations regarding the range of elastic parameters or the geometry of the measurement. Here, we use the P wave polarization (apparent P wave incidence angle) of teleseismic events to investigate the S wave velocity structure of the oceanic crust and the upper tens of kilometres of the mantle beneath single stations. In this study, we present an up to our knowledge new relation of the apparent P wave incidence angle at the ocean bottom dependent on the half space S wave velocity. We analyse the angle in different period ranges at ocean bottom stations (OBS) to derive apparent S wave velocity profiles. These profiles are dependent on the S wave velocity as well as on the thickness of the layers in the subsurface. Consequently, their interpretation results in a set of equally valid models. We analyse the apparent P wave incidence angles of an OBS data set which was collected in the Eastern Mid Atlantic. We are able to determine reasonable S wave velocity-depth models by a three step quantitative modelling after a manual data quality control, although layer resonance sometimes influences the estimated apparent S wave velocities. The apparent S wave velocity profiles are well explained by an oceanic PREM model in which the upper part is replaced by four layers consisting of a water column, a sediment, a crust and a layer representing the uppermost mantle. The obtained sediment has a thickness between 0.3 km and 0.9 km with S wave velocities between 0.7 km s−1 and 1.4 km s−1. The estimated total crustal thickness varies between 4 km and 10 km with S wave velocities between 3.5 km s−1 and 4.3 km s−1. We find a slight increase of the total crustal thickness from ∼5 km to ∼8 km towards the South in the direction of a major plate boundary, the Gloria Fault. The observed crustal thickening can be related with the known dominant compression in the vicinity of the fault. Furthermore, the resulting mantle S wave velocities decrease from values around 5.5 km s−1 to 4.5 km s−1 towards the fault. This decrease is probably caused by serpentinization and indicates that the oceanic transform fault affects a broad region in the uppermost mantle. Conclusively, the presented method is useful for the estimation of the local S wave velocity structure beneath ocean bottom seismic stations. It is easy to implement and consists of two main steps: (1) measurement of apparent P wave incidence angles in different period ranges for real and synthetic data, and (2) comparison of the determined apparent S wave velocities for real and synthetic data to estimate S wave velocity-depth models.