Common reflection point mapping of the mantle transition zone using recorded and 3-D synthetic ScS reverberations
Common reflection point mapping of the mantle transition zone using recorded and 3-D synthetic ScS reverberations
复制标题
DOI：
10.1093/gji/ggz467
复制
发表时间：
2019
影响因子：
2.8
通讯作者：
S. Haugland;J. Ritsema;Daoyuan Sun;J. Trampert;M. Koroni
中科院分区：
文献类型：
--
作者：
S. Haugland;J. Ritsema;Daoyuan Sun;J. Trampert;M. Koroni
南海混响偏移方法是基于对地幔过渡带（MTZ）多个南海反射的“共反射点”分析。我们研究了射线理论的传播时间、慢度和反射点是否足够精确，以估计MTZ的厚度H，由410和660公里相变之间的距离定义。首先，我们分析了35次地震产生的南海混响，这些地震是由中国的联合阵列、日本的Hi-NET和全球地震台网的数百个地震台站记录的。这一分析表明，H变化约30 km，因此，动力过程改变了东亚和西太平洋地区MTZ的大尺度结构。其次，我们将同样的方法应用于PREM和两个三维模型的谱元合成。一个3-D模型在410和660不连续面上包含了20度地形，否则将保留PREM速度模型。另一个模型结合了S20RTS的20度速度非均质性，使410和660保持不变。为了优化反射点覆盖范围，我们假设使用16个事件（其中4个是虚构的）的均匀站点网格来计算合成结果。使用PREM合成的图像与PREM结构相似，表明偏移方法是正确的。然而，ScS混响并不像预测的那样对H敏感——理论上是因为在410和660:H上具有20度地形的模型中，合成材料的迁移在分辨率图像中变化不到5公里，而在原始模型中变化10公里。此外，从S20RTS速度模型合成物的迁移和周期为15 s的sc混响的宽灵敏度核可以明显看出，全地幔剪切速度非均质性的影响相对较强。用射线理论的方法来模拟长周期的南海旅行时间似乎是不准确的，至少对于地震覆盖相对较少的大陆尺度地区来说是不准确的。附加的建模和与SS前驱和接收函数结果的比较应该依赖于各种结构的三维波形模拟，并最终实现全波理论。
The method of ScS reverberation migration is based on a ‘common reflection point’ analysis of multiple ScS reflections in the mantle transition zone (MTZ). We examine whether ray-theoretical traveltimes, slownesses and reflection points are sufficiently accurate for estimating the thickness H of the MTZ, defined by the distance between the 410- and 660-km phase transitions. First, we analyse ScS reverberations generated by 35 earthquakes and recorded at hundreds of seismic stations from the combined Arrays in China, Hi-NET in Japan and the Global Seismic Network. This analysis suggests that H varies by about 30 km and therefore that dynamic processes have modified the large-scale structure of the MTZ in eastern Asia and the western Pacific region. Second, we apply the same procedure to spectral-element synthetics for PREM and two 3-D models. One 3-D model incorporates degree-20 topography on the 410 and 660 discontinuities, otherwise preserving the PREM velocity model. The other model incorporates the degree-20 velocity heterogeneity of S20RTS and leaves the 410 and 660 flat. To optimize reflection point coverage, our synthetics were computed assuming a homogeneous grid of stations using 16 events, four of which are fictional. The resolved image using PREM synthetics resembles the PREM structure and indicates that the migration approach is correct. However, ScS reverberations are not as strongly sensitive to H as predicted ray-theoretically because the migration of synthetics for a model with degree-20 topography on the 410 and 660: H varies by less than 5 km in the resolved image but 10 km in the original model. In addition, the relatively strong influence of whole-mantle shear-velocity heterogeneity is evident from the migration of synthetics for the S20RTS velocity model and the broad sensitivity kernels of ScS reverberations at a period of 15 s. A ray-theoretical approach to modelling long-period ScS traveltimes appears inaccurate, at least for continental-scale regions with relatively sparse earthquake coverage. Additional modelling and comparisons with SS precursor and receiver function results should rely on 3-D waveform simulations for a variety of structures and ultimately the implementation of full wave theory.