Probing Depth-dependent Heterogeneities under Japan Using Transmitted Seismic Waves

使用传输的地震波探测日本地下的深度相关异质性

• 批准号: 0838359
• 负责人: Yingcai Zheng
• 金额: --
• 依托单位: University of California-Santa Cruz
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2009
• 资助国家: 美国
• 起止时间: 2009-01-01 至 2010-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0838359&HistoricalAwards=false
• 关键词: Probing; Depth; dependent; Heterogeneities; under

At present, the Earth?s interior dynamics are mainly inferred by detecting seismic wave velocity anomalies in the Earth using deterministic approaches such as seismic tomography. The anomaly is mapped point-wise within the Earth and it can be interpreted as a snapshot of the convective mantle and crust. However, the spatial resolution of such deterministic methods is limited by sparse geographic distribution of seismographs. Given the distribution of current seismograph networks, the length scale of the heterogeneity resolved by global tomography is on the order of several hundred kilometers. On the other hand, there are important geological processes, taking place on the Earth?s surface at much smaller length scales, from the order of tens of kilometers to meters. In order to gain a sound understanding of Earth dynamics, the ?missing? information at the high end (i.e., those small-scale heterogeneities) of the continuous heterogeneity spectrum must be supplemented. However, seismic tomography is far from achieving high-resolution deterministic models for structures below the crust, and a novel approach must be devised.In the work proposed here, a new stochastic inversion theory is formulated mathematically, which is based on wave propagation through random media, and it is able to yield statistical information on small-scale heterogeneities as an assemblage, such as the size distribution of heterogeneities, the wave scattering strength of the heterogeneity, and how they are distributed with depth. They physics behind the theory is that small-scale heterogeneities manifest themselves in transmitted seismic body waves as fluctuations of phase (or travel time, or arrival time) and wave amplitude across a seismic array. These observed fluctuations are then used to form coherence functions, which simultaneously depend on the distance between two seismic stations and the incident angles between two plane waves. The coherence function has sharp depth resolution and can be used to invert for the depth-dependent heterogeneity spectrum. This newly developed mathematical framework will be used to analyze data recorded by the HiNet seismic network in Japan. This densely instrumented large-aperture network has the potential to constrain heterogeneity spectra over depths from the surface to several hundred kilometers. The proposed work here may address several fundamental issues concerning: (1) the fate of subducted materials in the mantle (2) characterization of the seismic wave scattering properties for ?hot fingers?, source regions feeding the volcanoes under Japan, and non-volcanic regions; (3) seismic constraints on the slab-mantle interaction and evolution of the mantle wedge; (4) multi-scale analysis of mantle heterogeneities and at different depths may provide a means to isolate different physical and chemical mechanism that is responsible for each length scale. The statistical information on the small-scale heterogeneity is also critical in other disciplines of Earth sciences, for instance, petrology, geochemistry, tectonics and hydrodynamic modeling of the convective Earth. The mathematical methods developed here can potentially be used in other scientific areas related to wave propagation in random media, such as, ocean acoustics, electromagnetic wave propagation and communications, medical imaging, and helioseismology.

目前，地球？的内部动力学主要是通过使用确定性方法（如地震层析成像）检测地球中的地震波速度异常来推断的。该异常在地球内部逐点绘制，它可以被解释为对流地幔和地壳的快照。然而，这种确定性方法的空间分辨率受到地震仪稀疏地理分布的限制。考虑到目前地震仪网络的分布，全球层析成像解决的非均匀性的长度尺度是几百公里的数量级。另一方面，有重要的地质过程，发生在地球上？在更小的长度尺度上，从几十公里到几米。为了获得一个健全的了解地球动力学，？失踪了吗高端的信息（即，那些小尺度的不均匀性）必须被补充。本文提出了一种新的随机反演理论，该理论以波在随机介质中的传播为基础，能够将小尺度非均匀性作为一个集合体，得到非均匀性的统计信息，如非均匀性的大小分布、非均匀性的分布特征等。不均匀体的波散射强度，以及它们如何随深度分布。该理论背后的物理原理是，小尺度非均匀性在传输的地震体波中表现为相位（或旅行时间或到达时间）和地震阵列上的波幅波动。这些观测到的波动，然后用来形成相干函数，这同时取决于两个地震台站之间的距离和两个平面波之间的入射角。相干函数具有很高的深度分辨率，可以用来反演深度相关的非均匀性谱。这一新开发的数学框架将用于分析日本HiNet地震网络记录的数据。这种密集仪器化的大孔径网络有可能限制从地表到几百公里深度的异质性光谱。这里提出的工作可以解决几个基本问题：（1）在地幔俯冲物质的命运（2）地震波散射特性的表征？热手指？，（3）地震对板幔相互作用和地幔楔演化的制约;（4）对不同深度地幔非均匀性的多尺度分析可以提供一种方法来分离出不同的物理和化学机制，这些机制与每个长度尺度有关。小尺度非均匀性的统计信息在地球科学的其他学科中也是至关重要的，例如岩石学、地球化学、构造学和对流地球的流体动力学模拟。这里开发的数学方法有可能用于与随机介质中波传播相关的其他科学领域，例如海洋声学、电磁波传播和通信、医学成像和日震学。