Collaborative Research: Exploring the nature of deep-focus earthquakes in the Japan, Kuril, and Izu-Bonin subduction zones

合作研究：探索日本、千岛群岛和伊豆-小笠原俯冲带深源地震的性质

基本信息

批准号：
1802247
负责人：
Songqiao Wei
金额：
$ 33.05万
依托单位：
Michigan State University
依托单位国家：
美国
项目类别：
Continuing Grant
财政年份：
2018
资助国家：
美国
起止时间：
2018-08-15 至 2023-07-31
项目状态：
已结题

来源：
https://www.nsf.gov/awardsearch/showAward?AWD_ID=1802247&HistoricalAwards=false
关键词：
Collaborative Research Exploring nature deep

项目摘要

About 75% of all earthquakes occur in the upper 60 km of the Earth. The remaining events, known as intermediate and deep earthquakes, take place over a depth range of 60 to 700 km and are focused within lithospheric oceanic slabs descending into the mantle at convergent plate boundaries. The distribution of these events has provided unique and direct evidence of mantle convection, the driving force behind plate tectonics that shapes the surface of the Earth. Though the information derived from deep earthquakes have been essential for understanding the Earth's dynamic system as a whole, the physical mechanisms causing these events are still a mystery. In contrast to their shallow counterparts, deep earthquakes occur at depths where high temperatures and pressures should inhibit seismic brittle failure. Several mechanisms have been proposed to explain their occurrence, though differentiating between them has been difficult partly due to resolution limitations in both seismic velocity models, which are critical in constraining the geometry and internal physical properties of subducting slabs, and earthquake source models, which characterize the spatial and temporal evolution of source regions during seismic failure. The goal of this study is to improve seismic velocity structure and earthquake source imaging resolution in the Japan, Kuril, and Izu-Bonin regions, which host a significant number of deep earthquakes. The improved seismic images will clarify the spatial relationships between earthquake source properties and the internal structure of subducting slabs. These relationships will provide a new set of fundamental constraints for evaluating the viability of proposed deep earthquake source mechanisms. Through this project, undergraduate students will have opportunities to work on the proposed research activities, K-12 outreach events will be organized to encourage girls to pursue STEM field careers, and public lectures will be given on the work to adults who participate in lifelong learning programs. It is still unclear where deep-focus earthquakes nucleate and propagate within a slab, and as a result, details of the Earth's dynamic inner workings in the lower half of the upper mantle are still missing. Addressing this issue critically depends on accurate high-resolution images of both the slab internal structure and deep-focus earthquake source properties. Previous seismic image resolution and accuracy at depths below 300 km were limited from sparse data coverage and theoretical approximations used in traditional seismic tomography. Classical ray-theory based tomography images indicate that deep-focus hypocenters coincide with the highest wavespeed anomalies within the slab, traditionally viewed as the slab's cold core, where phase transformational faulting, involving the breakdown of metastable olivine, is considered as a likely cause of deep-focus earthquakes. However, with an unprecedented seismic data set in East Asia aided by the advanced full waveform tomography technique, the new images of the Japan, Kuril, and Izu-Bonin slabs (EARA2014) show that deep-focus earthquakes consistently occur near the top of high wavespeed regions, possibly indicating that these events occur near the top of the subducting slab. This intriguing observation motivated this proposal to further explore and resolve the fine-scale wavespeed variations and earthquake source properties in these slabs using high frequency full waveform information. The central hypothesis of this project is that deep-focus earthquakes nucleate and propagate along the top of the slab, where oceanic crust and a hydrous serpentine layer are located, i.e. away from the slab's cold core. In order to test this hypothesis, the following three specific goals will be pursued: (1) obtain a slab structural model with improved spatial resolution from the existing model EARA2014 using higher frequency seismic waveforms; (2) relocate deep-focus hypocenters and image deep-focus earthquake rupture propagation with the aid of the new tomographic slab model; (3) establish spatial relationships between the slab internal structure and deep-focus earthquake locations and rupture properties. If the central hypothesis of this project is supported by the proposed work, then there will be a paradigm shift in terms of our understanding of the nature of deep-focus earthquakes, and consequently, mechanisms other than phase transformational faulting need to be considered.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

大约75％的地震发生在地球上60公里处。其余事件（称为中间和深层地震）发生在60至700 km的深度范围内，并集中在岩石圈海洋板上，在收敛板边界下降落到地幔中。这些事件的分布提供了地幔对流的独特而直接的证据，这是塑造地球表面的板块构造背后的驱动力。尽管从深层地震中得出的信息对于理解整个地球动态系统至关重要，但导致这些事件的物理机制仍然是一个谜。与它们的浅层相比，高温和压力应抑制地震脆性失败的深度发生深层地震。已经提出了几种机制来解释它们的发生，尽管它们之间的区分很困难，部分原因是两种地震速度模型的分辨率限制，这对于约束俯冲板的几何形状和内部物理特性至关重要，地震源模型的几何形状和内部物理特性，这表征了在地震失败期间源区域的空间和时间进化的特征。这项研究的目的是改善日本，Kuril和Izu-Bonin区域的地震速度结构和地震源成像分辨率，这些区域占据了大量深层地震。改进的地震图像将阐明地震源特性与俯冲板的内部结构之间的空间关系。这些关系将为评估拟议深层地震来源机制的生存能力提供一套新的基本限制。通过该项目，本科生将有机会从事拟议的研究活动，K-12外展活动将鼓励女孩从事STEM现场职业，并将公开讲座向参加终身学习计划的成年人提供。目前尚不清楚地震在板块中成核并传播的地方尚不清楚，结果，地球下半部下半部的动态内部工作的细节仍然缺失。在关键上解决此问题取决于平板内部结构和深度焦点地震源特性的准确高分辨率图像。以前的地震图像分辨率和低于300公里的深度的准确性受到传统地震断层扫描中使用的稀疏数据覆盖率和理论近似的限制。基于经典的射线理论的层析成像图像表明，深对焦的低中心与平板内的最高波动异常相吻合，传统上被视为平板的冷核，在这种情况下，相位转化的断层涉及亚抗体橄榄石的崩溃，被认为是引起深焦点地震的可能原因。然而，在东亚的前所未有的地震数据得到了高级完整波形层析造影技术的帮助下，日本，库里尔和伊祖 - 波宁板的新图像（EARA2014）表明，深层地震始终出现在高波速区的顶部附近，可能表明这些事件发生在pop the the the the the the the the the the the the the the the the the the the the the top doctab topepeed地震。这种有趣的观察促使这一提议进一步探索和解决这些板的细尺度波动变化和地震源特性，使用高频全波形信息。该项目的中心假设是，深焦点地震有核并沿着平板的顶部传播，那里的海洋壳和含水蛇形层位于远离板的冷核。为了检验这一假设，将实现以下三个特定目标：（1）使用高频地震波形从现有模型EARA2014中获得改进的空间分辨率的平板结构模型；（2）借助新的断层板平板模型重新定位了深对焦的降压器和图像深焦点地震繁殖；（3）在平板内部结构与深层地震位置和破裂特性之间建立空间关系。如果拟议的工作支持该项目的中心假设，那么就需要考虑对深度关注地震的性质的理解，而除了相变性之外的机制以外的其他机制将被考虑。该奖项反映了NSF的法定任务，并通过评估基金会的范围和广泛的范围来反映了该奖项。