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

大约75%的地震发生在地球上60公里处。其余的事件，称为中深地震，发生在60至700公里的深度范围内，集中在岩石圈海洋板块内，在会聚的板块边界下降到地幔。这些事件的分布为地幔对流提供了独特而直接的证据，地幔对流是塑造地球表面的板块构造背后的驱动力。虽然从深源地震中获得的信息对于理解整个地球动力系统至关重要，但导致这些事件的物理机制仍然是一个谜。与浅源地震相反，深源地震发生在高温和高压应该抑制地震脆性破坏的深度。已经提出了几种机制来解释它们的发生，虽然它们之间的区分一直是困难的，部分原因是由于分辨率的限制，在地震速度模型，这是至关重要的约束的几何形状和内部物理性质的俯冲板，和地震源模型，其特征的空间和时间演变的震源区在地震故障。本研究的目的是提高地震速度结构和震源成像分辨率在日本，千岛群岛，和伊豆小笠原地区，主机的大量深地震。改进后的地震图像将阐明震源性质与俯冲板片内部结构之间的空间关系。这些关系将提供一套新的基本约束条件，以评估拟议的深源地震机制的可行性。通过这个项目，本科生将有机会从事拟议的研究活动，K-12外联活动将组织，以鼓励女孩追求STEM领域的职业生涯，并将对参加终身学习计划的成年人进行公开讲座。目前还不清楚深源地震在板块内的何处成核和传播，因此，上地幔下半部地球动态内部运作的细节仍然缺失。解决这个问题的关键取决于准确的高分辨率图像的板内部结构和深源震源属性。以前的地震图像分辨率和精度低于300公里的深度是有限的，从稀疏的数据覆盖范围和理论近似在传统的地震层析成像。经典的射线理论为基础的层析成像图像表明，深源震源符合板内的最高波速异常，传统上被视为板的冷芯，相变断层，涉及亚稳橄榄石的故障，被认为是深源地震的可能原因。然而，在先进的全波形层析成像技术的帮助下，东亚前所未有的地震数据集，日本，千岛群岛和伊豆-小笠原板块（EARA 2014）的新图像显示，深源地震始终发生在高波速区域的顶部附近，可能表明这些事件发生在俯冲板块的顶部附近。这一有趣的观察促使本提案使用高频全波形信息进一步探索和解决这些板块中的精细尺度波速变化和震源特性。该项目的核心假设是，深源地震成核并沿板块顶部沿着传播，即远离板块的冷核，板块顶部是大洋地壳和含水蛇纹层所在的地方。为了检验这一假设，将追求以下三个具体目标：（1）使用更高频率的地震波形，从现有的EARA 2014模型中获得空间分辨率更高的板状结构模型;（2）借助新的层析板状模型，重新定位深源震源并对深源地震破裂传播进行成像;（3）建立板片内部结构与深源地震位置和破裂性质之间的空间关系。如果该项目的核心假设得到了拟议工作的支持，那么我们对深源地震性质的理解将发生范式转变，因此需要考虑相变断层以外的机制。该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.