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部60公里处。其余的事件，被称为中地震和深地震，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，在板块交界处下降到地幔。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。尽管从深层地震中获得的信息对于理解整个地球的动力系统是必不可少的，但引起这些事件的物理机制仍然是一个谜。与浅层地震相比，深层地震发生在高温高压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，尽管区分它们一直很困难，部分原因是由于地震速度模型和震源模型的分辨率限制，地震速度模型对限制俯冲板块的几何形状和内部物理性质至关重要，而震源模型则表征地震破坏期间震源区域的时空演变。本研究的目的是提高日本、千岛岛和伊豆-波宁地区的地震速度结构和震源成像分辨率，这些地区发生了大量的深部地震。改进后的地震图像将明确震源性质与俯冲板块内部结构之间的空间关系。这些关系将为评估所提出的深震源机制的可行性提供一套新的基本约束。通过该项目，本科生将有机会参与拟议的研究活动，将组织K-12外展活动，鼓励女孩从事STEM领域的职业，并将为参加终身学习计划的成年人提供有关这项工作的公开讲座。目前还不清楚深震源地震在什么地方形成核并在板块内传播，因此，地球上地幔下半部分的动态内部工作细节仍然缺失。要解决这一问题，关键取决于板层内部结构和深震震源特性的精确高分辨率图像。以往300公里以下深度的地震图像分辨率和精度受到传统地震层析成像中稀疏数据覆盖和理论近似的限制。基于经典射线理论的断层扫描图像表明，深震震源与板块内的最高波速异常相吻合，通常被认为是板块的冷核，其中涉及亚稳橄榄石破裂的相变断裂被认为是深震地震的可能原因。然而，利用东亚前所未有的地震数据集，借助先进的全波形层析成像技术，日本、千岛岛和伊竹-小原板块（EARA2014）的新图像显示，深源地震始终发生在高波区顶部附近，这可能表明这些事件发生在俯冲板块顶部附近。这一有趣的观察激发了本研究的动机，即利用高频全波形信息进一步探索和解决这些板块的细尺度波速变化和震源特性。这个项目的中心假设是，深源地震沿着板块的顶部形成核并传播，那里是海洋地壳和含水蛇纹岩层的所在地，即远离板块的冷核。为了验证这一假设，我们将追求以下三个具体目标：(1)利用更高频率的地震波形，从现有模型EARA2014中获得空间分辨率更高的板结构模型；(2)利用新的层析板模型重新定位深震震源，对深震破裂传播过程进行成像；(3)建立板坯内部结构与深震源地震位置和破裂特性的空间关系。如果这个项目的中心假设被提议的工作所支持，那么就我们对深震源地震性质的理解而言，将会有一个范式的转变，因此，除了相变断层作用之外，还需要考虑其他机制。该奖项反映了美国国家科学基金会的法定使命，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.