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方60公里处。其余的地震被称为中深地震，发生在60至700公里的深度范围内，集中在岩石圈洋板内，在会聚的板块边界向下进入地幔。这些事件的分布为地幔对流提供了独特和直接的证据，地幔对流是板块构造背后的驱动力，塑造了地球表面。尽管从深部地震中获得的信息对于了解整个地球动力系统是必不可少的，但导致这些事件的物理机制仍然是一个谜。与浅层地震不同，深部地震发生在高温和高压应该会抑制地震脆性破坏的深处。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，部分原因是地震速度模型和震源模型的分辨率限制，前者对限制俯冲板块的几何和内部物理性质至关重要，后者描述了地震破坏期间震源区的空间和时间演化。这项研究的目标是提高日本、千岛和伊豆-博宁地区的地震速度结构和震源成像分辨率，这些地区是深部地震的发源地。改进后的地震图像将阐明震源性质与俯冲板块内部结构之间的空间关系。这些关系将为评估拟议的深源机制的可行性提供一套新的基本约束。通过该项目，本科生将有机会从事拟议的研究活动，将组织K-12外联活动以鼓励女孩从事STEM领域的职业生涯，并将向参与终身学习计划的成年人提供有关工作的公开讲座。目前还不清楚深源地震在板块内的哪里成核和传播，因此，地球上地幔下半部分的动态内部运作的细节仍然缺乏。解决这一问题关键取决于对板片内部结构和深源震源属性的准确高分辨率图像。由于传统地震层析成像中使用的稀疏数据覆盖范围和理论近似，以往在300公里以下深度的地震图像分辨率和精度受到限制。基于经典射线理论的层析成像图像表明，深源震源与板块内波速最高的异常重合，传统上被视为板块的冷核，在那里，相变断层，包括亚稳态橄榄石的破裂，被认为可能是深源地震的原因。然而，借助先进的全波形层析成像技术，东亚史无前例的地震数据集--日本板块、千岛板块和伊豆-博宁板块(EARA2014)的新图像显示，深源地震一直发生在高波速区域的顶部附近，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这一提议，利用高频全波形信息进一步探索和解决这些板块中的精细波速变化和震源属性。该项目的中心假设是，深源地震沿着板块顶部成核和传播，那里是洋壳和含水蛇纹层所在的位置，即远离板块的冷核。为了验证这一假设，将追求以下三个具体目标：(1)使用更高频率的地震波形，从现有模型EARA2014获得具有更高空间分辨率的板块结构模型；(2)借助新的层析成像板块模型，重新定位深源震源，并拍摄深源地震破裂传播的图像；(3)建立板块内部结构与深源地震位置和破裂性质之间的空间关系。如果该项目的中心假设得到拟议工作的支持，那么我们对深源地震本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。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.