Collaborative Research: What makes Low-Frequency Earthquakes low frequency?

合作研究：是什么让低频地震频率低？

• 批准号: 1920899
• 负责人: Rachel Abercrombie
• 金额: $ 15.9万
• 依托单位: Trustees of Boston University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2019
• 资助国家: 美国
• 起止时间: 2019-08-01 至 2024-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1920899&HistoricalAwards=false
• 关键词: Collaborative; Research; What; makes; Low

Both earthquakes, which cause damage, and slow slip events, which do not, release built-up stress at tectonic plate boundaries. While we have long studied the earthquake cycle, the recent discovery of slow slip events (lasting days to years) has upended our understanding of this cycle. There are still no answers to fundamental questions such as: how much tectonic motion is accommodated by slow slip? What physical conditions at plate boundaries determine whether an earthquake or slow slip occurs? How does this affect future earthquake hazard? Can a slow slip event trigger a major earthquake? In this international collaboration with scientists from Japan and New Zealand, the researchers will investigate these questions and others by studying the subduction of the Philippine Sea plate beneath Japan. They will use low-frequency earthquakes, which are unique seismic signature containing information about the source of slow slip. The study will use both low-frequency earthquakes and "normal" earthquakes to tease out the physics of slow slip at a higher-resolution than was previously possible, shedding light on the different styles of fault slip that occur on tectonic plate boundaries.Despite the importance of slow slip in the tectonic slip budget, there are not strong constraints on the mechanics of slow slip, or the structural properties of its source region, the highly deformed plate interface. Small, impulsive seismic events called low-frequency earthquakes (LFEs), commonly accompany slow slip. This unique type of seismicity provides a high-resolution lens through which to study slow slip, but we do not yet understand its deficiency in high-frequency content, which is intimately tied to the physics of slow slip. This study will conduct a localized, high-resolution analysis of LFEs in the Nankai Trough (Japan) to accurately image the LFE source and determine why their frequency content is lower than that of typical earthquakes. There are two potential end-member explanations for why LFEs are low frequency: (i) either the LFE source itself is fundamentally different from that of "normal" earthquakes, and does not radiate high-frequencies, or (ii) near-source attenuation in the highly sheared plate interface removes the high frequency content within the slow slip source region before it can be captured at seismometers located at the surface. The study will investigate this problem by performing a novel spectral analysis of crustal and intraslab earthquakes sandwiching LFEs at the plate interface in the Nankai Trough. This will enable high-resolution images of attenuation structure of the plate interface, allowing for the discrimination of the role near-source attenuation plays in generating LFEs' characteristic low-frequency signature. By precisely determining the effect attenuation has on LFE spectra observed at the surface, the researchers will be able to constrain the LFE source. A range of cluster-based spectral and time-domain approaches will be used to model the events, and search for differences between regions with and without LFEs. The research will also explore the time-dependent evolution of attenuation and LFE source parameters to evaluate how the plate interface changes throughout the slow slip cycle. Slow slip plays a significant role in the earthquake cycle, and this research will provide new and essential constraints on the LFE source and the structure of a major plate boundary, shedding light on the dynamics and physics of slow slip.This project is supported by the Geophysics program and the Prediction of and Resilience against Extreme Events (PREEVENTS) program.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.

造成破坏的地震和不造成破坏的缓慢滑动事件都释放了构造板块边界的累积应力。虽然我们长期以来一直在研究地震周期，但最近发现的缓慢滑动事件（持续数天至数年）颠覆了我们对这个周期的理解。对于一些基本的问题，例如：缓慢滑动能容纳多少构造运动，至今仍没有答案。板块边界的什么物理条件决定了是否发生地震或缓慢滑动？这将如何影响未来的地震灾害？慢滑动事件会引发大地震吗？在这次与日本和新西兰科学家的国际合作中，研究人员将通过研究菲律宾海板块在日本下方的俯冲来调查这些问题和其他问题。他们将使用低频地震，这是一种独特的地震特征，包含有关缓慢滑动源的信息。这项研究将使用低频地震和“正常”地震，以比以前更高的分辨率梳理出慢滑的物理学，阐明发生在构造板块边界上的不同类型的断层滑动。尽管慢滑在构造滑动预算中很重要，但对慢滑的力学或其震源区的结构特性没有很强的限制，高度变形的板块界面小的，脉冲地震事件称为低频地震（LFEs），通常伴随着缓慢的滑动。这种独特类型的地震活动提供了一个高分辨率的透镜，通过它来研究慢滑，但我们还不了解它在高频含量方面的不足，这与慢滑的物理学密切相关。这项研究将对日本南海海槽的低频地震进行局部化的高分辨率分析，以准确地对低频地震源进行成像，并确定其频率含量低于典型地震的原因。有两个潜在的端元解释为什么LFE是低频率的：（i）LFE源本身是从根本上不同于“正常”地震，并不辐射高频，或（ii）近源衰减在高度剪切板界面删除慢滑震源区域内的高频内容之前，它可以被捕获在位于地面的地震仪。该研究将通过对南海海槽板界面夹有LFE的地壳和实验室内地震进行新型谱分析来研究这个问题。这将使板界面的衰减结构的高分辨率图像，允许近源衰减在产生LFE的特征低频签名中所起的作用的区别。通过精确确定衰减对在表面观察到的LFE光谱的影响，研究人员将能够限制LFE源。一系列基于聚类的频谱和时域方法将用于对事件进行建模，并搜索有和没有LFE的区域之间的差异。该研究还将探索衰减和LFE源参数随时间的演变，以评估板块界面在整个缓慢滑动周期中的变化。慢滑动在地震周期中起着重要作用，这项研究将为LFE源和主要板块边界结构提供新的和必要的约束，揭示了慢滑的动力学和物理学。该项目得到了地球物理学计划和极端事件的预测和恢复的支持该奖项反映了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。