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

造成破坏的地震和不会造成破坏的慢滑事件都会在构造板块边界释放累积的应力。虽然我们长期研究地震周期，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。一些基本问题仍然没有答案，比如：有多少构造运动是由缓慢滑动调节的？板块边界的什么物理条件决定了是否发生地震或慢滑？这将如何影响未来的地震危险？一次缓慢滑动事件能引发一次大地震吗？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，包含有关慢滑源的信息。这项研究将同时使用低频地震和“正常”地震，以比以前更高的分辨率梳理出慢滑动的物理学原理，从而揭示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平衡中占有重要地位，但对于慢滑的力学机制，以及其震源区域——高度变形的板块界面的构造性质，并没有很强的约束。小型脉冲地震事件称为低频地震（LFEs），通常伴随着慢滑动。这种独特的地震活动类型为研究慢滑提供了一个高分辨率的透镜，但我们还不了解它在高频内容方面的不足，而高频内容与慢滑的物理特性密切相关。本研究将对南开海槽（日本）的LFE进行局部高分辨率分析，以准确成像LFE源，并确定其频率含量低于典型地震的原因。有两种潜在的端元解释可以解释为什么低频率低频地震：(i)低频率低频地震源本身与“正常”地震源根本不同，并且不辐射高频，或者（ii）在高剪切板界面中的近源衰减消除了慢滑震源区域内的高频内容，然后才能被地表地震仪捕获。该研究将通过对南开海槽板块界面处的地壳和岩内地震进行新的频谱分析来研究这一问题。这将使板界面衰减结构的高分辨率图像成为可能，从而可以区分近源衰减在产生lfe特征低频特征中所起的作用。通过精确地确定衰减对在地表观测到的LFE光谱的影响，研究人员将能够约束LFE源。一系列基于聚类的光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。研究还将探索衰减和LFE源参数的随时间演变，以评估板界面在整个慢滑循环中如何变化。慢滑在地震周期中起着重要的作用，本研究将对慢滑源和主板块边界结构提供新的和必要的约束，揭示慢滑的动力学和物理。该项目由地球物理计划和极端事件预测和恢复能力（PREEVENTS）计划支持。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。

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.