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积聚的应力。虽然我们对地震周期的研究由来已久，但最近发现的慢滑事件(持续数天到数年)颠覆了我们对这一周期的理解。基本的问题仍然没有答案，比如：慢滑在多大程度上适应了构造运动？板块边界的什么物理条件决定了是发生地震还是发生缓慢滑动？这对未来的地震风险有何影响？慢滑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研究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，包含了关于缓慢滑动来源的信息。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，揭示出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的重要性，但对慢滑的机制或其源区--高度变形的板块界面--的结构性质没有强烈的限制。被称为低频地震(LFE)的小型脉冲地震事件通常伴随着缓慢滑动。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，但我们还不知道它在高频内容上的不足，这与慢滑的物理学密切相关。这项研究将对日本南开海槽的LFE进行本地化的高分辨率分析，以准确地成像LFE震源，并确定它们的频率含量低于典型地震的原因。对于LFE是低频的原因，有两种可能的端元解释：(I)LFE震源本身与“正常”地震根本不同，不辐射高频，或(Ii)高剪切板块界面的近源衰减在慢滑震源区内的高频成分被位于地表的地震仪捕获之前将其移除。这项研究将通过对南开海槽板块界面夹着LFE的地壳和实验室内地震进行一种新的频谱分析来研究这一问题。这将使板界面的衰减结构的高分辨率图像成为可能，从而允许区分近源衰减在产生LFE的特征低频特征中所起的作用。通过精确地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。将使用一系列基于集群的频谱和时间域方法来模拟事件，并搜索具有和不具有LFE的区域之间的差异。这项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慢滑移周期中的变化。慢滑在地震周期中扮演着重要的角色，这项研究将对LFE源和主要板块边界的结构提供新的和必要的约束，揭示慢滑的动力学和物理学。该项目得到地球物理计划和极端事件预测和弹性(PREEVENTS)计划的支持。该奖项反映了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。

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.