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

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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.