Recent studies of slow earthquakes along plate boundaries have shown that tectonic tremor, low-frequency earthquakes, very-low-frequency events (VLFEs), and slow-slip events (SSEs) often accompany each other and appear to share common source faults. However, the source processes of slow events occurring in the shallow part of plate boundaries are not well known because seismic observations have been limited to land-based stations, which offer poor resolution beneath offshore plate boundaries. Here we use data obtained from seafloor observation networks in the Nankai trough, southwest of Japan, to investigate shallow VLFEs in detail. Coincident with the VLFE activity, signals indicative of shallow SSEs were detected by geodetic observations at seafloor borehole observatories in the same region. We find that the shallow VLFEs and SSEs share common source regions and almost identical time histories of moment release. We conclude that these slow events arise from the same fault slip and that VLFEs represent relatively high-frequency fluctuations of slip during SSEs.

中文翻译：

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在南开俯冲带，缓慢的浅层地震伴随着缓慢的滑动事件。

沿板块边界的慢地震的最新研究表明，构造震颤，低频地震，超低频事件（VLFE）和慢滑事件（SSE）经常彼此并存，并且似乎具有共同的震源断层。然而，由于地震观测仅限于陆基台站，因此在近海板块边界以下分辨率较差，在板块边界的浅部发生的慢速事件的发生过程尚不为人所知。在这里，我们使用从日本西南部南海海槽海底观测网络获得的数据来详细研究浅层VLFE。与VLFE活动同时发生的是，在同一地区的海底钻孔观测站通过大地观测观测到了指示浅层SSE的信号。我们发现浅层VLFE和SSE共享共同的源区，并且矩释放的时间历史几乎相同。我们得出的结论是，这些缓慢事件是由相同的断层滑动引起的，并且VLFEs表示SSE期间滑动的相对高频波动。