Slow earthquakes that are observed in the > 1 Hz frequency band are called tectonic tremor or low-frequency earthquakes (LFEs) and those in the 0.01–0.10 Hz band are called very-low-frequency earthquakes (VLFEs). These two phenomena are separated by large microseismic noise at 0.1–1.0 Hz. However, recent observations of the signal in this microseismic frequency band accompanying LFEs and VLFEs in the shallow part of the Nankai subduction zone suggest that LFEs and VLFEs are parts of the same broadband phenomenon, “broadband slow earthquakes”. Here, we report the observation of slow earthquake signals in the microseismic frequency band in the western Shikoku region of the Nankai subduction zone, Japan, by stacking many seismograms relative to the timing of the high-frequency LFE signals. We relocate LFE events detected by the Japan Meteorological Agency, use these LFE waveforms to construct synthetic templates, perform a matched-filter event detection analysis using these templates, stack the seismograms recorded by broadband high-sensitivity accelerometers relative to the timing of the detections, and compare the amplitude of the stacked waveforms at different frequency bands. The stacked waveforms have a continuous signal in the 0.015625 Hz (64 s) to 8 Hz frequency band, and support the idea that LFEs are just a small part of the broadband slow earthquake spectrum, which extends to the VLFE frequency band. Furthermore, the frequency dependency of the maximum amplitudes in this study is similar to that of slow earthquakes in the Cascadia subduction zone, and this is also explained by a Brownian slow earthquake model. However, the frequency dependency is inconsistent with the omega-square model, which is a model for ordinary earthquakes.

中文翻译：

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弥合低频和极低频地震之间的差距

在 > 1 Hz 频带内观察到的慢地震称为构造震颤或低频地震 (LFE)，而在 0.01–0.10 Hz 频带内观测到的缓慢地震称为极低频地震 (VLFE)。这两种现象被 0.1–1.0 Hz 的大微震噪声隔开。然而，最近对南海俯冲带浅部 LFEs 和 VLFEs 伴随的这个微震频带中的信号的观察表明，LFEs 和 VLFEs 是同一宽带现象的一部分，“宽带慢地震”。在这里，我们通过叠加许多与高频 LFE 信号时间相关的地震图，报告了对日本南海俯冲带四国西部地区微地震频带中慢地震信号的观测。我们重新定位日本气象厅检测到的 LFE 事件，使用这些 LFE 波形构建合成模板，使用这些模板执行匹配滤波器事件检测分析，将宽带高灵敏度加速度计记录的地震图与检测时间相关联，并比较不同频段叠加波形的幅度。叠加波形在 0.015625 Hz (64 s) 至 8 Hz 频段具有连续信号，支持 LFE 只是宽带慢地震频谱的一小部分的观点，该频谱扩展到 VLFE 频段。此外，本研究中最大幅度的频率依赖性与卡斯卡迪亚俯冲带慢地震的频率依赖性相似，这也可以用布朗慢地震模型来解释。然而，