Bridging the gap between low-frequency and very-low-frequency earthquakes

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.