Takemura, S.¹, Baba, S.², Yabe, S.³, Yamashita, Y.⁴, Shiomi, K.⁵, Matsuzawa, T.⁵, Detectability analysis of very low frequency earthquakes: Methods and application in Nankai using F-net and DONET broadband seismometers, Geophysical Journal International, 2024, 237 (1), 49-63, ggae033, https://doi.org/10.1093/gji/ggae033
¹ERI UTokyo, ²JAMSTEC, ³AIST, ⁴DPRI Kyoto Univ., ⁵NIED

　For a more quantitative discussion of slow earthquake activity, we evaluated the detectable limits of very low frequency earthquakes (VLFEs), which are seismic slow earthquakes observed in very low frequency (< 0.05 Hz) bands in the Nankai subduction zone. We performed numerical simulations using a local three-dimensional model and used the observed noise level of permanent broadband seismometers. First, we investigated the effects of the source-time functions on the maximum amplitudes of the VLFE signals at a certain station. The maximum amplitudes of the VLFE signals were controlled by the VLFE moment rate. The detectable limit of VLFEs at each source location can be defined as the lowest moment rate of detectable VLFEs, which radiate signals larger than the noise levels of any component at ≥ 3 stations. For inland seismometers only (Figure 1a), the detectable limits of VLFEs at deep (30–40 km) and shallow (≤ 10 km) depths were 10¹²–10^12.3 and 10^12.7 Nm/s, respectively. Due to the geometrical spreading of VLFE signals and large noise levels in horizontal components, offshore seismometers improved the detectability of shallow VLFEs in regions where seismometers were densely deployed (Figure 1b). Based on our detectability and published catalogs, shallow slow earthquakes are less active south-southwest off the Kii Peninsula, where geodetic studies expect mechanical coupling. The data on VLFE detectability along the Nankai Trough are uploaded at https://doi.org/10.5281/zenodo.7272617.