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On the occasion of my retirement, I summarized the main seismic observations I have been involved in. I started with the observation of micro-fractures in mines and progressed to the observation of very small earthquakes. Advances in electronic technology, especially the practical use of GPS satellites, made it possible to carry out long-term observations in mountainous areas without a power source, which had previously been impossible. In the Western Nagano Prefecture region, high-frequency seismic observations with 10 kHz sampling and trigger method were started in 1995 and continued until my retirement. From around 2008, a device named the “Manten System” was developed in collaboration with a group of small and medium-sized enterprises in the Kansai region. The Manten system can continuously record 250 Hz sampling for more than half a year using about 32 single dry cell batteries. They were deployed in western Nagano Prefecture, the central and northern Kinki region, the San-in earthquake zone and the northern part of the South Island of New Zealand, in collaboration with related research institutions. Some unexpected and remarkable earthquakes, such as the 2017 Kaikoura, New Zealand earthquake (Mw7.8), occurred in all of the above observation networks, and these observations provided important data on the process by which intraplate earthquakes are generated.

Additional file 1: Figure S1. Weakly smoothed S-wave velocity models with topography correction. This figure is relevant to Figure 7, but the smoothing parameter in velocity inversion is weaker. (a–f) S-wave velocity models overlaid with active faults (black lines). Figure S2. Strongly smoothed S-wave velocity models with topography correction. This figure is relevant to Figure 7, but the smoothing parameter in velocity inversion is stronger. (a–f) S-wave velocity models overlaid with active faults (black lines). Figure S3. Weakly smoothed S-wave velocity models without topographic correction. This figure is relevant to Figure S1. (a–f) S-wave velocity models overlaid with active faults (black lines). Figure S4. Moderately smoothed S-wave velocity models without topographic correction. This figure is relevant to Fig. 7. (a–f) S-wave velocity models overlaid with active faults (black lines). Figure S5. Strongly smoothed S-wave velocity models without topographic correction. This figure is relevant to Figure S2. (a–f) S-wave velocity models overlaid with active faults (black lines). Figure S6. S-wave velocity perturbation of the northern part of the Kinki region without topographic correction. (a) S-wave velocity perturbation at a depth of 1 km below sea level. Also shown are the locations of the Yamada Fault (YDF), Yamasaki Fault (YSF), Yagi-Yabu Fault (YGF-YBF), Mitoke Fault (MTF), Hanaore Fault (HOF) and the Biwako-seigan Fault Zone members (Chinai Fault, CF; Aibano Fault, AF; Kamidera Fault, KF; Katsuno, KtF; Hira Fault, HiF; Katata, KaF; Hiei Fault, HF; Zeze Fault, ZF). (b) S-wave velocity perturbation at a depth of 3 km below sea level. Solid black lines represent documented active faults.