Fig. 1

Location map of the 2016 Kumamoto earthquake and strong-motion records. a Epicenter (yellow star) and source mechanism (beach ball) of the 2016 Kumamoto earthquake occurred on April 15, 2016, and aftershocks (gray dots) occurred within 24 h. White circles and triangles indicate K-NET and KiK-net strong-motion stations and stations used for the back-projection analysis, respectively. Circles, which are located in the direction of N45°E from the epicenter of the mainshock, indicate stations of strong-motion waveforms shown in (b) and (c). The Japanese Islands and the focal area are shown in the top-left inset. b North–south component of acceleration waveforms. Black and red traces show waveforms at K-NET and KiK-net stations and K-NET station OIT009, respectively. Station codes are shown in the right side of each trace. Thick gray lines indicate an identified seismic phase different from phases of the mainshock, and S-wave propagation of apparent velocity 3.5 km/s from station OIT009. c Vertical component of 20 Hz high-pass filtered acceleration waveforms. Thick gray lines indicate an identified seismic phase different from phases of the mainshock, and P-wave propagation of apparent velocity 6.3 km/s from station OIT009