Earthquake in the Kumamoto Region, Kumamoto Prefecture on July 28, 2026 - Analysis of the Source Process Using Far-Field Body Waves (provisional) -
The source process of the earthquake in Kumamoto region, Kumamoto Prefecture on July 28, 2026, at 16:27 (Japan Standard Time) was calculated using far-field body waves, with broadband seismic waveform records obtained from the EarthScope Consortium's Data Management Center (DMC) (Note 1).
The rupture initiation point was set at the epicenter location reported by the Japan Meteorological Agency (32°37.5′N, 130°40.7′E, depth 16 km). To analyze the fault plane, an NE-SW trending plane (strike 212°, dip 75°, slip 164°) from the two planes in the USGS W-phase solution was assumed. The maximum rupture propagation velocity was set at 2.0 km/s. The theoretical waveforms were calculated using the CRUST2.0 (Bassinet et al., 2000) and IASP91 (Kennett and Engdahl, 1991) subsurface models.
The main results are as follows (these results are preliminary and may be updated in the future).- The main rupture zone extended approximately 30 km along strike and approximately 10 km along dip.
- The main slip propagated from the rupture initiation point into the shallow zone, with a maximum slip of 2.7 m (calculated using a shear modulus of 30 GPa based on surrounding structures).
- The main rupture duration was approximately 20 seconds.
- The moment magnitude (Mw) was 6.8. These results can be accessed at https://www.data.jma.go.jp/eqev/data/world/about_srcproc.html
(Note 1) Program used for the analysis
M. Kikuchi and H. Kanamori, "Note on Teleseismic Body-Wave Inversion Program,"
http://www.eri.u-tokyo.ac.jp/ETAL/KIKUCHI/
Japan Meteorological Agency
[Evaluation of the 2026 Kumamoto Earthquake (August 12, 2026)]