Earthquake Offshore of Sanriku on April 20, 2026 - Source Process Analysis Using Far-Field Body Waves (Preliminary Report)-

A source process analysis was performed for the earthquake offshore of Sanriku at 16:52 on April 20, 2026 (Japan Standard Time). This analysis was based on far-field body waves recorded by broadband seismometers from the EarthScope Consortium Data Management Center (DMC) (Note 1).

The rupture initiation point was set at the epicenter provided by the Japan Meteorological Agency (39°50.5′N, 143°9.4′E, depth = 19 km). The fault plane analysis utilized an N-S nodal plane (strike 179°, dip 17°, and slip angle 64°) from the two nodal planes in the JMA CMT solution. The maximum rupture propagation velocity was set at 2.0 km/s. To calculate theoretical waveforms, the crustal structure models CRUST2.0 (Bassin et al., 2000) and IASP91 (Kennett and Engdahl, 1991) were used.

The main results are as follows (note that these are provisional and may be updated in the future).

The main rupture zone extended approximately 50 km along the strike direction and approximately 50 km in the dip direction.
The main slip expands around the rupture initiation point, with a maximum slip of 3.9 m (calculated using a rigidity modulus of 49 GPa based on the surrounding structures).
The main rupture duration was approximately 20 seconds.
The moment magnitude (Mw) was 7.6.

For results interpretation, please refer to https://www.data.jma.go.jp/eqev/data/world/about_srcproc.html.

(Note 1) The program used for 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 Earthquake Offshore of Sanriku on April 20, 2026 (April 21, 2026)]