Overview
- Researchers reanalyzed Japan’s 2011 Tōhoku earthquake records and identified an ScS shear wave that returned from the core–mantle boundary roughly 13–16 minutes after the mainshock and coincided with a near‑simultaneous eastward GPS offset across Japan.
- Hundreds of GEONET GNSS stations recorded step‑like eastward shifts up to about 5–6 millimeters that the team interprets as a permanent, continent‑scale response rather than a temporary oscillation.
- Modeling by the authors suggests the returning ScS pulse triggered gradual slip spread over thousands of kilometers of plate boundary, releasing energy comparable to an approximately magnitude 7.5 earthquake while producing little local shaking.
- The researchers tested and rejected alternatives such as GNSS processing errors, continued mainshock rupture, and a large submarine landslide but note a key limitation is sparse offshore GPS coverage that prevents mapping the full extent of the slip.
- If confirmed in other large quakes, the result would add a previously unrecognized delayed hazard to earthquake science and prompt calls for more offshore GNSS, targeted seismic analyses, and modeling to assess how deep‑reflected waves can (re)activate stressed faults.