Overview
- A multinational team published its findings Thursday showing that seismic S‑waves from the 2011 M9.0 Tohoku quake reflected at the mantle–outer core boundary and returned about 15 minutes later to produce eastward ground movements up to 5–6 millimetres across much of Japan.
- The result rests on one‑second‑resolution analysis of Japan’s dense GNSS and seismometer networks, which recorded near‑simultaneous, millimetre‑scale eastward displacements at stations from Tohoku to Hokkaido and Kyushu.
- The reflected wave, known as ScS, is an S‑wave that bounces off the mantle–outer core interface roughly 2,900 km deep and then returns to the surface, delivering a broad, short‑duration dynamic load to plate boundaries.
- Authors interpret the synchronous motions as evidence that the ScS arrival induced slow slips on multiple plate interfaces, possibly including the Nankai Trough area, which can change fault stress without producing strong shaking.
- Although the measured shifts were small and produced little direct harm, researchers say this is the first instrumental record of core‑reflected waves driving widescale crustal movement and that the finding should inform models of post‑mainshock deformation and seismic hazard.