Overview
- The Tōhoku earthquake on March 11, 2011 produced millimeter‑scale eastward moves across Japan that peaked at about 5–6 millimeters, as recorded by Japan’s dense Geonet GNSS network.
- Geonet records show transverse shear waves reflected at the mantle–core boundary arrived roughly 13 minutes after the mainshock and coincided with the abrupt eastward shifts.
- A Science paper led by Sunyoung Park combined the GNSS observations with depth‑resolved simulations of slip at 20–60 km and found the best match when slip occurred along multiple plate‑boundary zones over roughly 3,000 kilometers.
- The modeled ruptures involved breaks between the Pacific and Okhotsk plates in the northeast and between the Philippine and Eurasian plates in the southwest, and released energy comparable to about a magnitude 7.5 event.
- The authors say the mainshock likely reduced fault friction so the reflected waves could trigger delayed, remote slip and they warn this mechanism is a previously unrecognized seismic hazard that requires follow‑up work and potential updates to hazard models.