Full Breakdown
Core-Reflected Seismic Wave Nudged Japan Eastward After the 2011 Tohoku-Oki Earthquake
6/19/2026, 10:28:55 PM
Nationwide Eastward Displacement
Fifteen minutes after the magnitude-9.0 Tohoku-Oki earthquake on 11 March 2011, GPS stations across Japan recorded a uniform eastward step of roughly five to six millimetres. The motion was simultaneous, not linked to any aftershock, and persisted as a permanent offset.
GNSS Detection
Analysis of the Global Navigation Satellite System network showed a step-like displacement lasting about 100–200 seconds at hundreds of sites from Hokkaido to Kyushu. The signal’s spatial uniformity ruled out localized landslides or processing artefacts.
Core-Reflected ScS Wave
The researchers identified a shear-wave (ScS) that descended through the mantle, reflected off the liquid outer core, and returned after a 13–15 minute round-trip. Despite attenuation, the wave re-energised two plate-boundary segments spanning ~3 000 km, releasing energy comparable to a magnitude-7.5 quake but producing only millimetre-scale surface motion.
Research Team and Timeline
The study was led by Sunyoung Park, assistant professor of geophysical sciences at the University of Chicago, with collaborators Hiroo Kanamori (Caltech) and Luis Rivera (University of Strasbourg). After ruling out alternative explanations, the team published the results in *Science* on 18 June 2026.
Seismic-Hazard Implications
The finding defines a new hazard class: core-reflected waves can reactivate stressed faults minutes after the main shock. Global hazard models may need to incorporate delayed, distributed slip, especially for other megathrust zones such as the San Andreas system.
Official Assessment and Peer Commentary
The authors label the observation “extraordinary” and a “new type of seismic hazard.” Independent seismologists note the offshore GPS network is sparse, limiting confidence in the full offshore extent of the slip.
Data Gaps and Uncertainties
Reported travel times differ between 13 minutes (ScienceAlert) and 15 minutes (University of Chicago). The surface-displacement duration is cited as 100–200 seconds in some analyses and about three minutes in others, indicating modelling uncertainties.
Verbatim Quotes
- “The authors’ observations of triggered slip from this source across an area six to seven times greater than the area that broke during the main shock is extraordinary.” — Sunyoung Park, University of Chicago
- “We think the vigorous shaking from the original Tohoku earthquake might have already weakened the plate boundaries, making them more susceptible to be moved when the core-reflected wave came by,” — Sunyoung Park, University of Chicago
- “We see a small 5-millimetre eastward step that happens nearly simultaneously and with similar size across almost all of Japan, without any ordinary earthquake at that exact time.” — Sunyoung Park, University of Chicago
- “We need to recognize that additional ground deformation can occur even several minutes after the strong shaking of the mainshock has ended,” — Sunyoung Park, Associate Professor, University of Chicago
Outlook
The team will add offshore GNSS stations, embed ScS-wave detection in global networks, and test the mechanism in other megathrust settings to evaluate its worldwide relevance.
