- The 2011 magnitude‑9 quake permanently shifted nearly all of Japan about 5–6 mm (≈0.2 in) eastward, detected roughly 15 minutes after the main shock.
- Researchers say the nudge was caused by seismic waves that dove to Earth’s liquid outer core, bounced back, and pushed the crust — the broadest country‑wide ground shift ever recorded.
- The rebound released energy comparable to a magnitude‑7.5 quake; because the core round‑trip takes about 15 minutes, scientists say this delayed, widespread effect might be anticipated and is a newly recognized seismic hazard.
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