A seismic wave from Japan’s 2011 magnitude 9.0 Tohoku-Oki earthquake traveled nearly 2,900 kilometres to the core-mantle boundary, reflected back roughly 13 minutes later and shifted the country eastward by about six millimetres, according to research published in Science. The displacement was too minor for direct observation, yet Japan’s dense GPS network recorded the simultaneous nationwide movement. For 15 years the signal remained unexplained, as it matched neither the mainshock nor any aftershock or other known event. The study attributes the shift to a returning ScS shear wave. Led by Sunyoung Park of the University of Chicago with Hiroo Kanamori and Luis Rivera, the team showed that the wave descended through the mantle, reflected off the liquid outer core and returned with enough energy to trigger minor fault slips. The round trip spanned nearly 5,800 kilometres. The Tohoku-Oki quake’s exceptional size produced an unusually strong ScS wave whose timing matched the GPS anomaly. The arriving wave caused small slips along already stressed plate boundaries, releasing energy comparable to a magnitude 7.5 event. Earlier explanations were ruled out because they failed to account for the uniform timing and nationwide pattern. The finding marks the first observation of such a deep wave leaving a measurable surface effect and highlights the value of dense monitoring networks for studying Earth’s interior.
