Entire Japan shifted – now scientists know why

The massive Tohoku-Oki earthquake in 2011, with a magnitude of 9.0, caused widespread devastation, including the tsunami that led to the Fukushima nuclear disaster. The quake itself shifted Japan's main island, Honshu, approximately 2.4 meters to the east.

However, about fifteen minutes after the initial earthquake, the island experienced a second, smaller shift of up to 5-6 millimeters eastward, according to satellite data. This secondary movement puzzled researchers because it occurred without any known aftershock at the time.

Sunyoung Park, the lead author of the study, explained that a seismic wave from the main earthquake traveled thousands of kilometers into the Earth's core. This wave then bounced back towards the surface, causing the edges of tectonic plates to shift.

The energy released by this reflected wave was equivalent to a magnitude 7.5 earthquake. Researchers believe this finding introduces a novel seismic risk: these waves can significantly impact an already affected area minutes after the main shock and cause movement over a large region.

"This adds a completely new dimension to seismic risk that we were not aware of before," Park stated. The study, published in the journal Science, highlights a previously unknown phenomenon that could influence how seismic hazards are assessed in the future.