All of Japan was moved – now scientists know why

TT

Published 10.56

 

Tsunami waves reach land in Natori, Japan during the earthquake on March 11, 2011. Photo: Kyodo News/AP/TT

The 2011 mega-earthquake caused Japan to mysteriously move a second time – several minutes after the quake. Now scientists believe they understand why.

The so-called Tohoku-Oki earthquake in 2011 had a magnitude of 9.0 and caused great devastation, including the tsunami that caused the meltdown at the Fukushima nuclear power plant. The earthquake caused the main island of Honshu to move 2.4 meters to the east.

But about a quarter of an hour after the main earthquake, the island moved again – at most 5–6 millimeters to the east, according to satellite data.

“Usually you see this kind of movement when the actual earthquake occurs. But here there was no known aftershock at this time, which made us curious,” lead author Sunyoung Park tells the University of Chicago website.

She and her colleagues have concluded that a seismic wave from the quake traveled thousands of kilometers into the planet’s core, from where it bounced back up to the surface and caused the edges of the tectonic plates to shift. The energy is equivalent to a strong earthquake with a magnitude of 7.5.

According to the researchers, the discovery represents a new kind of threat from strong earthquakes. These waves can have a massive impact on the already affected area several minutes after the main quake, and also cause movements over a large area.

“This adds a whole new angle to seismic risk that we didn’t know about before,” says Park.

The study is published in the scientific journal Science.