Japan appears to have shifted further east in recent years. The 2011 earthquake in Japan that triggered a tsunami and the Fukushima reactor disaster moved the country east by up to 6 millimetres, according to a new study.
Researchers examined how earthquake waves reflected from Earth’s core triggered movements along tectonic plate boundaries.
According to the findings, published by researchers at the University of Chicago in the journal Science in June, the affected area stretched across around 3,000 kilometres. That makes it the largest rupture zone ever recorded after a single event.
Devastating quake with consequences
The so-called Tohoku earthquake of March 11, 2011, struck in the sea east of Japan. At magnitude 9.0 on the moment magnitude scale, it was the strongest quake ever recorded in Japan.
The tsunami it triggered flooded more than 500 square kilometres on the Pacific coast and left more than 20,000 people dead or missing. Waves up to 14 metres high inundated the Fukushima I nuclear power plant. Several core meltdowns occurred after the cooling systems failed. Large quantities of radioactive material were released.
Japan’s Geonet earth observation system draws on data from 1,300 stations distributed across Japan and from global navigation satellite systems. About 13 minutes after the main quake, it recorded the arrival of so-called transverse waves that had been reflected at the boundary between the Earth’s mantle and core.
Shortly afterwards, larger parts of Japan were shifted east by a few millimetres relative to a reference point in China near the border with Russia. The researchers put the maximum value at 5 to 6 millimetres. According to the study, this was the first observation of such an event.
How the eastward movement came about
It was still not clear, however, how the eastward movement came about. The researchers suspect that the powerful earthquake had reduced friction on fault zones in the Earth’s crust and that the earthquake waves ultimately triggered the sudden shift.
Park’s team simulated various possible instances of what are called slip events, the sliding of rock masses along a fault line, at depths of 20 to 60 kilometres and compared the results with the Geonet data recorded.
This allowed them to rule out that these slips had occurred only in the area of the main quake’s epicentre. If that had been the case, the displacement would have had to decrease clearly with distance from the epicentre, but that was not the case.
A simulation in which the slips occurred along the fault zones between four tectonic plates came much closer to the Geonet recording data: in the north-east between the Pacific Plate and the Okhotsk Plate and to the south-west between the Philippine Plate and the Eurasian Plate.
On the entire area in which the simulations indicated slips occurred, the study authors wrote: “Its overall length is similar to that of mainland Japan (~3,000 km), exceeding the mainshock rupture length by six to seven times.”
The researchers consider their findings relevant to dealing with earthquakes. “The observation underscores the importance of accounting for this previously unrecognized source of seismic hazard from potential (re)activation of the mainshock area and surrounding region, even tens of minutes after the mainshock.”