North-east Japan quake rattles same fault as last year

The large earthquake today 245 kilometres off the east coast of Japan involved the same fault as in last year's devastating Tohoku earthquake and tsunami, which killed 15,000 and wrecked the Fukushima Daiichi nuclear facility.

The latest quake, of magnitude 7.3, struck at 5.18 pm Japanese time and was centred 160 kilometres southeast of the epicentre of the Tohoku quake.

Reports spoke of buildings swaying violently in Tokyo and in many cities along the east coast. There have been no reports of casualties, although evacuations were ordered in some cities.

A tsunami 1 metre high washed ashore at 6 pm Japanese time in Ishinomaki, Miyagi prefecture, a region which suffered severe damage last year. But tsunami warnings along the east coast of Honshu island from the far north down to Tokyo were lifted around two hours after the quake.

Japan's Nuclear Regulation Authority said that all nuclear facilities, including Fukushima, were undamaged.

"Same fault zone"

Randy Baldwin, a geophysicist at the US Geological Survey (USGS) National Earthquake Information Center in Golden, Colorado, told New Scientist that today's quake originates from the same fault zone as the Tohoku quake. "It appears to be at the same interface, where the Pacific plate moving towards Japan is diving beneath the Okhotsk plate," says Baldwin.

According to Baldwin, the quake is one-fiftieth the size of the magnitude 9 Tohoku quake, which released 350 times as much energy. By mid-morning UK time, USGS had detected five aftershocks, the largest of which was of magnitude 6.2.

He says there is always a possibility that the quake could be the prelude to an even larger one. "But as a rule, most aftershocks are smaller than the first."

Baldwin stresses the extreme seismic activity around Japan, which straddles four continental plates all rubbing against one another. "Since 1978, there have been 15 quakes of magnitude 7 or higher," he says. "These happen pretty regularly."

Brian Baptie of the British Geological Survey (BGS) agrees that aftershocks are likely to be smaller, but notes that the devastating quake in 2011 was preceded a few days earlier by a magnitude-7 quake in the same zone. Whether large quakes trigger other "megaquakes" through a geological domino effect is hotly debated.

The BGS has produced a map showing the epicentres of the Tohoku event and today's quake.

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