Geologists suspect that today’s quake was actually an aftershock of a much weaker magnitude-7.2 quake on 9 March. They warn that seismic stress has not yet relaxed in some particularly vulnerable parts of Japan, including the Tokyo region, 400 kilometres south of today’s epicentre.
Both quakes are the result of the Pacific tectonic plate sliding beneath the Japanese islands. At eight centimetres per year, convergence along this subduction zone is extremely fast in geological terms.
Increased stress
“Although certainly very big, today’s quake was not totally unexpected,” says John McCloskey, a geophysicist at the University of Ulster in Coleraine, UK. “Technically, it was in fact an aftershock of the weaker quake earlier in the week — even though it may sound odd that an aftershock can be stronger than the main shock.”
“The previous quake, although much smaller, significantly increased stress in the fraction of the fault zone that ruptured today,” says McCloskey.
The sequence of quakes has probably also affected the stress field further south along the fault zone, critically increasing the earthquake risk in the Tokyo region, he says.
“There is a strong interaction of quakes along a subduction zone, and we can certainly expect a number of major aftershocks in the next weeks,” he says. “Some may be as large as, or even stronger than, the quake that last month devastated Christchurch in New Zealand. And chances are that another very large shock could occur to the south near Tokyo.”
Today’s quake seems to have abruptly displaced the seafloor off Japan by a few metres, which caused the tsunami.
“This was the largest tsunami ever measured by US National Oceanic and Atmospheric Administration tsunamographs in the open ocean, with maximum crest height of over 2 metres,” says Costas Synolakis, director of the University of Southern California’s Tsunami Research Center in Los Angeles.
According to local reports, the tsunami reached up to 10 metres in height when it hit Japan’s Sanriku coast barely 30 minutes after the quake. Along flat coasts, the tsunami propagated hundreds of meters inland, sweeping away homes, cars and roads. Sendai airport was completely flooded. The number of people killed in the tsunami is as yet unclear, but reports suggest it could be several hundred.
http://www.nature.com/news/2011/110311/full/news.2011.156.html
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