70% chance Tokyo region to be hit by large quake: institute

TOKYO - The University of Tokyo's Earthquake Research Institute said Monday the risk of a big earthquake hitting the southern Kanto region including Tokyo in the next four years is as high as 70 percent.

The increased risk, according to Naoshi Hirata, a professor at the institute and a member of the research team who compiled the latest report, is due to the disruption caused to tectonic plates following the magnitude-9.0 earthquake that struck on March 11.

The earthquake leveled much of Japan's eastern seaboard and triggered a huge tsunami that sparked one of the world's worst- ever nuclear disasters at a power plant in Japan's northeast.

The institute said that it had noticed an increase in seismic activity in the southern Kanto region in the months following March's massive temblor and said that quakes with of more than magnitude-3.0 have been occurring five times more frequently than usual.

The research team said that in light of the increased frequency of quakes in the region, there is a 70 percent likelihood is that a magnitude-7.0 quake in the region will hit the region over the next four years.

The institute also said recently that last March's Great East Japan Earthquake and a smaller earlier quake may have been caused by a phenomenon known as "slow slips".

Slow slips are when two plates or other underground layers slowly move past one another, creating warps in the surrounding ground, the institute said in a recent scientific journal on the matter.

It said that is the first time that slow slips have been confirmed to have occurred before other warning signs such as smaller earthquakes.

"We confirmed that if a slow slip occurs and there is warped ground ahead of it, an earthquake can be triggered," said Aitaro Kato, assistant professor and earthquake researcher.

According to the research team, the Great East Japan Earthquake occurred when a tectonic plate to the east of the Sanriku region sunk under another plate and the energy of stored warps in the ground was released.

The institute's conclusions were reached after analyzing the Japan Meteorological Agency's data on some 333 earthquakes from mid-February 2011 to the main quake on March 11. It also noted that 1,083 additional small earthquakes had also occurred.

It also said that the earthquakes' vibration waves greatly increased from two to five kilometers per day from mid-to-late February, to about 10 kilometers per day on March 11.