Magnitude 9 Nankai Trough quake would violently rock Tokyo, Osaka high-rises


If a magnitude 9.0 quake were to occur along the Nankai Trough off central and western Japan, it could be so powerful as to violently rock high-rise buildings in cities as far from the epicenter as Tokyo and Osaka, a new set of estimates showed Saturday.

The National Research Institute for Earth Science and Disaster Prevention in Tsukuba, Ibaraki Prefecture, projected that tall buildings in Tokyo would sway at a speed 10 times faster than during the 9.0-magnitude Great East Japan Earthquake that struck off Tohoku in 2011. It also forecast the speed of oscillation for high-rises in Osaka could be several dozen times faster.

The institute for the first time simulated ground motion in the event of a magnitude 9.0 Nankai Trough quake, which researchers estimate has a 60 to 70 percent chance of striking within the next 30 years.

Such a powerful temblor would generate projected ground movement for as long as 20 seconds, according to the institute, which called for enhanced safety measures at high-rise buildings in the two metropolises.

Specifically, the institute forecast that 200- to 250-meter-tall buildings near the Tokyo Metropolitan Government’s offices could sway at speeds of between 0.50 and 6 meters per second, compared with 0.50 to 1 meter during the March 11, 2011, temblor.

Structures of a similar height near the Osaka Prefectural Government’s offices are projected to swing at speeds of 0.50 to 3 meters per second, compared with a maximum of 20 cm in the 2011 jolt.

The institute cited as reasons the greater proximity of the envisioned quake’s epicenter to Japan’s shore and its shallower center than the temblor that struck deep under the Pacific, off Tohoku. The shock waves are also expected to pass through softer rock formations, it said.

During the 2011 mega-quake, an Osaka Prefectural Government building, located some 770 km from the epicenter, suffered damage at 360 points in its ceilings and walls.

The Nankai Trough runs from Suruga Bay in Kanto’s Shizuoka Prefecture to the Hyuganada sea off Miyazaki Prefecture in Kyushu.

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