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Japan – Quakes’ long-period motions may damage high-rises

Tuesday, May 1, 2012

A group of researchers says long-period ground motions will shake high-rise buildings in Japan for more than 20 minutes if massive earthquakes hit off the country’s Pacific coast within minutes of each other.

The group led by University of Tokyo Professor Takashi Furumura says such slow but severe swaying could seriously damage high-rises in the capital and elsewhere.

The group estimated the duration of such motions with periods of several seconds or longer in the event of Tokai, Tonankai and Nankai earthquakes hitting around the same time along the Nankai Trough, off western Japan.

Three such megaquakes could hit almost simultaneously, or several years apart. Such quakes have occurred every 100 to 150 years.

The group says the swaying could last for more than 20 minutes in Tokyo if Tonankai and Nankai quakes hit together, followed by a Tokai jolt 10 minutes later.

Swaying in Osaka and Nagoya could reportedly last more than 20 minutes if a Tokai earthquake hits 5 minutes after a Tonankai jolt, followed by a Nankai shock 5 minutes later.

The group says long-period motions in Tokyo and Osaka will be larger and last longer than those from the quake that shook northeastern Japan on March 11th, 2011.

Architecture experts say steps are needed to curb swaying of tall buildings.

Tokyo University of Science Professor Haruyuki Kitamura says vibration control systems installed in high-rises effectively lowered the severity and duration of ground motions caused by the March 11th quake.

Tuesday, May 01, 2012