‘Long-period’ quake warning urged

by Jun Hongo

Staff Writer

Experts reviewing the impact of “long-period ground motion” on tall structures such as high-rises following the March 11 earthquake are calling for creating a new system that would enhance warnings in the event of a major temblor and subsequent aftershocks.

Unlike the direct impact of an earthquake, long-period ground motion can at times be amplified by geographical conditions and other factors. Such slow-cycle tremors can impact a much greater area and cause serious damage, especially to large structures such as oil tanks and tall buildings.

The experts, operating under the Meteorological Agency since November, revealed Tuesday that people in skyscrapers in Tokyo, 400 km from the epicenter, felt intense shaking compared with people at ground level.

A survey of approximately 800 people in Tokyo conducted by the group found that more than 80 percent of those above the 20th floor of buildings around Tokyo Bay found it hard to remain standing when the quake hit on March 11.

That by definition would equal upper 6 on the nation’s seismic intensity scale to 7, even though Tokyo on that day was registered as upper 5.

“I knew that there was an earthquake on the coast of Miyagi Prefecture, but there wasn’t any information regarding the impact on Tokyo. The jolting was extremely intense,” a person who was in a Tokyo skyscraper March 11 responded.

“An adjacent building was swaying so hard that I feared it might hit our building,” another respondent said.

Numerous people in the upper levels of high-rises in Tokyo felt symptoms akin to sea-sickness, according to the survey.

The Meteorological Agency panel also found that even those inside tall buildings constructed on firm stratum in western Tokyo felt the amplified shaking. More than 50 percent of people above the 20th floor of Kogakuin University in Shinjuku Ward found it difficult to remain on their feet, the survey showed.

Elevators in high-rise buildings located as far away as Osaka also shut down due to the nature of the long-period ground motion.

The agency aims to continue studying the phenomenon and seek measures to predict it. The panel set fiscal 2013 as the target to launch warnings of long-period ground motion that would include the level of intensity in tall buildings.