No one died in the recent earthquake that struck Tottori Prefecture, but the temblor was strong enough to cause extensive damage. Several thousand people are still in evacuation centers, which means it is not safe for them to live in their homes. Most of these houses were probably built before the latest building standards were put in place to minimize damage caused by large quakes, but what most people don’t know is that these standards are not uniform throughout Japan.
Since they were first implemented in 1952, the standards have been revised twice: first in 1981, and a second time in 2000. In 1952, the government devised a map of Japan that illustrated the perceived differences in earthquake frequency and force from one region to another. The most dangerous areas were the Kanto, Chubu, Kansai and eastern Tohoku regions. Other areas, such as Shikoku and Chugoku, were considered slightly less dangerous, Kyushu even less so.
These findings are referenced by builders when designing structures. Specific areas are given a numeric rating from 0 to 1.0, with 1.0 indicating the most dangerous situation in terms of quake frequency and force. In those areas, designers must incorporate into their buildings the maximum quake-proof standards set by the government. In a region with a quake rating of 0.9, however, designers are only required to incorporate 90 percent of the maximum standard into their buildings. As the rating drops, so does the strictness of the standard.
While earthquake-proof standards for buildings have been strengthened twice over the past 60 years, the ratings for quake susceptibility in given areas have not. But according to a recent report on NHK, many of the major quakes that have hit Japan in the past 40 years have occurred in areas with ratings of less than 1.0 — Niigata, Tottori (twice), Fukuoka and, most devastatingly last spring, Kumamoto, where 20 condominium buildings built after the last quake-proof standard was implemented were declared too damaged to be livable any more.
In Kumamoto, in fact, two powerful quakes occurred within a short period of time and were both characterized as being sōteigai (beyond reasonable expectations). This word became notorious when it was used to describe the Great East Japan Earthquake, which was in an area rated 1.0. Kumamoto is rated 0.8; but, in essence, sōteigai is itself no longer a rare description. The recent quake in Tottori, which is rated 0.9, could also be described as exceptional. Experts now say Japan has entered a long-term cycle of stronger and more frequent earthquakes.
In the NHK report, use of the rating system was pegged to the construction of apartment buildings and condominiums. They focused on one 13-story condo that was so badly damaged in the Kumamoto quake that walls between some units collapsed, revealing the steel framework behind them. Inspectors looked at the architectural diagrams and found that the design was for a rating of 0.9, meaning 90 percent of the government standard. That translates as 10 percent fewer steel rods in the underlying structure of the building than for a building built to 1.0.
Developers take advantage of the rating system not only to reduce construction costs, but also to offer larger units for sale. With fewer rods, less concrete is necessary and rooms can be slightly bigger and marketed that way. Engineers estimated that the residents of the building will have to pay ¥300 million to repair the damage.
Another collective housing problem exposed by the Kumamoto quake was the ineffectiveness of menshin technology — rubber shock absorbers installed in the foundation of a structure that alleviate the effect of vibrations. It was found that in a number of cases in Kumamoto, these dampers were deformed by the quake and thus rendered useless. The dampers were designed to withstand vibrations of relatively short wavelengths, but the Kumamoto quake was shallow, and thus produced vibrations of longer wavelengths. The designer estimated that in a strong quake the dampers would move off their center to a maximum radius of 60 centimeters, but during the recent temblor they actually moved to a maximum radius of 90 centimeters, and in doing so compromised the whole structure of the buildings. Again, the physical effects of sōteigai quakes were not considered sufficiently.
The problem with collapsed detached houses was slightly different in nature. NHK reported that in the area of Kumamoto most affected by the quake, 1,042 wooden houses had been built in accordance with the newest quake-proof standards implemented in 2000, and of these 99 collapsed. When inspectors compared the structures of the collapsed houses to those still standing, the main difference was a relative lack of walls on the first floor of two-story houses.
The 2000 standards require that walls have lateral cross-beams attached at the bottom and top of the frame by metal brackets. The Asahi Shimbun reported that in 13 of the collapsed homes, the wrong type of brackets were used, which would seem to be a violation of the standards. However, other houses collapsed even though they used proper brackets.
As with the condo issue of taking advantage of fewer rods to make larger rooms, most of the collapsed houses had bigger living-dining-kitchen areas on the first floor, meaning large spaces without walls. With less support holding it up, the second floor basically pancaked onto the first when shaking intensified.
One inspector told NHK that new homebuyers demand larger common spaces and builders are trying to satisfy that demand while adhering to quake-proof standards. But the only way to guarantee structural integrity is to have walls going through both floors, meaning walls on the second floor must be complemented by contiguous walls on the first floor. Either that, or design the house with the open LDK space on the second floor, which is counter to standard living customs. Another problem is the desire for more and larger windows, which decreases the number of lateral crossbeams in the overall structure.
When NHK asked the Ministry of Land, Infrastructure, Transport and Tourism about revising the standards, a representative said that they hadn’t yet “confirmed whether or not regional standards had an effect on damage,” but that they planned to study the matter “over the long term.”
Obviously, homeowners and those who plan to be homeowners need to understand the situation themselves. If they are building a new home, they should talk to their builders about quake-proofing measures. Assessments for existing structures by certified engineers usually cost about ¥50,000, though related improvements will run upward of ¥1 million.
Japan is one of the most advanced countries in the world when it comes to earthquake-proof technology, but individuals still need to take proactive advantage of it.
Philip Brasor and Masako Tsubuku blog about Japanese housing at www.catforehead.wordpress.com.