Majority of wooden homes built before 2000 need earthquake-retrofitting

According to the Japan Wood-frame House Earthquake Reinforcement Business Cooperative, made up of contractors and architects that provide earthquake-retrofitting services, 93% of the 4,000 homes that have had earthquake-resistance inspections carried out over the past 15 years in Tokyo were at risk of collapse in an earthquake that produced a seismic-intensity (shindo) level of upper 6 or higher (*Note: The seismic intensity scale is difference to magnitude). The average age of a house surveyed by the Cooperative was 37 years.

One of those homes was a 55-year old house in Setagaya Ward which was already found to have leaning pillars, wall cracks, and no reinforcing in the foundations. It was determined to be of high risk of collapse in a Shindo 6+ earthquake. The owner had assumed the house would be fine, since it did not suffer any obvious damage in the March 2011 earthquake. 

Of the home-owners who have had earthquake-resistance inspections, about 30% chose to go ahead with reinforcing work or rebuilding.  The main obstacle is the high cost in retrofitting an older house. Financial assistance programs offered by local governments are often insufficient. The Cooperative estimates that the average cost to retrofit a 2-story wooden house is around 1.67 million Yen (approx. US$15,000).

For homes built to the old earthquake codes (between 1950-1980), 97.3% were found to fall short of meeting current earthquake-resistance standards. Over 85% were considered at high-risk of collapse in a strong earthquake. For those homes, the average estimated cost of retrofitting was 1.89 million Yen.

Homes built between 1981 and May 2000 also underperformed, with 85.9% not meeting current standards. A total of 64% were considered to be at high-risk of collapse in a Shindo 6+ tremor. 

Japan’s earthquake codes underwent two major recent changes. The first was in June 1981, and the second, which introduced stricter standards for wooden houses, was introduced June 2000. In the 2016 Kumamoto earthquakes, approximately 20% of the wooden homes built between 1981 and 2000 either collapsed or suffered major damage. 

Source: Japan Wood-frame House Earthquake Reinforcement Business Cooperative Data Report, March 4, 2021.

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