Safer housing reduces worst-case earthquake scenario in Tokyo by 30 ~ 40%

Improvements in housing in Tokyo have reduced the expected level of damage from a severe earthquake by around 30 ~ 40%. On May 25, the Tokyo Metropolitan Government released an updated report on a ‘worse case scenario’ earthquake. This is the first update in 10 years.

The report is based on 8 different earthquake scenarios with magnitudes ranging from 7 to 9, including an earthquake with an epicenter directly underneath Tokyo, and a magnitude 9 megaquake alongside the Nankai Trough. The worst-case scenario assumes a magnitude 7.3 earthquake in the central-southern area of Tokyo occurring at 6pm in winter.

Under this assumption, areas within 11 of the 23 special wards (all on the eastern side of Tokyo) would experience a seismic intensity of up to 7, and over 60% of Tokyo would experience a seismic intensity of 6+ and higher. 

As many as 82,200 buildings would be destroyed (down from a previous estimate of 120,000) by the earthquake itself, and around 80% of those are estimated to be buildings built to the old earthquake codes.

 A further 112,200 would be destroyed by fire (down from a previous estimate of 200,000).  The death toll would be 6,150 (the previous estimate was 9,641), and there would be 2.99 million evacuees (down from a previous estimate of 3.39 million).  Forecast casualties and displaced resident numbers are down despite Tokyo’s population increasing by as many as 870,000 over the past decade.

The reason for the reduction in damage estimates is due to the ongoing earthquake-proofing of buildings and neighborhoods. In 2010, 81.2% of buildings in the Tokyo metropolitan area were estimated to be earthquake-proof. In 2020, this number increased to 92%. The total size of districts with densely-packed wooden houses has also dropped by almost half over the past 10 years to 8.6 hectares. These districts typically present a fire hazard.

As of 2020, half of Tokyo’s wood-frame buildings were built within the past 30 years. 8.2% were constructed before 1962. Wooden buildings accounted for 70.3% of Tokyo’s building stock. For non-wooden buildings, 6.5% were built before 1971, while 78% were completed in 1981 and later. 

Even a repeat of the devastating 1923 Great Kanto earthquake that produced a magnitude 7.9 and resulted in over 70,000 dead (44,000 of those died in a firestorm in Yokoami Park in Sumida ward) and 200,000 buildings destroyed would not have the same impact today. Current simulations estimate 1,777 deaths and 55,000 buildings destroyed.

The Nankai Trough mega-quake, predicted to occur somewhere off the coast of Japan between Shizuoka and Kyushu, would not cause any expected deaths or building destruction in Tokyo, even with a magnitude 9-class tremor at the epicenter.

In the event of a major earthquake occurring out to sea that causes a tsunami, estimated tsunami heights are between 2 ~ 2.6 meters for Tokyo Bay. Most of Tokyo’s urban area is outside of a tsunami inundation risk zone, and with measures such as sea walls and flood gates in place. 

Even with earthquake-resistant housing, residents are strongly encouraged to take additional measures such as securing large furniture items so that they won’t fall down in a quake, and learning how to operate fire extinguishers.  

Source: The Tokyo Metropolitan Government, May 25, 2022.

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