The two earthquakes that hit the southwesternmost island of Kyushu last month have killed at least 40 people and injured more than 1,500, according to the Japan Meteorological Agency.
The first quake had a magnitude of 6.5 while the second earthquake which hit two days later and which has a magnitude-7.3, was deadlier compared to the first. The epicentre was in Kumamoto Prefecture in the centre of Kyushu Island.
Before the M7.0 mainshock, many buildings were assessed to have structural damage that may make them uninhabitable and 20 buildings collapsed, according to a note by catastrophe risk modelling firm AIR Worldwide.
The note said: “Given the main shock’s shallow depth and proximity to population centres, it is expected to cause significant additional damage, particularly to Japan’s traditional wood frame building stock.”
AIR Worldwide said: “Kumamoto Prefecture is home to roughly 25% of Japan’s semiconductor production, including some of the country’s leading manufacturers. In addition, there are more than 100 semiconductor-related enterprises located in the prefecture. The area also has automobile, steel, and ship manufacturers. The quake could cause business interruption losses and further impact supply chains, but that has yet to be seen.”
Manufacturers including Honda Motor and Sony Corp halted production at plants in southwestern Japan following the first quake to inspect for any damage, reported Reuters. Toyota Motor was also said to have cancelled its shifts at a Lexus assembly plant and two components plants about 93 miles north of Kumamoto after the first quake too, as it checked to see whether suppliers in the region would be able to continue providing parts.
The damage caused by the quakes and others in its wake has included collapsed buildings, landslides, blackouts that have left more than 200,000 households without power, and water outages in 400,000 homes. In Chuo Ward, Kumamoto City’s busy shopping and restaurant area, the quake caused strong sideways shaking, knocking down lockers and shelves at stores and hotels. No damage to the nuclear power plants in Kagoshima and Saga prefectures has been reported.
Japan’s buildings’ materials composition
According to AIR Worldwide, buildings in Japan are predominantly wood, steel, or steel-reinforced concrete. Residential exposures are dominated by single-family homes, nearly 90% of which are of wood construction. Apartment complexes in Japan are usually of steel or steel-reinforced concrete construction; only about 24% are of wood construction. Among commercial buildings, wood is even rarer and is only used for about 12% of the building stock with most of them being steel or steel-reinforced concrete.
The small industrial building stock is primarily that of steel or light metal with about 20% made of concrete. Older buildings in Japan are typically quite vulnerable to shake damage, as many were built under the misguided view that a higher degree of stiffness prevents building failure. As research and construction techniques have improved, building codes have been updated to increase safety.