Kumamoto suffers double quake

Because of its position on the boundary of several tectonic plates, Japan has suffered more than most from powerful earthquakes. In the south of the country, the Philippine plate pushes itself five centimetres further under the Eurasian plate every year. The resultant rock stress triggered a series of quakes starting on 14 April 2016. On that day, a foreshock with a moment magnitude of 6.2 hit the island of Kyushu, and was followed by smaller aftershocks and, finally, the main shock on 16 April with a magnitude of 7.0 (see map on page 20). The series of earthquakes caused numerous landslides, and many old buildings in particular suffered severe damage. Large industrial facilities were at a standstill for days. In a number of places there was ground liquefaction, which caused very serious damage to buildings.

The quakes occurred on known 'crustal faults' at a shallow depth of around ten kilometres, mainly in the Futagawa-Hinagu fault zone. This type of fault, far away from the actual plate boundaries, is frequently caused by internal deformation of the tectonic plates as a result of external pressure. Despite their lower magnitude and longer return periods compared to subduction quakes – where one plate pushes under another – crustal quakes are often more destructive because they occur nearer populated areas. Unlike the foreshock, the fracture caused by the main shock in Kumamoto reached the surface. The ground opened up in several places, and there were local horizontal slips of more than two metres.

According to the Japan Meteorological Agency (JMA), both quakes generated exceptionally high ground acceleration of over 10 m/s2. For the same region to suffer severe earthquakes in close succession is also considered quite a rare occurrence.

Official building standards have been in force in exposed regions of Japan since 1924. They have been updated many times. There were major changes, for example, in 1981 (after the 1978 Miyagi quake), following which, though a building may suffer damage from strong ground motion, it should not be capable of collapsing. There were many smaller changes in the years thereafter, relating, for example, to the stability of wooden buildings in 2000 and the requirement that all buildings under construction be inspected by an independent body and checked for compliance with the building standards, in 2006.

The series of earthquakes in April caused large losses in the Kumamoto prefecture and surrounding towns (e.g. Mashiki). There were 69 deaths, and many people were injured. Almost 300,000 had to be evacuated after the main shock. Some 8,000 buildings collapsed and more than 140,000 were damaged, 24,000 severely. A large proportion of the buildings that collapsed were wooden buildings with heavy roof structures built according to the pre-1981 building standards. Several cultural heritage sites (including Kumamoto Castle and the Aso Shrine) were damaged, as was infrastructure (roads, bridges and railway lines), either directly by the quake or by subsequent landslides.

Firms producing cars, electronic components and pharmaceuticals are based in the industrial area to the northeast of Kumamoto. Though the structural damage to buildings tended to be minor, production at several sites was brought to a standstill, at least in the week following the quake, causing worldwide interruptions in the supply chain for downstream production facilities. The industrial losses in Kumamoto once again highlight the extent to which just-in-time production is utterly dependent on a steady supply of individual components.

The overall loss for the two earthquakes in Japan amounts to around US$ 31bn, of which US$ 6bn was insured, making Kumamoto Japan’s third-costliest earthquake after Tohoku in 2011 and Kobe in 1995 (see table). Residential buildings and their contents accounted for over half of the overall losses – and almost three quarters of the insured losses.

As there has been a considerable rise in the number of buildings insured, the insured losses in 2016 were significantly higher than for the Kobe quake in 1995. The proportion of households insured against earthquakes with private insurance companies has more than tripled since Kobe, from 9% to 29%. Nevertheless, due to the persistently low insurance density in Japan, the uninsured portion of the losses was higher than for comparable disasters in other industrialised countries like New Zealand.