Reconnissance report on geotechnical and geological aspects of the 14-16 April 2016 Kumamoto earthquakes, Japan

On 16 April 2016, a moment magnitude (Mw) 7.0 earthquake struck the Island of Kyushu, Japan. Two major foreshocks (Mw 6.2 and Mw 6.0) contributed to devastation in Kumamoto City, Mashiki Town and in the mountainous areas of the Mount Aso volcanic caldera. This report summarises geotechnical and geological aspects of the earthquakes that were observed during a field investigation conducted by the NZSEE Team in collaboration with Japanese engineers and researchers. Many houses and other buildings, roads, riverbanks, and an earth dam, either on or adjacent to the surface fault rupture or projected fault trace, were severely damaged as a result of both the strong ground shaking and permanent ground displacement. In the Mount Aso volcanic caldera, traces of medium to large scale landslides and rock falls were frequently observed. A number of landslides impacted homes and infrastructure, and were reported to have killed at least 10 people out of the 69 confirmed deaths associated with the earthquake. In a few suburbs of Kumamoto City and in Mashiki Town, localised liquefaction took place, causing lateral spreading, differential settlements of the ground and riverbanks, sinking and tilting of buildings, foundation failures, cracks on roads, and disruption of water and sewage pipe networks. The overall effects from liquefaction related hazards appeared relatively minor compared to the damage caused by shaking, landslides and surface fault rupture. Based on the field survey, key findings are highlighted and recommendations to NZ engineering practice are made in the report.