Damage to abandoned lignite mines induced by 2003 Miyagi-Hokubu [Japan] earthquakes and some considerations on its possible causes

The seismic response and stability of areas situated above the abandoned lignite mines in Japan due to urbanization in recent years are of great concern. The July 26 2003 Miyagi-Hokubu earthquake (Mj6.2) caused some damage in the area of abandoned lignite mines nearby Yamamoto town, which is just on the epicenter of the earthquake. The author had the chance to visit the damaged area in October, 2003. This article describes several cases of damage to the area of abandoned lignite mines and some peculiar phenomena such as the sinkholes. sloshing-induced sand boiling, the settlement of filled shafts caused by the M6.2 earthquake and its aftershocks, which are of great interest in environmental city planning and new developments in such areas. In addition, some model tests on abandoned lignite mines were performed by using a shaking table to explain peculiar phenomena observed during this earthquake. The ground water in abandoned lignite mines are thought to be contributing to the stability of the abandoned mines under static conditions. However, this earthquake showed that the ground-water may have negative effects on the stability of the abandoned mines under dynamic conditions. Furthermore, if the filling material used to grout underground openings such as shafts, adits, rooms is cohesionless, the settlement and lateral flow of the filling material may occur, which may subsequently cause the settlement in such areas. The sloshing phenomena depend upon the geometry of the abandoned mines and the characteristics of the earthquake. The sloshing phenomena may particularly influence the small mines by nearby in-land earthquakes while the large mines may be influenced by the off-shore earthquakes.