We analyzed induced earthquakes and underground structures from the viewpoint of energy, and specifically used existing natural fracture densities for the underground structure in this paper. We examined the existing literature regarding the natural fracture density at six sites (Ogachi, Hijiori, Soultz, Cooper Basin, Basel and Newberry) clarified its relation with the maximum magnitude of induced earthquakes. The result showed that the maximum magnitude of induced earthquakes induced by hydraulic fracturing can be estimated by investigating the natural fracture density of geological core samples from wells.
　The close relationship between the maximum induced earthquake energy and the natural fracture density was revealed by incorporating the average rock area calculated from the natural fracture density and the open-hole length of the well.
　We demonstrated that the maximum induced earthquake energy is related not only to the natural fracture density but also to the energy of the water injected for hydraulic fracturing. We also stated the importance of the maximum induced earthquake energy chart which relates the water injection energy to the maximum induced earthquake energy with a parameter describing the natural fracture density is important.