| Abstract |
Fukuoka City, in Fukuoka Prefecture, Japan, was hit by an earthquake measuring 7.0 on the Richter scale on 20 March 2005. The hypocenters of the main shock and the aftershocks exist in the offshore area of Fukuoka City and align with an extension of the Kego fault, one of the active faults in Fukuoka Prefecture. On the other hand, there are some low-to-moderate temperature hot springs along the Kego fault. Therefore, we conducted some numerical model simulations for development of a non-volcanic hydrothermal system caused by a high permeability fracture zone formed by seismic activity. We treated the Fukuoka earthquake as a model and set a permeable fracture zone that has a height of 13 km and exists 3 km below ground surface based on the hypocentral distribution. The width, extension and permeability of the fracture zone were set as parameters of the simulations. The results of the simulations up to 100,000 years after formation of the fracture zone show that relatively strong convection occurs in the fracture zone in the early stage of the development process, and changes to weaker one. And temperature homogenization in the zone and groundwater flow outside of the zone progress with time. The result of the simulation up to 1,000,000 years shows that weak convection spreads and cools over a wide area out of the fracture zone. Therefore, it is inferred that a non-volcanic hydrothermal system will be formed after formation of a permeable fracture zone without any kind of special heat source, and the distributions of underground temperature and groundwater flow will change in the long term even if no hydrothermal feature appears just after the seismic events. |