Where the sea plate subducts under the land plate, large earthquakes occur periodically at the plate boundary and the huge branch fault.It is thought that when the pressure of water (fluid pressure) around the fault that causes an earthquake rises, the fault becomes slippery and an earthquake occurs, but direct measurement is difficult and research has not progressed.

　This time, the research group focused on the quartz veins distributed around the Nobeoka thrust fault in Miyazaki prefecture, which is said to have had a huge earthquake in the past.Quartz veins are silica that is dissolved in water in the crust and precipitates as quartz in rock cracks, and this phenomenon may change the location, flow direction, fluid pressure, etc. of underground water.Therefore, we have developed a calculation model that can calculate the formation time of quartz veins.

　Calculations were performed under the conditions of a depth of 10 km and a temperature of 250 ° C, which corresponds to the Nobeoka thrust fault.As a result, it was found that it takes about 6 to 60 years to form a quartz vein (average size of the fault), and that most of the relatively large cracks become quartz veins in 300 years or less.

　The results of this calculation correlate with the time scale of the repetitive period (100 to 1000 years) of large earthquakes that occur in huge branch faults such as the Nankai Trough.This is the world's first research result that quantitatively shows the close relationship between the process of increasing the fluid pressure, which is prone to earthquakes, and the process of quartz vein formation.In addition, the evaluation of temporal and spatial changes in underground fluid pressure can be applied to the sustainable use of geothermal energy.

Paper information: [Scientific Reports]