Geological constraints on dynamic changes of fluid pressure in seismic cycles

Fluid pressure along faults plays a significant role in fault behaviors in seismic cycles in subduction zones. When a thermal pressurization event occurs, the fluid pressure rises; conversely, when a fault-valve behavior event occurs, the fluid pressure falls. The two models have different time scales and may coexist in an event. Therefore, the purpose of this study is to quantify the change in fluid pressure from a natural fault zone, focusing on the underplating thrust zone accompanied with extension veins in the Mugi mélange, the Cretaceous Shimanto Belt, SW Japan. The fault zone we studied is thought to be related to underplating consisting mainly of basalt rocks. In the mélange just above the fault zone, mineral veins filling the extension cracks and cutting the mélange structures show network texture. The network veins cut each other, suggesting that the development of the mineral veins was in repeated multiple stages. In this study, the dike method was applied to the mineral veins, and the paleo-stress and driving fluid pressure ratio (P *) were estimated. P * is defined as the maximum fluid overpressure normalized by differential stress for the extension cracks. Based on the results of stress analysis and outcrop observation, we show that the mineral veins record seismic cycles where stress states exchange from reverse to normal fault stress regimes. We constrained the tensile strength and depth to be 6.94-9.38 MPa and 5.14-5.33 km, using P* and the fluid pressure from fluid inclusions from previous study. We also constrained the dynamic fluid pressure increase to be 6.7-9.0 MPa and 8.3-11.3 MPa in reverse and normal fault stress regimes, based on these estimated values. The fluid pressure increase could be a dynamic phenomenon because the estimated the maximum fluid pressure exceeds over lithostatic pressure, which cannot be a stable condition. We also showed that the fluid pressure during mineral vein formation differs between the reverse fault stress regime and the normal fault stress regime, which could be related to seismic cycles. Our study constrained the dynamic change in fluid pressure quantitatively in a seismic cycle. To fully evaluate fluid pressure fluctuations in the fault zones, investigations on other modes of failure, such as shear veins and hybrid extensional shear veins, must be expanded.