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 stress state changes with seismic cycles from a reverse fault regime to a normal fault regime, as observed in both geophysical observations and geological records. Fluid pressure has been estimated for both modern accretionary prisms and exhumed accretionary complexes. However, changes in fluid pressure on seismogenic faults have not been connected to seismic cycles. Here, we quantitatively show the dynamic change in fluid pressure in a seismogenic fault with geological evidence from an exhumed accretionary complex. We found extensional veins related to seismogenic fault records that exchanged stress states the during seismic cycles. We also constrained the fluid pressure quantitatively, both at an increasing stage during an event and at a decreasing stage after an event. In this procedure, we propose new methods to constrain the magnitude of vertical stress and rock tensile strength.