Experimental investigation of strength recovery under fluid flow

Abstract:

The geofluid, especially water, is abundant inside the crust and flows universally along cracks, pore, and faults. Due to the physical-chemical interaction between fluid and rock, the strength of the fault changes with time. The fluid is considered to play a major role in the occurrence of earthquake including slow slips. However, how the flow rate and pressure of the geofluid affect the strength of the fault has not been fully studied. In this study, we performed slide-hold-slide test under condition of water flow (flow SHS test) using a fluid pressure controlled rotary shear apparatus installed in JAMSTEC Kochi to verify the hypothesis that the flow of water suppresses the increase of fault strength (strength recovery). Indian sandstone crushed to 125-250 μm was used as a simulated fault gouge, and the flow SHS test was performed under the condition of flowing water at a flow rate of 0.0cc/min and 0.6 cc/min to compare the strength recovery rate. The experiment was conducted under the conditions of a slip velocity of 5 μm/s, a normal stress of 3 MPa, a water pressure of 0.3 MPa, and a hold time of 2 seconds to 12 hours.In case of without the flow, the frictional strength recovered in proportion to the logarithm of time, as in previous studies (e.g. Dietrich, 1972). On the other hand, the frictional strength did not almost recover in the presence of flow. The water pressure tended to gradually increase with the hold time. When the strength recovery is expressed by the effective friction coefficient, the increase rate of the friction coefficient was almost the same as that when there was no flow. Similar results were obtained when ceramic balls were used for the fault gouge.The fact that the fault strength does not recover in flow SHS experiments is thought to be since the flow suppresses the growth of asperity inhibiting water-rock reaction in the contact area, or because the increasing contact area induced buildup of fluid pressure, which may prevent the strength recovery.In this presentation, we will report the preliminary results of the further experiments in which the fluid pressure is controlled and with several size of ceramic balls to verify the relationship between the moving fluid and strength recovery in more detail.