17:15 〜 18:45
[SCG46-P12] Slip characteristics of fault damaged by rapid decompression under hydrothermal conditions.
キーワード：急減圧、せん断すべり、き裂、花崗岩
The buildup and release of fluid pressure within the seismogenic zone play an important role in the changes in tectonic stress and fault strength during the interseismic period. In particular, the extremely rapid decrease in fluid pressure associated with dilational jog formation due to fault slip is termed flash vaporization, which has been recognized as a driver of mineralization. In addition, when fluid decompression occurs, surrounding rocks are cooled by the phase change of the fluid. Although hydrothermal rapid decompression associated with the phase change of fluid could cause fracturing within the rock in the vicinity of the fault, the effect of fractures induced by hydrothermal rapid decompression on the fault strength and slip characteristics remains unclear. Here we conduct laboratory experiments of hydrothermal rapid decompression under high-temperature and high-pressure conditions and shear-slip experiments.
We used cylindrical-shaped Iidate granites (30 mm for diameter and 40 mm for height) with pre-cut fault at an angle of 45° to the axis of the sample. Before the shear-slip tests, the pre-cut samples and distilled water sealed inside the chamber were heated up to 300℃, and then hydrothermal rapid decompression was generated by opening the valve connected to the inside of the chamber. Then, the shear-slip experiments were conducted on the pre-cut samples damaged by the hydrothermal rapid decompression at temperature of 150℃, axial stress of 55 MPa and confining pressure of 30 MPa. The differential stress was increased by decrease in the confining pressure in steps. In addition, axial displacement and acoustic emissions (AE) were also measured simultaneously during the experiments.
From the experimental results, the shear-slip of the sample damaged by rapid decompression initiated at the differential stress of 29 MPa, and the friction coefficient calculated from the stress acting on the fault is 0.35. Then, continuous slow slips (~ 0.1 μm/s) with small AE energy were observed with an increase in differential stress. On the other hand, in the shear-slip experiment on the sample with thermally cracking by natural cooling, rapid unstable slips with significant AE energy were observed. Given that rapid decompression is caused by fluid phase change, it is expected that fracturing of a rock surface in contact with the fluid causes weakening of the slip surface. In addition, observation of the rapid decompression sample after the shear-slip experiment showed that parts of the slip surface had become finer-grained. Therefore, it was shown that hydrothermal rapid decompression causes a large number of microcracks distributed on the slip surface, resulting in a significant reduction in strength, and that shearing with the collapse of the slip surface causes continuous slow slop.
We used cylindrical-shaped Iidate granites (30 mm for diameter and 40 mm for height) with pre-cut fault at an angle of 45° to the axis of the sample. Before the shear-slip tests, the pre-cut samples and distilled water sealed inside the chamber were heated up to 300℃, and then hydrothermal rapid decompression was generated by opening the valve connected to the inside of the chamber. Then, the shear-slip experiments were conducted on the pre-cut samples damaged by the hydrothermal rapid decompression at temperature of 150℃, axial stress of 55 MPa and confining pressure of 30 MPa. The differential stress was increased by decrease in the confining pressure in steps. In addition, axial displacement and acoustic emissions (AE) were also measured simultaneously during the experiments.
From the experimental results, the shear-slip of the sample damaged by rapid decompression initiated at the differential stress of 29 MPa, and the friction coefficient calculated from the stress acting on the fault is 0.35. Then, continuous slow slips (~ 0.1 μm/s) with small AE energy were observed with an increase in differential stress. On the other hand, in the shear-slip experiment on the sample with thermally cracking by natural cooling, rapid unstable slips with significant AE energy were observed. Given that rapid decompression is caused by fluid phase change, it is expected that fracturing of a rock surface in contact with the fluid causes weakening of the slip surface. In addition, observation of the rapid decompression sample after the shear-slip experiment showed that parts of the slip surface had become finer-grained. Therefore, it was shown that hydrothermal rapid decompression causes a large number of microcracks distributed on the slip surface, resulting in a significant reduction in strength, and that shearing with the collapse of the slip surface causes continuous slow slop.