5:15 PM - 6:45 PM
[SSS05-P19] Dilatancy and elastic wave properties around a fault during stick-slip cycle
Keywords:Stick-Slip, Dilatancy, Elastic wave, Granite
1. Background and Motivation
Earthquakes are stick-slip in fault movement. Stick-slip is a frictional behavior of repeated stationary and unstable slip (Brace and Byerlee, 1966). Just before an earthquake occurs, that is, during the transition from stationary state to slip, dilatancy (volume expansion) may occur around the fault, and the velocity and attenuation rate of elastic waves transmitted through the fault plane may change (Scholz, 1993). Investigating such phenomena that occur around a fault before slip should be useful for understanding earthquake precursor phenomena. Therefore, this study attempts to clarify the changes in strain around a fault and the transmission characteristics of elastic waves during stick-slip by conducting laboratory rock friction experiments.
2. Research Methods
Aji granite, shaped into a cylinder with a diameter of 40 mm and a length of 80 mm, was used as the sample. The sample was cut at an angle of 30° from the axial direction and the cut surface was polished with Carborundum #3000 (average grain size 4 μm). The sample was saturated with ion-exchanged water, and friction experiments were conducted by loading the sample axially at a constant speed in the axial direction under confining pressure. The confining pressure was 100 MPa, the pore pressure was 1 MPa, and the axial displacement rate was 0.03 mm/min. Stick-slip was generated on the pre-cut surface, and the strain near the pre-cut surface and the transmitted elastic wave in the direction perpendicular to the surface were measured. Strain measurements were made by placing two sets of 1 mm long biaxial strain gages (axial and circumferential) on the specimen on both side of the fault plane, 1 mm away from the plane. A pair of piezoelectric elements (frequency 1.0 MHz) was used for elastic wave measurements to investigate how velocity and amplitude change during stick-slip. The input voltage signal was a square wave of 100 V and 1 kHz, measured at 1 sec intervals.
3. Results and Discussion
During the period when no slip occurred, axial strain decreased (contracted) and circumferential strain increased (elongated) with axial displacement. In the case off the strain gages on one side (above the fault plane), with a few exceptions, an increasing rate the circumferential strain increased and the axial strain initiated to increase just before slip. This might suggest dilatancy near the fault plane. On the other hand, with a few exceptions, strain gauges of rocks below the fault plane showed an increasing rate of circumferential strain increase and axial strain decrease immediately before slip. Changes in axial strain were different on the upper and lower sides of the fault plane. For the transmission elastic waves, no obvious changes were detected in the waveforms immediately before and after slip, when a significant change in the slip surface condition would be expected; Shreedharan et al. (2021) performed similar experiments for direct-shear type specimens with a gouge, and found that, except for short-term significant changes during slip, the changes in velocity and amplitude are small, at most 0.1 % and 0.32 %, respectively, and are not likely to be recognized simply by comparing waveforms. In the future, cross-correlation analysis of the transmitted elastic waveforms will be performed to investigate whether there are small changes during the period when slip does not occur.
References
Brace, W.F., Byerlee, J.D. (1966) Stick-Slip as a Mechanism for Earthquakes, Science 26, 153 (3739), 990-992.
Scholz, C.H. (1993) The Mechanics of Architectures and Faulting, Cambridge University Press.
Shreedharan, S., Bolton, D.C., Rivière, J., Marone, C. (2021) Competition between preslip and deviatoric stress modulates precursors for laboratory earthquakes, Earth and Planetary Science Letters, 553 (2021) 116623.
Earthquakes are stick-slip in fault movement. Stick-slip is a frictional behavior of repeated stationary and unstable slip (Brace and Byerlee, 1966). Just before an earthquake occurs, that is, during the transition from stationary state to slip, dilatancy (volume expansion) may occur around the fault, and the velocity and attenuation rate of elastic waves transmitted through the fault plane may change (Scholz, 1993). Investigating such phenomena that occur around a fault before slip should be useful for understanding earthquake precursor phenomena. Therefore, this study attempts to clarify the changes in strain around a fault and the transmission characteristics of elastic waves during stick-slip by conducting laboratory rock friction experiments.
2. Research Methods
Aji granite, shaped into a cylinder with a diameter of 40 mm and a length of 80 mm, was used as the sample. The sample was cut at an angle of 30° from the axial direction and the cut surface was polished with Carborundum #3000 (average grain size 4 μm). The sample was saturated with ion-exchanged water, and friction experiments were conducted by loading the sample axially at a constant speed in the axial direction under confining pressure. The confining pressure was 100 MPa, the pore pressure was 1 MPa, and the axial displacement rate was 0.03 mm/min. Stick-slip was generated on the pre-cut surface, and the strain near the pre-cut surface and the transmitted elastic wave in the direction perpendicular to the surface were measured. Strain measurements were made by placing two sets of 1 mm long biaxial strain gages (axial and circumferential) on the specimen on both side of the fault plane, 1 mm away from the plane. A pair of piezoelectric elements (frequency 1.0 MHz) was used for elastic wave measurements to investigate how velocity and amplitude change during stick-slip. The input voltage signal was a square wave of 100 V and 1 kHz, measured at 1 sec intervals.
3. Results and Discussion
During the period when no slip occurred, axial strain decreased (contracted) and circumferential strain increased (elongated) with axial displacement. In the case off the strain gages on one side (above the fault plane), with a few exceptions, an increasing rate the circumferential strain increased and the axial strain initiated to increase just before slip. This might suggest dilatancy near the fault plane. On the other hand, with a few exceptions, strain gauges of rocks below the fault plane showed an increasing rate of circumferential strain increase and axial strain decrease immediately before slip. Changes in axial strain were different on the upper and lower sides of the fault plane. For the transmission elastic waves, no obvious changes were detected in the waveforms immediately before and after slip, when a significant change in the slip surface condition would be expected; Shreedharan et al. (2021) performed similar experiments for direct-shear type specimens with a gouge, and found that, except for short-term significant changes during slip, the changes in velocity and amplitude are small, at most 0.1 % and 0.32 %, respectively, and are not likely to be recognized simply by comparing waveforms. In the future, cross-correlation analysis of the transmitted elastic waveforms will be performed to investigate whether there are small changes during the period when slip does not occur.
References
Brace, W.F., Byerlee, J.D. (1966) Stick-Slip as a Mechanism for Earthquakes, Science 26, 153 (3739), 990-992.
Scholz, C.H. (1993) The Mechanics of Architectures and Faulting, Cambridge University Press.
Shreedharan, S., Bolton, D.C., Rivière, J., Marone, C. (2021) Competition between preslip and deviatoric stress modulates precursors for laboratory earthquakes, Earth and Planetary Science Letters, 553 (2021) 116623.