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[SCG43-P04] Does Kaiser effect exist at upper mantle pressures and temperatures?
Keywords:intermediate earthquakes, acoustic emission,, in-situ measurements, Kaiser effect
The subduction zone produces a major fraction of the Earth’s seismic activity. The mechanisms of intermediate and deep-focus earthquakes are fundamentally different from those of shallow (< 40 km) earthquakes. This is because the frictional strength of silicate rocks is proportional to the confining pressure and it exceeds the upper limit of the stress level in the upper mantle. To understand the process triggering intraslab earthquakes, some experimental studies on faulting of slab-forming rocks have been conducted at upper mantle pressures using a D-DIA apparatus. Acoustic emission (AE) events satisfy the same scaling relationship as natural earthquakes in which seismic moment is inversely proportional to the cube of corner frequency (Yoshimitsu et al., 2014), namely, AE is a good simulation of actual earthquake events. One of characteristic phenomena of microfracturing of rocks is a hysteresis of AE numbers related to the stress history (i.e., Kaiser effect). Kaiser effect could be a clue to evaluate the tectonic stress memorized in seismic zones. Previous experiments on Kaiser effect have been conducted at low pressures and room temperature (e.g., Kurita and Fujii, 1979). However, the elevated confining pressure and temperature may affect the process of microcracking, including Kaiser effect. Note that the existence of Kaiser effect has been confirmed in elastic deformation. Here, we conduct a series of cyclic loading experiments using a D-DIA apparatus to evaluate whether Kaiser effect exists in ductile rocks at upper mantle pressures and temperatures.
We conducted in-situ uniaxial deformation experiments on as-is olivine aggregates at pressures 1-3 GPa and temperatures 700-1250 K with a constant displacement rate using a deformation-DIA apparatus, installed at BL04B1, SPring-8. A deformation process followed by a 30-min annealing process was repeated at high pressures and high temperatures. Pressure, stress, and strain were measured in situ by using x-ray diffraction patterns and radiographs. AEs were also recorded continuously on six sensors, and three-dimensional AE source location were determined.
Stress increased with strain at the beginning of sample deformation, and it reached the yielding point at strains of ~0.05. AEs from the deforming sample were detected when stress exceeded ~1 GPa and the amplitude of AE is positively correlated with stress (and strain). During the annealing process, stress continuously decreased and AE activity completely ceased. This observation satisfies the definition of Kaiser effect. A high stress magnitude (>1 GPa) was maintained during the annealing process at 1150 K, but stress was decreased to almost zero during the annealing process at 1250 K. AE activity restarted when the deformation process was restarted. The stress magnitude at which AE activity restarted was far below the previous stress maximum, even though AE radiation should be inhibited if stress is below the previous stress maximum. A possible explanation for the cause of the discrepancy between our results and Kaiser effect is crack healing during the annealing process. Considering the fact that not only stress increase but also strain increase is essential for radiation of AEs in our experiments, sliding along crack surfaces rather than opening of cracks could be the origin of AEs at high pressures and high temperatures.
We conducted in-situ uniaxial deformation experiments on as-is olivine aggregates at pressures 1-3 GPa and temperatures 700-1250 K with a constant displacement rate using a deformation-DIA apparatus, installed at BL04B1, SPring-8. A deformation process followed by a 30-min annealing process was repeated at high pressures and high temperatures. Pressure, stress, and strain were measured in situ by using x-ray diffraction patterns and radiographs. AEs were also recorded continuously on six sensors, and three-dimensional AE source location were determined.
Stress increased with strain at the beginning of sample deformation, and it reached the yielding point at strains of ~0.05. AEs from the deforming sample were detected when stress exceeded ~1 GPa and the amplitude of AE is positively correlated with stress (and strain). During the annealing process, stress continuously decreased and AE activity completely ceased. This observation satisfies the definition of Kaiser effect. A high stress magnitude (>1 GPa) was maintained during the annealing process at 1150 K, but stress was decreased to almost zero during the annealing process at 1250 K. AE activity restarted when the deformation process was restarted. The stress magnitude at which AE activity restarted was far below the previous stress maximum, even though AE radiation should be inhibited if stress is below the previous stress maximum. A possible explanation for the cause of the discrepancy between our results and Kaiser effect is crack healing during the annealing process. Considering the fact that not only stress increase but also strain increase is essential for radiation of AEs in our experiments, sliding along crack surfaces rather than opening of cracks could be the origin of AEs at high pressures and high temperatures.