11:15 〜 11:30
[SCG46-09] In situ time-resolved X-ray observation of faulting at high pressures using a CdTe detector
キーワード：スラブ内地震、アコースティック・エミッション、その場観察実験、軟化、カイザー効果
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, many experimental studies on faulting of slab-forming rocks have been conducted at upper mantle pressures using a D-DIA apparatus or a Griggs rig. Previous studies have revealed that shear localization induced by dehydration of hydrous minerals (e.g., Okazaki & Hirth, 2016; Ferrand et al., 2017) or adiabatic shear heating (e.g, Kelemen & Hirth, 2007) is essential for the occurrence of faulting at high pressures. Although acoustic emission (AE) monitoring technique for D-DIA apparatuses enabled us to discuss the process of microcracking at high pressures, mechanical behavior at the onset of faulting is still unclear due to low time-resolution stress/strain measurements using a synchrotron X-ray. The cause of bottleneck in stress/strain measurements is a long exposure time required for the acquisition of a two-dimensional X-ray diffraction pattern of minerals. Considering that the timescale of stress drop associating faulting is sometimes as short as ~0.01 sec (e.g., Okazaki & Katayama, 2016), observations of precursors occurring prior to the faulting require a significant improvement on in situ stress/strain measurements. To improve the time resolution of stress/strain measurements, we installed a series of new devices at BL04B1, SPring-8. In this talk, we will report recent progresses on in situ measurements for faulting in rocks at high pressures.
We conducted in situ uniaxial deformation experiments on as-is olivine aggregates at pressures 1-3 GPa and temperatures 700-1250 K using a deformation-DIA apparatus, installed at BL04B1, SPring-8. Constant strain-rate deformation runs and cyclic loading runs (i.e., a deformation process followed by a 30-min annealing process was repeated) were performed. Pressure, stress, and strain were determined from two-dimensional X-ray diffraction patterns and radiographs of monochromatic X-rays (energy 60 keV), those were acquired using a WidePix CdTe detector (Advacam Co.) equipped with an X-ray camera. The CdTe detector, which requires 40-80 s of exposure time, enabled us to observe a few precursors of faulting at high pressures. 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 the magnitude of stress. At strains higher than 0.1 (i.e., beyond the yielding point), both softening (i.e., decrease in stress and/or increase in strain rate) and a decrease in AE rate were observed prior to the occurrence of faulting. Faulting was followed by a stress drop which continued for a few minutes. Cyclic loading experiments demonstrated that stress continuously decreased and AE activity completely ceased (i.e., the Kaiser effect) when the deformation was halted and then the annealing process started. AE activity restarted when the deformation was restarted. Our results suggest that AE rate is a proxy of softening, which is followed by faulting.
We conducted in situ uniaxial deformation experiments on as-is olivine aggregates at pressures 1-3 GPa and temperatures 700-1250 K using a deformation-DIA apparatus, installed at BL04B1, SPring-8. Constant strain-rate deformation runs and cyclic loading runs (i.e., a deformation process followed by a 30-min annealing process was repeated) were performed. Pressure, stress, and strain were determined from two-dimensional X-ray diffraction patterns and radiographs of monochromatic X-rays (energy 60 keV), those were acquired using a WidePix CdTe detector (Advacam Co.) equipped with an X-ray camera. The CdTe detector, which requires 40-80 s of exposure time, enabled us to observe a few precursors of faulting at high pressures. 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 the magnitude of stress. At strains higher than 0.1 (i.e., beyond the yielding point), both softening (i.e., decrease in stress and/or increase in strain rate) and a decrease in AE rate were observed prior to the occurrence of faulting. Faulting was followed by a stress drop which continued for a few minutes. Cyclic loading experiments demonstrated that stress continuously decreased and AE activity completely ceased (i.e., the Kaiser effect) when the deformation was halted and then the annealing process started. AE activity restarted when the deformation was restarted. Our results suggest that AE rate is a proxy of softening, which is followed by faulting.