[S-CG40] Science of slow-to-fast earthquakes

[SCG40-14] Three-dimensional location analysis on acoustic emissions and faults in metastable olivine under pressure-temperature conditions of deep subducted slabs

*Tomohiro Ohuchi¹, Masato Hoshino², Kentaro Uesugi², Satoshi Okumura³, Yuji Higo², Noriyoshi Tsujino² (1.Geodynamics Research Center, Ehime University, 2.Japan Synchrotron Radiation Research Institute, 3.Graduate School of Science, Tohoku University)

Keywords:deep earthquake, olivine, acoustic emission, fault, X-ray computed micro-tomography

The frequency of seismic activity becomes lower at greater depths due to the positive pressure dependency of frictional strength. Surprisingly, the depth dependence of seismic activity turns to increase at depths between 400 and 660 km (e.g., Florich, 1989). To investigate the role of phase transitions of olivine on deep earthquakes, many experimental studies have been conducted using germinate olivine at 1-2 GPa (Mg₂GeO₄: e.g., Green and Burnley, 1989) or fayalite at 4-9 GPa (Fe₂SiO₄: Officer and Secco, 2020). It becomes increasingly clear that propagation of spinel-filled anticracks (Green and Burnley, 1989) or nano-shear bands filled with spinel nanograins (Shubnel et al., 2013; Wang et al., 2017) is likely to be the cause of strain localization followed by throughgoing faulting. Both three-dimensional (3D) fault distribution and locations of acoustic emission (AE) hypocenter are critical data when we try to understand the fracturing process (e.g., Lei et al., 2004). Wang et al. (2017) examined the relationship between 3D fault distribution and locations of AE hypocenter in germinate olivine deformed at 4 GPa and 973-1173 K. They reported that AE events occur near the final fault planes with increasing magnitude in the late stage of deformation. The latent heat release via the phase transition would promote adiabatic instability. The latent heat via the phase transition(s) in silicate olivine is much higher than that in the germanate analogue (Kirby et al., 1996). The fracturing process in the deep subducted slabs needs to be considered based on the experimental results on silicate olivine.
We conducted in situ uniaxial deformation experiments on Mg_1.8Fe_0.2SiO₄ olivine aggregates at pressures of 17-22 GPa and temperatures of 870-1120 K using a D-DIA apparatus at BL04B1/SPring-8. Pressure, stress, and strain were measured by using X-ray diffraction patterns and radiographs. The 3D microstructures of the recovered samples were observed using X-ray radiography and fast X-ray computed micro-tomography at BL20B2/SPring-8. We found that throughgoing faulting sometimes occur at temperatures ~1100 K, accompanied by intense acoustic emissions at the onset of fault slip (displacement of ~100 micrometer). Many AE hypocenters were located around the final fault planes. Occurrence of large AEs repeated around a limited part of the fault. Microstructures of the recovered sample revealed that the fault gouge consists of pulverized old olivine grains (larger than a few micrometers) and nanoparticles of olivine and wadsleyite/ringwoodite. Pulverization of old olivine grains seems to proceed with kinking of old olivine grains, microcracking and nucleation of high-pressure phases. Formation of a throughgoing mode-II crack was observed at 19 GPa and 1120 K. Even though the slip of the mode-II crack was absent, many AEs continuously radiated around the crack planes. A lesser degree of pulverization of old olivine grains and formation of nanograins proceeded around the mode-II crack. Our observations suggest that the detected AEs are related to the onset of growth of cracks (i.e., formation of mode-II cracks), rather than fault slip. Initiation of fault slip would require not only formation of mode-II cracks but also nanograins along the cracks.