13:45 〜 15:15
[SCG46-P05] Acoustic emission monitoring at high pressures using small sensors optimized for D-DIA apparatuses
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
To understand the process triggering intraslab earthquakes occurring at depths greater than ~40 km, some recent experimental studies on faulting of rocks have been conducted using a D-DIA apparatus combined with an acoustic emission (AE) monitoring system (e.g., Schubnel et al., 2013). As supported by scaling laws such as the relationship between seismic moment and corner frequency (Yoshimitsu et al., 2014), AE, which is an acoustic wave radiated from a propagating microcrack, is equivalent to a natural earthquake due to an unstable slip of a natural fault. The sample size is typically less than 3mm in diameter in high-pressure and high-temperature experiments using a multi-anvil apparatus, while the uncertainty in AE hypocenter location is limited to ±1mm (e.g., Ohuchi et al., 2017). To explore the nanoseismological analysis, Wang et al., (2017) applied waveform Cross-Correlation (CC) and a double-difference relocation algorithm to AE waveforms monitored at high pressures. They reported that the number of relocated AE hypocenters was about 10 times higher than that of AE hypocenters determined by a conventional method and the location uncertainty decreased down to ±10μm. They compared spatial distribution of faults and AE hypocenters to discuss the rupture process.
In this study, we examined two methods to improve the accuracy in AE hypocenter location determined in high-pressure and high-temperature experiments: i) to minimize the size of AE sensor (down to 2mm in diameter) which is inevitably correlated to the location uncertainty; and ii) application of waveform CC combined with a source relocation algorithm (based on Template-Matching method: Lei et al., 2022).
In order to test the reliability of our AE sensors, we conducted cold-compression runs of quartz beads samples (diameter: 0.8-3 mm; length 1-4 mm) at pressures up to 3 GPa using a deformation-DIA(D-DIA) apparatus "MADONNA" combined with an AE monitoring system installed at Geodynamics research center (GRC), Ehime University. AEs radiated from the sample were detected by six AE sensors glued on behind the 2nd-stage anvils. We successfully determined the locations of AE hypocenters with uncertainty of ~±0.4mm, showing that the uncertainty is much better than that in previous studies using conventional AE sensors (~±1.0mm: Ohuchi et al., 2017). Note that any source relocation algorithm was not applied to our cold-compression runs.
We conducted in situ deformation experiments on olivine aggregates using a D-DIA apparatus "SPEED-Mk.II" combined with an AE monitoring system at the BL04B1 beamline of SPring-8. The experimental conditions correspond to those of the interior of subducting slabs (700-900°C, ~3 GPa). Throughout the deformation runs, semi-brittle flow associating AE radiation dominated the sample shortening was observed with increasing AE frequency. At 900℃, a decrease in AE rate followed by the occurrence of huge AEs and softening were observed at strains higher than 0.28. We applied our source relocation algorithm to huge AE events. We found that the uncertainty in AE hypocenter location could be drastically improved via the algorithm. Future numerical analyses using the relocation algorithm may help understanding the process of faulting at high pressures.
In this study, we examined two methods to improve the accuracy in AE hypocenter location determined in high-pressure and high-temperature experiments: i) to minimize the size of AE sensor (down to 2mm in diameter) which is inevitably correlated to the location uncertainty; and ii) application of waveform CC combined with a source relocation algorithm (based on Template-Matching method: Lei et al., 2022).
In order to test the reliability of our AE sensors, we conducted cold-compression runs of quartz beads samples (diameter: 0.8-3 mm; length 1-4 mm) at pressures up to 3 GPa using a deformation-DIA(D-DIA) apparatus "MADONNA" combined with an AE monitoring system installed at Geodynamics research center (GRC), Ehime University. AEs radiated from the sample were detected by six AE sensors glued on behind the 2nd-stage anvils. We successfully determined the locations of AE hypocenters with uncertainty of ~±0.4mm, showing that the uncertainty is much better than that in previous studies using conventional AE sensors (~±1.0mm: Ohuchi et al., 2017). Note that any source relocation algorithm was not applied to our cold-compression runs.
We conducted in situ deformation experiments on olivine aggregates using a D-DIA apparatus "SPEED-Mk.II" combined with an AE monitoring system at the BL04B1 beamline of SPring-8. The experimental conditions correspond to those of the interior of subducting slabs (700-900°C, ~3 GPa). Throughout the deformation runs, semi-brittle flow associating AE radiation dominated the sample shortening was observed with increasing AE frequency. At 900℃, a decrease in AE rate followed by the occurrence of huge AEs and softening were observed at strains higher than 0.28. We applied our source relocation algorithm to huge AE events. We found that the uncertainty in AE hypocenter location could be drastically improved via the algorithm. Future numerical analyses using the relocation algorithm may help understanding the process of faulting at high pressures.