Grain size dependency of the phase transformational faulting mechanism responsible for deep-focus earthquakes
Abstract
Deep-focus earthquakes occur at the depth from 440 km to 660 km in the slabs. By the geophysical observations (e.g. Zhan et al., 2014) and deformation experiments (e.g. Green and Burnley, 1989), the phase transformational faulting mechanism is presumed as the precursor of deep-focus earthquakes. This mechanism is that shear instability occurs at fine-grained spinel nucleated by the phase transformation from metastable olivine in the subducting slab. This shear instability depends on grain size, but previous studies have not referred to the grain size dependency of metastable olivine on the phase transformational faulting mechanism (e.g. Burnley et al., 1991). Therefore, we prepared the two sets of germanate olivine (an analogue material of silicate olivine) aggregates with different grain sizes: sample A: <10 μm and B: >30 μm. We used a Griggs-type solid-confining media deformation apparatus. Confining pressure, temperature and strain rate are 1.2 GPa, 500~800 ℃, 2.0 ×10-4 s-1, respectively. As a result of deformation experiments using fine-grained germanate olivine (sample A), we did not observe any shear instability event. Also, micro-Raman spectroscopy clarified that metastable olivine underwent the phase transformation to spinel at temperature higher than 600 ℃ although it did at temperature higher than 1000 ℃ in the previous study (Burnley et al., 1991). Shear instability does not occur because the difference in grain sizes between the initial olivine and nucleated spinel in this study was smaller than those in previous studies. Considering the rate equation of the phase transformation (Cahn, 1956), small grain size promotes the transformation and causes lower temperature at which olivine undergoes the transformation in this study than that in the previous study. These results indicate that the phase transformational faulting mechanism has strong dependency of grain size. In the presentation, we discuss the effect of grain sizes from results of sample B.
This work was supported by the JSPS Japanese-German Graduate Externship. [References] Burnley, P., H. W. Green and D. J. Prior (1991), JGR, vol 96, 425-443. Chan (1956), Acta Metallurgica, vol 4, 449-459. Green, H. W. and P. Burnley (1989), Nature, vol 341, 733-737 Zhan et al. (2014), EPSL, vol 385, 89-96.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.S13C0460S
- Keywords:
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- 3613 Subduction zone processes;
- MINERALOGY AND PETROLOGY;
- 7209 Earthquake dynamics;
- SEISMOLOGY;
- 7215 Earthquake source observations;
- SEISMOLOGY;
- 8170 Subduction zone processes;
- TECTONOPHYSICS