Japan Geoscience Union Meeting 2022

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[E] Oral

S (Solid Earth Sciences ) » S-CG Complex & General

[S-CG43] Shallow and intermediate depth intraslab earthquakes: seismogenesis and rheology of the slab

Thu. May 26, 2022 9:00 AM - 10:30 AM 103 (International Conference Hall, Makuhari Messe)

convener:Saeko Kita(International Institute of Seismology and Earthquake Engineering, BRI), convener:Tomohiro Ohuchi(Geodynamics Research Center, Ehime University), Marina Manea(Computational Geodynamics Laboratory, Geosciences Center, National Autonomous University of Mexico), convener:Kurama Okubo(National Research Institute for Earth Science and Disaster Resilience), Chairperson:Marina Manea(Computational Geodynamics Laboratory, Geosciences Center, National Autonomous University of Mexico), Tomohiro Ohuchi(Geodynamics Research Center, Ehime University)

9:30 AM - 9:45 AM

[SCG43-09] Effect of grain size on the olivine-spinel phase transformation faulting mechanism for deep-focus earthquakes

*Sando Sawa1, Nobuyoshi Miyajima2, Julien Gasc3, Jun Muto1, Alexandre Schubnel3, Marie Baïsset4, Hiroyuki Nagahama1 (1.Department of Earth Science, Tohoku University, 2.Bayerisches Geoinstitut, University of Bayreuth, 3.Laboratoire de Géologie, École Normale Supérieure, Paris, 4.ISTEP, Sorbonne Université)

Keywords:deep-focus earthquake, olivine-spinel phase transformation, grain size, acoustic emission

Deep-focus earthquakes occur at depths from 410 km to 660 km in subduction zones. Geophysical observations suggest that the phase transformational faulting could be the precursor of deep-focus earthquakes (e.g., Zhan et al., 2014). Phase transformational faulting occurs due to deformation localization on dense spinel phases (wadsleyite and ringwoodite), which underwent the phase transformation from olivine. Previous deformation experiments indicated that the phase transformation rate controls the occurrence of transformational faulting (Burnley et al., 1991). Although the grain size of olivine affects the phase transformation rate (Cahn, 1956), there are no deformation experiments that quantitatively assess the effect of grain size on the transformational faulting. Therefore, we conducted deformation experiments of germanate olivine (Mg2GeO4) aggregates with grain sizes ranging from a few microns to more than hundreds of microns. The olivine-structured Mg2GeO4 is an analog material of olivine that undergoes the same phase transformations at a much lower pressure. We used Griggs-type deformation apparatuses with solid-confining medium installed at Tohoku university and École Normale Supérieure. The confining pressure, temperature, and strain rate are 1.2-1.5 GPa, 700-992 ℃, and 4.2×10-5-2.0×10-4 s-1, respectively. As a result, samples deformed at higher than 800 ℃ showed strain weakening with no difference in the peak stress between the coarse-grained and fine-grained aggregates. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) showed that spinel grains nucleated at the grain boundaries in the fine-grained aggregates. Meanwhile, slip bands composed of fine-grained spinel form along the (100), (011), and (010) planes of olivine. We will also present the b value in the Gutenberg-Richter law obtained from deformation experiments with a transducer to detect acoustic emissions.
[References]
Zhan et al. (2014), EPSL, vol 385, 89-96.
Burnley et al. (1991), JGR, vol 96, 425-443.
Cahn (1956), Acta Metallurgica, vol 4, 449-459.