[S-CG43] スラブ内地震とその発生メカニズム

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

*澤 燦道¹、宮島 延吉²、Gasc Julien³、武藤 潤¹、Schubnel Alexandre³、Baïsset Marie⁴、長濱 裕幸¹ (1.東北大学大学院理学研究科地学専攻地圏進化学講座、2.Bayerisches Geoinstitut, University of Bayreuth、3.Laboratoire de Géologie, École Normale Supérieure, Paris、4.ISTEP, Sorbonne Université)

キーワード：深発地震、オリビンスピネル相転移、粒径、AE測定

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.