[S-CG48] Science of slow earthquakes: Toward unified understandings of whole earthquake process

[SCG48-P31] Evidence for semi-brittle flow in mantle-wedge serpentinites under high pore fluid pressure and their implications for deep low frequency earthquakes

片岡 賢悟¹、永田 有里奈¹、*平内 健一¹、大柳 良介²、岡本 敦³、道林 克禎⁴ (1.静岡大学理学部地球科学科、2.国立研究開発法人海洋研究開発機構、3.東北大学大学院環境科学研究科、4.名古屋大学大学院環境学研究科地球環境科学専攻)

キーワード：スロー地震、低周波地震、マントルウェッジ、蛇紋岩

In southwest Japan, deep low frequency earthquakes occur at the base of the shallow serpentinized mantle wedge. In order to understand deformation styles of the mantle wedge serpentinite, we conducted structural and petrological analysis of a paleo-mantle wedge body that has experienced serpentinization at pressure–temperature conditions corresponding to source areas of low-frequency earthquakes, in the Sanbagawa belt, central Shikoku. The serpentinite body has a block-in-matrix structure that result from pervasive fragmentation. At outcrop and thin section scales, the distribution of blocks follows a power law distribution with exponent 2.32 < D < 2.37 (2-D). The block has experienced complete serpentinization, characterized by randomly-oriented antigorite aggregates. The matrix also consists of antigorite, which was formed due to dissolution–precipitation process of the block in the presence of H₂O. Antigorite aggregates in the matrix along localized shear planes is characterized by a strong crystallographic preferred orientation with a dominant [010](001) slip system, suggesting dislocation glide on (001) planes. We suggest that deep slow earthquakes occur by semi-plastic deformation of the serpentinized mantle wedge, under low effective normal stresses.