Japan Geoscience Union Meeting 2019

Presentation information

[E] Poster

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

[S-CG50] Intraslab and intraplate earthquakes

Thu. May 30, 2019 3:30 PM - 5:00 PM Poster Hall (International Exhibition Hall8, Makuhari Messe)

convener:Saeko Kita(International Institute of Seismology and Earthquake Engineering, BRI), Tomohiro Ohuchi(Geodynamics Research Center, Ehime University), Marcel Thielmann(Bavarian Geoinstitute, University of Bayreuth), Ryo Okuwaki(Research Institute of Earthquake and Volcano Geology, Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology)

[SCG50-P06] High-pressure deformation experiments on peridotite gouges under hydrothermal conditions, using a deformation-DIA apparatus

*Maki Semba1, Ken-ichi Hirauchi1, Tomohiro Ohuchi2, Katsuyoshi Michibayashi3 (1.Department of Geosciences, Faculty of Science, Shizuoka University, 2.Geodynamics Research Center, Ehime University, 3.Department of Earth and Planetary Sciences, Nagoya university)

Keywords:deformation-DIA apparatus, peridotite, serpentinization, frictional strength

In subducting oceanic plates, the focal depth of intraslab earthquakes is deeper (>40 km) than that of interplate earthquakes. Since the normal stress on faults (e.g., outer-rise faults) hosting the intraslab earthquakes is too high to generate brittle shear failure, there may be some mechanisms to weaken effective fault strength. In addition, the hypocenter distribution of intraslab earthquakes forms double-planed structure of the deep seismic zone, and the location of the lower plane is in accord with the stability limit of serpentine (antigorite), suggesting shear deformation of antigorite-bearing peridotite gouges in the presence of hydrothermal water. Here, in order to understand the influence of hydrous minerals on the fault strength, we performed deformation experiments on simulated peridotite gouges with compositions of dunite (100% olivine) and harzburgite (70% olivine/30% orthopyroxene) under hydrothermal conditions, using a deformation-DIA apparatus. The experiments were conducted at a temperature of 580°C, a confining pressure of 2.5 GPa, and shear strain rates of 2.70 × 10-5 to 1.03 × 10-4 s-1. In the deformed peridotite gouges, localized shear zones such as R1 and B shears were observed, and these shears were accompanied with newly formed hydrous phases (serpentine or talc). Olivine grains have a crystal preferred orientation characterized by a slip system of [001] (010). These results indicate that semi-brittle flow is a major deformation mechanism of the peridotite gouges. Considering low coefficients of friction of serpentine and talc, we can suggest that the infiltration of seawater along outer-rise fault planes and subsequent hydrothermal alteration result in effective shear strengths low enough to produce intraslab earthquakes.