[S-CG43] スラブ内地震とその発生メカニズム

[SCG43-01] Metamorphism-facilitated faulting in deforming orthopyroxene: implications for global intermediate-depth seismicity

*Yanbin Wang¹、Feng Shi²、Jianguo Wen³、Tony Yu¹、Lupei Zhu⁴、Taizi Huang⁵、Kelin Wang⁶ (1.Center for Advanced Radiation Sources, University of Chicago、2.State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Sciences, China University of Geosciences, Wuhan, Hubei, China、3.Center for Nanoscale Materials, Argonne National Laboratory, Argonne, IL, USA、4.Department of Earth & Atmospheric Sciences, Saint Louis University, St. Louis, MO, USA、5.School of Earth and Ocean Sciences, University of Victoria, Victoria, B.C., Canada 、6.Pacific Geoscience Centre, Geological Survey of Canada, Sidney, B.C., Canada)

キーワード：intraslab seismicity, metamorphism, laboratory earthquake simulation , Acoustic emission, synchrotron

Intermediate-depth earthquakes that occur between ~60 and 300 km depths in subducted oceanic slabs often form a double band of seismicity. The upper band of seismicity (UBS) is predominantly within the basaltic crust and is thought to be facilitated by processes related to dehydration reactions. More controversial is the faulting mechanism of the lower band of seismicity (LBS), located within the subducted mantle lithosphere which consists primarily of olivine and orthopyroxene (Opx). High-pressure petrological studies have shown that as the oceanic lithosphere subducts to below 60-70 km depth, the Opx component undergoes an exothermic metamorphic reaction, characterized by continuous garnet exsolution to depths of ~250 km. Here, we simulate the mechanical behavior of syn-deformational metamorphosing Opx in oceanic lithosphere using controlled deformation experiments combined with acoustic emission detection. We show that deforming Opx under the pressure and temperature conditions corresponding to those of LBS fail consistently by macroscopic faulting, accompanied with numerous acoustic emission events which are laboratory analogs of earthquakes. Microstructural analysis shows that syn-deformational metamorphism proceeds by garnet exsolution both within Opx grains and along grain boundaries. Ultrafine-grained reaction product (grain size as small as 50 nm) facilitates strain localization and weakening, promoting slip instability. An examination of thermal structures in 9 subduction zones shows that temperature conditions in these LBSs are consistent with the kinetic onset of brittle behavior observed in the laboratory, considering the tradeoff between onset temperature and strain rate. These results suggest that metamorphosing Opx likely plays an important role in intraslab earthquake processes.