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<p>Deep-focus earthquakes (&gt; 300 km depth) are widely distributed in global subduction zones, but their mechanism is still debated because the fine structure of deep subducting slabs is unclear. Here we use a modified double-difference tomography method to investigate 3-D P and S wave velocity (V<sub>P</sub>, V<sub>S</sub>) structures of the subducting Pacific slab beneath central Japan. A three-layered anomalous belt is revealed in the deep slab: the portion at 330-380 km depths shows low V<sub>P</sub>, low V<sub>S</sub> and high V<sub>P</sub>/V<sub>S</sub> ratio, whereas the up-dip (&lt;330 km depth) and down-dip (&gt;380 km depth) portions exhibit low V<sub>P</sub>/V<sub>S</sub> ratios. From top to bottom, these layers correspond to three distinct olivine states: (meta)stable α-olivine, unstable/transforming olivine, and stabilized β-olivine (wadsleyite). The high-V<sub>P</sub>/V<sub>s</sub> zone may indicate water in and around the metastable olivine wedge, suggesting that deep-focus earthquakes could be triggered by metastable olivine phase transition, promoted by deep dehydration.</p>

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Fine slab structure and mechanism of deep earthquakes beneath central Japan

  • 期刊论文

  • 出版模式：

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  • 发表日期：

    2026年2月11日

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  • Xinxin Zhang,
  • Guoming Jiang,
  • Dapeng Zhao,
  • Guibin Zhang

摘要

Deep-focus earthquakes (> 300 km depth) are widely distributed in global subduction zones, but their mechanism is still debated because the fine structure of deep subducting slabs is unclear. Here we use a modified double-difference tomography method to investigate 3-D P and S wave velocity (VP, VS) structures of the subducting Pacific slab beneath central Japan. A three-layered anomalous belt is revealed in the deep slab: the portion at 330-380 km depths shows low VP, low VS and high VP/VS ratio, whereas the up-dip (<330 km depth) and down-dip (>380 km depth) portions exhibit low VP/VS ratios. From top to bottom, these layers correspond to three distinct olivine states: (meta)stable α-olivine, unstable/transforming olivine, and stabilized β-olivine (wadsleyite). The high-VP/Vs zone may indicate water in and around the metastable olivine wedge, suggesting that deep-focus earthquakes could be triggered by metastable olivine phase transition, promoted by deep dehydration.