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.
Article link: https://doi.org/10.1038/s43247-026-03280-x
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