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Abstract Although transformational faulting in the rim of the metastable olivine wedge is hypothesized as a triggering mechanism of deep-focus earthquakes, there is no direct evidence of such rim. Variations of thebvalue – slope of the Gutenberg-Richter distribution – have been used to decipher triggering and rupture mechanisms of deep earthquakes. However, detection limits prevent full understanding of these mechanisms. Using the Japan Meteorological Agency catalog, we estimatebvalues of deep earthquakes in the northwestern Pacific Plate, clustered in four regions with unsupervised machine learning. Theb-value analysis of Honshu and Izu deep seismicity reveals a kink at magnitude 3.7–3.8, where thebvalue abruptly changes from 1.4–1.7 to 0.6–0.7. The anomalously highbvalues for small earthquakes highlight enhanced transformational faulting, likely catalyzed by deep hydrous defects coinciding with the unstable rim of the metastable olivine wedge, the thickness of which we estimate at$$\sim$$ $~$1 km.