Mature serpentinized faults cannot host depth-dependent slow earthquakes

Deep slow earthquakes (e.g., episodic tremor and slip, ETS) nucleate under elevated pore–fluid pressure at serpentinized subduction interfaces1,2 and exhibit a characteristic depth dependence3​. A key question is whether this trend, and the debated linear versus cubic scaling laws of slow earthquake4,5, originate from the mature fault core or the mantle wedge corner—two structurally distinct domains within the subduction interface. Whereas previous experiments on antigorite conflate thermal and mineralogical changes6-8, we isolate the role of composition via experiments on synthetic olivine-antigorite gouges. We demonstrate that increasing antigorite content linearly reduces stick-slip displacement, indicating its instability-inhibiting potential. However, it fails to produce the systematic changes in stress drop and recurrence interval characteristic of natural ETS3. Therefore, mature, serpentinized fault cores cannot host depth-dependent slow earthquakes. Instead, the mantle wedge corner—where intact, heterogeneous rocks deform—is the probable source8. Our work establishes that linear scaling arises from the mature fault core, while cubic scaling is evident in the heterogeneous mantle wedge corner, redefining the origin of slow earthquakes and resolving the controversy surrounding their scaling laws.