Deep seismic tremor in subduction zones has been suggested to repeat at a regular interval, migrate at various velocities and be modulated by tidal stress. Here, evidence is presented that a time-invariant interface property — possibly the ratio of brittle to ductile areas — controls tremor behaviour in the Nankai subduction zone, Japan. Where tremor duration is short, tremor is more strongly affected by tidal stress and migration is inhibited. Where tremor lasts longer, diffusive migration occurs with a constant diffusivity.
