Variations in the Characteristic Amplitude of Tectonic Tremor Induced by Long-Term Slow Slip Events

Long-term slow slip events (L-SSEs) often excite short-term slow slips events (S-SSEs) and tectonic tremor in the zone of episodic tremor and slip (ETS). However, the factors controlling the occurrence of primary versus excited tremor events remain unclear. To elucidate these factors, we analyzed tectonic tremor events in and around the Bungo Channel (Nankai subduction zone), where L-SSEs are known to excite tremor and S-SSEs in the ETS zone. We focused on the spatial distribution of the characteristic amplitude (CA) of tremor, determined from the duration-amplitude distributions of tremor events, as an indicator of the properties of the tremor source. CAs are large in L-SSE slip areas and small in adjacent areas. The difference between CA values during tremor-excitation periods (L-SSEs) and the intervening periods (ΔCA) is positive in the slip area, negative in adjacent areas, and tends toward zero in the far field. We suggest that the heterogeneous distributions of CA and ΔCA reflect the heterogeneous effective strengths of tremor patches, which might be related to petrological properties, and stress and pore-fluid pressure variations induced by L-SSEs, respectively. The upward migration of fluid from the ETS zone along the plate interface might modulate the effective stress and strength states of tremor patches during L-SSEs.