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. Plain Language Summary: In subduction zones, slow earthquakes have inspired great interest in the connection between slow and megathrust earthquakes. Long‐term slow slip events (L‐SSEs) are known to excite short‐term slow slip events and tectonic tremor. To understand the factors controlling the occurrence of primary and excited tremors, we investigated the characteristic amplitude (CA) of tremor events (representing the properties of a tremor source) during tremor‐excitation periods and the intervening periods. CAs are larger in L‐SSE slip areas than in adjacent areas during both periods. This contrast may result from differences in pore‐fluid pressure arising from differences in petrological properties between the two areas. Moreover, relative to CAs during the intervening periods, CAs during tremor‐excitation periods are larger in the slip area, smaller in adjacent areas, and almost the same far from the slip area. These variations in CA are the combined effects of (1) stress changes accompanying L‐SSEs and (2) upward fluid migration along the plate interface from the tremor source area during L‐SSEs, because fluid migration reduces the pore‐fluid pressure and increases the strength of tremor source areas. Our findings emphasize that CA can be a useful tool for monitoring fluid migration in the source areas of tremor events. Key Points: Long‐term slow slip events change characteristic amplitudes of tectonic tremor in the episodic tremor and slip zoneVariations in characteristic amplitude reflect the heterogeneous effective strengths of tremor patchesFluid migration during long‐term slow slip events might control the stress state and strength of tremor patches [ABSTRACT FROM AUTHOR]