Slow earthquakes illuminating interplate coupling heterogeneities in subduction zones

Slow earthquakes are mainly distributed in the vicinity of seismogenic zones of megathrust earthquakes and relationships between both types of earthquakes are expected. We examined the activity of very low frequency earthquakes (VLFEs), classified as one type of slow earthquakes, around Japan because they have the potential to clarify detailed spatiotemporal slip behaviors at the plate boundaries. The distribution of the shallow VLFE activity rate is heterogeneous along trench axes and exhibits an anticorrelation relationship with the spatial distribution of the interplate coupling rate, whereas deep VLFEs are distributed only in weakly coupled areas and the spatial variation of the activity rate is small. Furthermore, VLFEs are mainly hosted by low seismic velocity anomalies. Thus, slow earthquakes can be triggered by a decreased effective stress due to the high pore fluid pressure within regions with weak interplate coupling and their activity can be an indicator of interplate slip behavior.