LAUSR

We investigate the temporal evolution of the background seismicity rate related to the subduction of the Pacific plate in northeast Japan, at latitudes 34o to 42o, for the 1/1/1990 - 9/3/2011 period. Two declustering methods are used to identify robust features. We find that the dominant behaviour is a lowering down of activity, especially in the northern half of our studied area, where changes appear related to the cycle of M7.5+ earthquakes, in particular the 1968 Tokachi and the 1994 Sanriku earthquakes. Acceleration of background seismicity is observed offshore the Kanto region, and could mark a long term decoupling of the Pacific and the Philippine Sea plates over a 100 km-long segment along the Sagimi Trough, which cannot be simply explained by the earthquake cycle model. Our analysis further suggests that changes in background seismicity are plausibly related to changes in seismic coupling, and thus further strengthens the recent observation that seismic coupling does vary at the time scales of tens of years, for the Japanese subduction zone.