Non-volcanic tremor resulting from the combined effect of Earth tides and slow slip events

Slow slip events(1) are accompanied by swarms of non-volcanic tremor(2,3) along the subduction zone of the Philippine Sea plate in southwest Japan: the swarms often occur with a periodicity of about 12 or 24 h (refs 4,5). These episodic events are considered to be a manifestation of stress relaxation at the subducting plate interface(1,6,7). Here, we analyse seismic data to record the locations and durations of non-volcanic tremor swarms that occurred in May 2005 and February 2006. We evaluate the magnitude of stress changes produced by slow slip events as well as the effective normal stress on the plate interface where slow slip events occur. We find that the observed periodicity in tremor occurrence originates from a combined effect of the periodic stress due to Earth tides and the transient stress due to slow slip events. Our calculations show that non-volcanic tremor is sensitive to stress change. This phenomenon can therefore be effective in monitoring the process of stress relaxation at subducting plate interfaces.