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

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Slow slip events1 are accompanied by swarms of non-volcanic tremor2,3 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 interface1,6,7. 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.

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Figure 1: Distribution of epicentres of NVT swarms.
Figure 2: Comparison of observed NVT occurrences and theoretical tidal Coulomb failure stress.
Figure 3: Comparison of calculated seismicity rate and observed NVT occurrences.


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We thank Y. Fukao, K. Shimazaki, T. Shimamoto and J. E. Vidale for comments. This work was supported by a Grant-in-Aid for Scientific Research, the Ministry of Education, Sports, Science and Technology, Japan (to N.S.).

Author information

R.N. carried out the inversion based on the seismicity rate theory. N.S. developed the tremor detecting method and supervised all of the research. H.T. calculated the theoretical tidal stress.

Correspondence to Ryoko Nakata.

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Nakata, R., Suda, N. & Tsuruoka, H. Non-volcanic tremor resulting from the combined effect of Earth tides and slow slip events. Nature Geosci 1, 676–678 (2008) doi:10.1038/ngeo288

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