Professor Junichi Nakajima of Tokyo Institute of Technology and Associate Professor Naoki Uchida of Tohoku University analyzed the waveforms of earthquakes that occur around the upper boundary of the Philippine Sea Plate in the southwestern part of Ibaraki Prefecture.It was clarified that water is discharged to the shallow part due to the "slow slip" that occurs at the plate boundary in a cycle of about one year.

 Slow slip is a phenomenon in which a fault moves slowly over several days to several years.It often occurs periodically around the epicenter area of ​​a large earthquake at the plate boundary, and is said to play an important role in stress accumulation in the epicenter area.On the other hand, when water moves due to slow slip, the strength of the surrounding interplate boundary may be significantly reduced, so elucidation of the behavior of water is extremely important for predicting the occurrence of interplate earthquakes.

 Professor Nakajima et al. Estimated in detail the temporal changes in seismic activity and seismic attenuation near the upper boundary of the Philippine Sea Plate in the southwestern part of Ibaraki Prefecture, and aimed to elucidate the behavior of water due to slow slip.

 As a result, it was found that the activity of repeated earthquakes becomes active in a cycle of about one year, the damping characteristics of the seismic waves just above the plate boundary increase in synchronization with the activity, and the shallow seismic activity becomes active several months later. ..This is because slow slip at the plate boundary occurs in a cycle of about one year, and the water at the plate boundary is discharged to the upper plate, and the water rises to the shallow part over several months and is inside the upper plate. It can be explained that it triggered an earthquake.This is similar to an artificial water injection experiment, and it is possible that a "natural water injection experiment" has occurred underground in the Kanto region.

 This result is expected to show a new direction for the study of the interaction between slow slip and interplate earthquakes and to contribute to the elucidation of the mechanism of occurrence of interplate earthquakes.

Paper information:[Nature Geoscience] Repeated drainage from megathrusts during episodic slow slip

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