Slow slip is a type of "slow earthquake" that releases strain at a fault slip speed slower than a normal earthquake.It was also observed just before the XNUMX off the Pacific coast of Tohoku Earthquake, and the slow slip area moved significantly during the mainshock, which contributed to the enormous tsunami damage.However, since most of the slow slip slip area is below the seafloor, there were many unclear points about the details.

　In the "Hikrangi subduction zone" off the eastern coast of North Island, New Zealand, the Pacific plate subducts under the Australian plate at a rate of 3 to 6 cm per year, and slow slips occur every 18 to 24 months near the plate boundary.Crustal movements due to slow slip have been observed on land in New Zealand so far, and it is known that the amount of crustal movements is about 1 to 3 cm and lasts for about a week.

　In this international joint research by Japan, the United States, and New Zealand, a seafloor pressure gauge was installed in this area in 2014 and successfully recovered in 2015.As a result of the analysis, it was found that the slow slip faults are distributed to the shallower part than the estimation on the ground.It suggests that strains that cause earthquakes may be accumulated even at the plate boundary, which was previously considered to be unable to accumulate strains.

　In the future, while continuing similar pressure observations, we plan to conduct seafloor GPS observations to investigate steady plate movements and verify the uniqueness of repeated slow slips.In this sea area, excavation by the deep-sea exploration vessel "Chikyu" is also being considered.The company aims to elucidate the entire mechanism by acquiring slow-slip tomographic materials and combining them with the findings obtained from observations on land and on the seafloor.