11:30 〜 11:45
[SIT14-09] Double Seismic Discontinuities at the Base of the Mantle Transition Zone beneath the Northern Japan Sea
Near the northeastern edge of China, double seismic discontinuities at the base of the mantle transition zone were indicated by the receiver function studies (Niu and Kawakatsu, 1996; Sun et al., 2020). The present study examines the possible existence of the double discontinuities beneath the northern Japan Sea by observing seismic waves triplicated at depths of around 660 km. Japanese seismic stations recorded the P waves from deep-focus earthquakes around Sakhalin. A clear triplication of P wave appears in a similar manner to iasp91. In addition, signals following the triplication are observed in vertical components, which appear to be due to another triplication. The signals are not limited at stations in particularly local areas, and their apparent slowness suggests that they can traverse the bottom of the mantle transition zone below the northern Japan Sea, away from the descending Pacific slab. The double discontinuities seem to spread in a wide region from the eastern edge of the stagnant Pacific slab toward the trench side in the east. Whereas the 660 km seismic discontinuity is considered as the consequence of a phase transition of the olivine system, the appearance of double discontinuities can be associated with phase transitions of the non-olivine system in the mantle. The characteristics and spatial extent of the double discontinuities are important for understanding the nature of the mantle.