[S-CG52] Oceanic plate as inputs to subduction zone: evolution process of the oceanic plate

[SCG52-11] Seismological constraints on the depth range of dehydration reactions in Pacific plate

*Kazuki Miyazaki¹, Junichi Nakajima¹ (1.Department of Earth and Planetary Sciences, Tokyo Institute of Technology)

Keywords:receiver function, Japan, oceanic Moho

The water in the basaltic oceanic crust is supplied into the Earth’s interior through plate subduction. Hydrous minerals in the subducting oceanic crust dehydrates during phase transitions, and significant quantities of water are released during the transformation to eclogite. The seismic velocity of the hydrated basaltic crust is much slower than that of the slab mantle, but eclogite is indistinguishable from the slab mantle by seismic velocity. By focusing on sharp seismic velocity boundaries, it is possible to estimate the location of the phase transformation from hydrous to anhydrous minerals within subducting oceanic crust. In addition, the numerical simulations of slab subduction suggests that fluid dehydrated from the subducting plate forms a thin serpentine layer at the bottom of mantle wedge (Iwamori, 1998; Iwamori, 2000). In the Tohoku region, Kawakatsu and Watada (2007) clearly imaged the oceanic Moho and the bottom of the serpentine layer as distinct velocity discontinuities. However, it is difficult to evaluate the deep extension of the oceanic Moho and serpentine layer beneath Tohoku region because the deeper part of the Pacific plate is located in the Japan Sea area. Here, we investigate the depth variation of the oceanic Moho and serpentine layer beneath Kanto region from receiver function imaging.
We applied an instrumental response correction (Maeda et al., 2011) to waveform data and calculated RFs with a water level of 0.001 in a frequency range of 0.1-0.5Hz. For the RF calculation, we used the tele-seismic waveform data of magnitude 5.5 or greater that occurred during a period from April 2005 to March 2023 and epicentral distances between 30°-90°. After, we discarded RFs with low signal-to-noise ratio, migrated RFs into space-domain using the iasp91 1D velocity model (Kennett and Engdahl, 1991).
As a result, we found that the oceanic Moho shows a distinct velocity contrast down to 150-200 km. This result suggests that hydrous phases in the oceanic crust exist to deeper part beneath the Kanto region compared to the Tohoku region. The deep preservation of the hydrous minerals is consistent with the locations of dehydration in the hydrous minerals predicted from the thermal model in Iwamori (2000), where he considered the shielding effect of the Pacific slab from the overlying Philippine Sea plate. In addition, we observed a continuous positive amplitude layer immediately above the subducting Pacific plate down to 300-350 km depth. Because the positive amplitude layer is located about 30 km above the seismic zone, we interpret it to be a hydrous layer that is probably formed by fluids released from the subducting plate.