[S21-P] Poster

[S21-P-12] Seismological evidence of slab dehydration based on a high-resolution receiver function image of the subducting Philippine Sea plate beneath western Shikoku, southwest Japan

^○Katsuhiko Shiomi, Tetsuya Takeda, Tomotake Ueno (NIED, Tsukuba, Japan)

In the Nankai region, southwest Japan, low-frequency earthquakes (LFEs) are very active at the down-dip limit of the megathrust source region. To reveal the spatial relationship between underground structure and LFE activity, we newly constructed high-resolution receiver function (RF) profiles of the subducting Philippine Sea (PHS) slab and overriding continental plate. In this study, we installed 30 short-period seismographs in western Shikoku for one year. The survey line was aligned in the NNW-SSE, orthogonal to iso-depths of the slab, and it was passed through above one of the dominant LFE clusters. We applied the RF analysis to not only these temporal stations but also nine NIED Hi-net/F-net stations near the survey line. To enhance converted phase amplitudes at a dipping oceanic Moho (OM), we excluded the RFs estimated from earthquakes located in the up-dip direction (SSE) of our survey line. We clearly confirmed the existence of the north dipping OM and the continental Moho (CM) from the profiles. The OM was lying at 30 km depth beneath the Pacific coastline, and inclined to the north with dip of 7-degree. In the northern part of the survey line, the flat CM was located at 30 km depth. At the area where the OM reached 40 km depth, the CM became shallow to the south and gradually unclear. the precisely detected LFEs were distributed to the deep extension of the upper boundary of the PHS slab. The harmonic analysis of RFs implied anisotropic rocks existence just above the OM only at the north of LFE zone. The pressure and temperature there corresponds to the condition for phase transition from blueschist to eclogite with dehydration. Our observation implies that water dehydrated from the oceanic crust rises to the inter-plates, and it causes the LFEs.
Acknowledgements: This study was partially supported by JSPS KAKENHI Grant Number JP16H06475.