Depths of the 410-km and 660-km discontinuities in and around the stagnant slab beneath the Philippine Sea: Is water stored in the stagnant slab?

We determined the depths of the 410-km and 660-km seismic discontinuities (called “410” and “660,” respectively, hereafter) beneath the Philippine Sea and the northwestern Pacific by the receiver function method; we used the broadband data obtained from broadband ocean-bottom seismographs. We found a very deep “660” at 691 km in the stagnant slab beneath the Philippine Sea. In the surrounding Philippine Sea regions and northwestern Pacific, the “660” appears at depths of 659–674 km. Comparison of the “660” depth and P-velocity anomalies indicated that temperatures in the stagnant slab are lower than that in the mantle transition zone (MTZ) beneath the northwestern Pacific by about 500 K. The temperature in the MTZ beneath the Parece Vela Basin and West Philippine Sea Basin, which surrounds the stagnant slab, is lower than that in the MTZ beneath the Pacific by about 200 K. The water content in the stagnant slab (∼0.2 wt% H2O) is higher than that in the other regions; however, the large uncertainty in the estimated water content (±0.2–0.3 wt% H2O) prevents us from arriving at a definitive conclusion regarding the presence of water in the stagnant slab. Therefore, it is desirable to use other geophysical parameters (e.g., electrical conductivity) along with the seismic parameters in order to obtain an accurate estimate of the water content in the MTZ.