Detection of the structure near the 410 km and 660 km discontinuities in Japan subduction zone from the waveform triplication

Abstract:

Slab subduction plays an important role in the mantle material circulation [Stern, 2002], and can also affect the feature of the 410 km and 660 km seismic discontinuities (410 and 660) [Lebedev et al., 2002]. Japan subduction zone is a natural laboratory for studying the mantle composition and velocity structure associated with the deep subduction of the Pacific plate. In this study, triplicated waveforms of an intermediate-depth earthquake at the Hokkaido of Japan (2011/10/21, 08:02:37.62, 142.5315°E, 43.8729°N, M_b6.0, relocated depth: 188 km) are retrieved from the dense Chinese Digital Seismic Network (CDSN). P and S waveforms are filtered with the band of 0.05-1.0 Hz and 0.02-0.5 Hz, respectively, and then integrated into the displacement data. The relative traveltime and synthetic waveform fitting is applied to mapping the deep structure. The best fitting models are obtained through the trial and error tests. We find a 15 km uplift of the 410 and a 25 km depression of the 660, indicating the cold environment caused by the subduction slab; both the 410 and 660 show the sharp discontinuity, but a smaller velocity contrast than the IASP91 model [Kennett and Engdahl, 1991]. Atop the 410 and 660, there are high-velocity layers associated with the subduction (or stagnant) slab. We also find a low-velocity anomaly with the thickness of ~65 km below the 660, which may relate to the slab dehydration or the hot upwelling at the top of the lower mantle. The seismic velocity ratio (V_P/V_S) shows a lower zone at the depth of ~210-395 km, showing the consistency with the low Poisson’s ratio signature of the oceanic plate; a higher zone at the depth of ~560-685 km, implying the hydrous mantle transition zone.