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S-to-P Conversions from Mid-mantle Slow Scatterers in Slab Regions: Observations of Deep/Stagnated Oceanic Crust?

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Abstract

The fate of a subducted slab is a key ingredient in the context of plate tectonics, yet it remains enigmatic especially in terms of its crustal component. In this study, our efforts are devoted to resolve slab-related structures in the mid-mantle below eastern Indonesia, the Izu–Bonin region, and the Peru area by employing seismic array analysing techniques on high-frequency waveform data from F-net in Japan and the Alaska regional network and the USArray in North America. A pronounced arrival after the direct P wave is observed in the recordings of four deep earthquakes (depths greater than 400 km) from three subduction systems including the Philippines, the Izu–Bonin, and the Peru. This later arrival displays a slightly lower slowness compared to the direct P wave and its back-azimuth deviates somewhat from the great-circle direction. We explain it as an S-to-P conversion at a deep scatterer below the sources in the source region. In total, five scatterers are seen at depths ranging from ~930 to 1500 km. Those scatterers appear to be characterised by an ~7 km-thick low-velocity layer compared to the ambient mantle. Combined evidence from published mineral physical analysis suggests that past subducted oceanic crust, possibly fragmented, is most likely responsible for these thin-layer compositional heterogeneities trapped in the mid-mantle beneath the study regions. Our observations give a clue to the potential fate of subducted oceanic crust.

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Acknowledgements

We gratefully acknowledge IRIS and F-net for making waveform data available. Figures were made with GMT (Wessel and Smith 1995) and GNUPLOT. We thank Prof. Wim Spakman (editor), Dr. Sebastian Rost, and three anonymous reviewers for the careful and constructive review comment. This work was supported by the National Science Foundation of China (Grants 91428309, 41761134051) and State Key Laboratory of Marine Geology, Tongji University (No. MGK1705).

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  1. Department of Marine Sciences, Ocean College, Zhejiang University, Zhoushan, 316000, China

    Xiaobo He & Yixian Zheng

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  1. Xiaobo He
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Correspondence to Xiaobo He.

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He, X., Zheng, Y. S-to-P Conversions from Mid-mantle Slow Scatterers in Slab Regions: Observations of Deep/Stagnated Oceanic Crust?. Pure Appl. Geophys. 175, 2045–2055 (2018). https://doi.org/10.1007/s00024-017-1763-z

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