Using high-dense regional body waves for three deep earthquakes that occurred around Russia-China border, we investigate both S and P wave velocity structures in the mantle transition zone beneath Northeast China and northern part of North China Craton, where the northwestern Pacific plate is imaged to subhorizontally lie above the 660-km discontinuity. We observe an increasing trend of S-P travel time residuals along the epicentral distance within a distance range of 11-16.5°, indicating a velocity anomaly in MTZ. We seek the simplest model that explains the observed broadband waveforms and relative travel times of triplication for a confined azimuth sector. Both SH and P data suggest a ~140±20km high velocity layer lying above a slightly depressed and broad 660-km discontinuity. Shear velocity reduction of ~2.5%zone is required to compensate for the significantly large relative time between AB and CD triplicate branches and the increased trending of S-P travel time residuals as well. The MTZ, as a whole, is featured by low shear velocity and high Vp/Vs ratio. A water-rich mantle transition zone with 0.2-0.4wt% of H2O may account for the discrepancy between the observed Vp and Vs velocity structures. Our result supports the scenario of a viscosity-dominated stagnant slab with an increased thickness of ~140km, which was caused by the large viscosity contrast between the lower and upper mantles. The addition of water and eastward trench retreat might facilitate stagnation of the subducting Pacific slab beneath Northeast China.
Bibliographical noteFunding Information:
This research is supported by NSFC (J. Li, Grant 41074034 ). We thank Risheng Chu and an anonymous referee for constructive and valuable comments, which improved the manuscript greatly. Thanks to Dr. Shengqiang Li of China Earthquake Administration for providing the focal mechanism solution for event 20081022 with Mw 4.7. Lingling Ye has joined the discussion and contributed to earlier work. Thanks to Prof. Fenglin Niu's discussion on travel time analysis and seismic structure beneath northeast China. We thank the Data Management Centre of China National Seismic Network at Institute of Geophysics, China Earthquake Administration and IRIS DMS, for providing seismic data. Sac2000 and GMT are used in basic data processing and plotting of some figures.
- Mantle transition zone structure
- Northwestern Pacific subduction zone
- Seismic wave speed
- Stagnant slab
- Water bearing mantle transition zone
- Waveform modeling