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Scientists Find Lateral Variations of Shear-wave Velocity in D'' Layer beneath Indian-Eurasian Plate Collision Zone

Apr 01, 2020

The core-mantle boundary (CMB) near 2900 km depth is the most heterogeneous layer in the Earth with rapid variations in temperature and chemical composition. It is also the origin of mantle convective upwelling magma and final destination of subduction plate fragments.
The formation of Tibetan Plateau has experienced a long history of tectonic evolution. Previous studies indicate that the seismic wave speed is relatively high in the lowermost mantle right above the CMB (i.e., the D'' layer) beneath East Asia.
Recently, researchers led by Prof. BAI Ling from the Institute of Tibetan Plateau Research (ITP) of the Chinese Academy of Sciences conducted a study to image the small-scale structure of the D'' layer beneath the India-Eurasian plate collision zone. Their findings were published in Geophysical Research Letters.
The study is mainly based on the waveform data of eight earthquakes in the Indian Ocean and the cross-correlation method with respect to the 3D theoretical waveforms of the CMB reflections.
The researchers obtained approximately 1000 high-quality ScS-S travel time residuals δt3D. These time differences showed a lateral change of up to 10s, indicating that within a horizontal range of about 300 km at the D'' layer, the shear wave velocity has a lateral change of up to 3-7%, which is significantly larger than that obtained by previous seismic tomographic image with changes within 0-1.5%.
This study supports the previous works in seismic tomography, geochemistry and laboratory experiment that the dominated high velocity anomalies beneath the Tibetan areas are associated with the subduction of the Neo-Tethyan and Pacific plates and the Paleo-Tethyan and Paleo-Pacific plates.
The results further showed that there are small-scale low-velocity anomalies in the southeast and northwest parts of the study area, indicating that the formation of the CMB is affected not only by the subduction of oceanic plates, but also the interactions of the outer core and the lowermost mantle.
The study revealed a shear-wave velocity variation of up to 7% over a distance about 300 km at the D'' layer in the continental collision zone, which provides new constraints on the understanding of the temperature and chemical structure of the CMB area as well as the revolution history of the Tibetan Plateau and its surrounding areas.
Contact

LIU Xiaoqian

Institute of Tibetan Plateau Research

E-mail:

Lateral Variations of Shear‐Wave Velocity in the D″ Layer Beneath the Indian‐Eurasian Plate Collision Zone

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