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A research team utilized teleseismic double-difference tomography technology to uncover the morphological changes of the Pacific subducting slab in the mantle transition zone beneath Northeast China.

In addition, they also reported its controlling effects on Changbaishan volcanic field (CVF) volcanism and deep earthquakes. The study is published in .

Cenozoic volcanoes are widely distributed in the northeast region of China, among which the CVF is the largest one. The CVF is a typical intraplate volcano. Currently, the formation mechanism of the CVF is still controversial. In addition, there is another remarkable geological feature in northeast China, a deep earthquake cluster about 300 km east of the CVF.

It is usually thought that the formation mechanisms for deep earthquakes include dehydration, embrittlement, adiabatic shear instability and so on. These mechanisms are mainly controlled by temperature conditions, but the thermochemical structure of the subduction zone along the strike remains basically unchanged in Northeast Asia. Hence, it is also unclear why the deep earthquakes concentrate in CVF.

The researchers, led by Prof. Zhang Haijiang from the University of Science and Technology of China (USTC), in collaboration with Dr. Robert Myhill from the University of Bristol, utilized seismic arrival data from the Northeast Asia region received by global seismic stations, and constructed a high-resolution three-dimensional velocity model of over 1,000 kilometers in depth using multi-scale double-difference tomography.

The results indicated that beneath the CVF, there was a phenomenon of a subducting slab locally inserting into the lower mantle, while its northern and southern sides were stagnant in the mantle transition zone.

The regions where the subducting slabs descend coincide with the local depressions of the 660-kilometer discontinuity interface in the mantle transition zone obtained from receiver function imaging.

The morphology changes of the subducting slabs from lying flat to descending and then to lying flat again from north to south within the mantle transition zone provides space for the upwelling of hot material beneath the slab, thus providing a deep mantle source for the volcanic activity of CVF.

In addition, the deep earthquake clusters concentrated along the curved part of the Pacific plate where it subducted through the 660, suggesting that the localized strong deformation produced by the slab when it subducts locally was the key mechanism for the occurrence of deep earthquakes.

This discovery provides a new perspective on the physical mechanism of deep-source earthquakes, and for the first time unifies the CVF volcanism with deep-source earthquakes, attributing them to the deep dynamical processes caused by the partial subduction of the Pacific plate through the 660-kilometer interface.