Cenozoic volcanoes are widely distributed in the northeast region of China, among which the Changbaishan volcanic field (CVF) is the largest one. The CVF is a typical intraplate volcano. There is another geological feature in northeast China, a deep earthquake cluster about 300 km east of the CVF. 

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

In a study published in Nature Communications, a team led by Prof. ZHANG Haijiang from the University of Science and Technology of China of the Chinese Academy of Sciences, and Dr. Robert Myhill from the University of Bristol, utilized tele-seismic double-difference tomography technology to uncover the morphological changes of the Pacific subducting slab in the mantle transition zone beneath Northeast China, and revealed its controlling effects on CVF volcanism and deep earthquakes.

The researchers utilized seismic arrival data from the Northeast Asia region collected by global seismic stations, and constructed a high-resolution three-dimensional velocity model of over 1000 kilometers in depth using multi-scale double-difference tomography.

They found 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 coincided 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 provided 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 subducted locally was the key mechanism for the occurrence of deep earthquakes.

The findings of this study provide a new perspective on the physical mechanism of deep-source earthquakes, and for the first time unify 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.