The School of Earth and Space Sciences of the University of Science and Technology of China, the Institute of Precision Measurement Science and Technology Innovation of the Chinese Academy of Sciences and international peers have developed a new method of SKKKP B focal extension seismic phase to detect the ultra-low speed region of the core mantle boundary, revealing that there are small and medium-sized ultra-low speed regions in the large-scale high-speed anomaly region of the global core mantle boundary, providing key information for the study of the formation mechanism of the ultra-low speed region. On March 4th, the related research results were published online in Nature Geoscience under the title of Detections of ultra-low velocity zones in high velocity lower mantle linked to induced slabs.

As one of the most important boundaries within the Earth's interior, the core mantle boundary layer connects the liquid outer core with the solid mantle, making it one of the most complex regions in terms of physical structure and dynamic processes. It is of great significance for the study of the evolution of the Earth's magnetic field, changes in Earth's rotation, and the Earth's reference framework. In the past few decades, seismological observations have shown the existence of multi-scale non-uniform structures above the nuclear mantle boundary, including large low-speed provinces, small and medium-sized ultra-low speed zones, and smaller scattering bodies. As the most extreme anomalous structure with physical properties above the nuclear mantle boundary, the shear wave velocity in the ultra-low velocity zone can decrease by up to 50%, the longitudinal wave velocity can decrease by up to 25%, and the thickness variation range is from a few kilometers to tens of kilometers, while the lateral scale variation range is between tens of kilometers to thousands of kilometers. The structure and formation mechanism of ultra-low velocity zones are important aspects of fundamental research on mantle mineral composition, nuclear mantle material, energy, and angular momentum exchange.

The spatial distribution characteristics of the ultra-low speed zone can reveal its formation mechanism. Previous seismological observations have shown that most of the ultra-low velocity areas are distributed within and at the edges of large low-speed provinces, but there is still doubt as to whether there are widespread ultra-low velocity areas in the high-speed anomaly areas of the core mantle boundary. This work focuses on this scientific issue and develops a new method for SKKKP B focal extension seismic phase detection of the ultra low velocity zone at the core mantle boundary, expanding the sampling range of the high-speed anomaly area at the core mantle boundary.

The downward propagating shear waves triggered by natural earthquakes pass through the core mantle boundary and are converted into liquid outer core longitudinal waves. They undergo two reflections on the inner side of the core mantle boundary and then propagate in the form of mantle longitudinal waves after leaving the outer core. Finally, they are recorded by surface stations to form the SKKKP seismic phase. Based on the standard Earth reference model, the observable epicenter distance of SKKKP waves calculated by ray theory should be less than 67 degrees (i.e. less than the epicenter distance of focal point B). However, actual measurement data shows that the observable epicenter distance of SKKKP waves can extend to 90 degrees. The team collected waveform data from 143 global seismic events with focal depths greater than 300 kilometers and body wave magnitudes greater than 6 recorded by seismic arrays in North America, Europe, and China from 2000 to 2020. They conducted a multi index characteristic analysis of arrival time, amplitude, polarization, slowness, frequency, and selected over 7900 SKKKP wave event station pairs. The study synthesized seismic maps through a series of theoretical calculations and compared them with observed data. It was found that the ultra-low velocity zone structure above the core mantle boundary is the cause of the SKKKP B focal extension seismic phase.

Based on the above methods, this study discovered ultra low velocity zones around large low-speed provinces in the Pacific and Africa, and detected ultra low velocity zones within previously understudied high-speed anomaly areas of the core mantle boundary, such as Central America, central and western Asia, Alaska, and Greenland. This study, combined with existing geodynamic simulation results, reveals the formation mechanism of ultra-low velocity zones within the high-speed anomaly area of the core mantle boundary. That is, when a subducting plate enters the lower mantle, its top oceanic crust can gradually separate from the underlying plate and sink to the core mantle boundary. At the same time, the melting point of the oceanic crust material is lower than that of the surrounding mantle rocks, so partial melting may occur to form ultra-low velocity zones.

The research work was supported by the National Natural Science Foundation of China and the Chinese Academy of Sciences strategic leading science and technology project.

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