Researchers from the University of Science and Technology of China (USTC) of the Chinese Academy of Sciences and the collaborators proposed a new method, named the SKKKP B focal extended seismic phase technique, and unveiled a series of ultralow velocity zones (ULVZs) in the large high-speed anomaly region of the Earth's core-mantle boundary (CMB) using this technique. The study was published in Nature Geoscience. 

The CMB is crucial for the Earth's magnetic field and rotation. It is known that this boundary harbors complex structures, including the ULVZs, characterized by significantly slowed seismic wave velocities. The origin and structure of these zones are key to unravel and understand some secrets in the related fields, especially the Earth’s science. 

Seismological observations indicated that ULVZs primarily reside within and around large low-velocity provinces (LLVPs), while their presence in high-velocity anomalies remains unclear. To anwser this question, researchers introduced a novel SKKKP B focal extension seismic phase technique to detect ULVZs at the CMB. 

The technique focused on analyzing the SKKKP seismic core phase—a specific pattern of wave propagation and reflection at the CMB. By conducting an in-depth analysis of SKKKP waves, researchers uncovered that the ULVZs are responsible for the unexpectedly large observable distances of these waves. 

Besides, researchers discovered ULVZs not only around the Pacific Ocean and below Africa but also in less explored high-speed anomaly areas such as Central America, Alaska, and Greenland, and Central and Western Asia. 

Further explorations suggested that ULVZ formation might be linked to tectonic plate movement. As a subducting plate descends into the lower mantle, the oceanic crust of the mantle, with a lower melting point, may separate from the underlying plate and sink to the CMB, potentially leading to partial melting and ULVZ creation. 

This study enhances the understanding of the Earth's inner workings, offering us a closer look at our mysterious planet.