Han Guangjie et al. - EPSL: Study on the structure of the low-velocity layer overlying the 410-km discontinuity. The structure of the low-velocity layer located above the mantle transition zone is an important discovery in the field of geophysics in the past 20 years.
The detection of the low-velocity layer structure overlying the 410-km discontinuous surface (the spatial distribution, thickness changes, low-velocity anomaly size, etc.) of the low-velocity layer and the study of its formation mechanism are hot issues in the study of the structure, physical properties and dynamics of the deep mantle.
It is of great significance to understand issues such as mantle convection patterns, material migration within the Earth, melt distribution, and the fate of subducted plates in the deep Earth.
The northwest Pacific subduction zone is the most typical subduction area in the world, where the deep subduction of slabs has diverse forms and complex structures.
Previous studies have carried out some research on the structure of the low-velocity layer overlying the 410-km discontinuous surface in this area. The distribution of the detected low-velocity layer is relatively discrete, and there is a lack of research on the distribution of the low-velocity layer under the continent where there are few sea areas and stations.
Dr. Han Guangjie and researcher Li Juan from the Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, Chinese Academy of Sciences, used the triple seismic phase waveform fitting method to study the velocity structure near the 410-km discontinuity in the northwest Pacific subduction area.
The low-velocity layer structure distributed above the mantle transition zone was successfully detected.
The relative arrival times and amplitude differences of the three seismic phases, including the location of the wave train characteristic points arranged according to the epicentral distance, can be used to effectively constrain the velocity structure near the discontinuity.
The researchers mainly used the National Digital Seismic Network data, combined with the NECsaids mobile array observation data, to analyze the P-wave and S-wave waveforms of five medium-depth earthquake areas that occurred in the Sea of ??Japan region (Figure 1).
Due to the coupling relationship between the size of the velocity anomaly and the thickness of the low-velocity layer, researchers combined waveform fitting and grid search methods to find the optimal solution within a certain range of parameter space.
Considering the two-dimensional effect of the subducting slab, the researchers also discussed the differences between the 1D and 2D velocity models. Within a certain azimuth angle range, the 1D model can perform better even in subduction zones with complex structures.
It reflects the structural characteristics of the 410-km overlying low-velocity layer.
Figure 1 The seismic stations used for the study consist of the National Seismic Network (black inverted triangle) and the NECsaids mobile seismic array (pink triangle).
The red source balls represent the five seismic events used for research. The scattered points in the figure are the study area. The inset in the lower right corner shows the P-wave tomography results in the area (Y oshio and Masayuki, 2013, JGR). The regional optimal velocity model clearly shows
, there is a 55-80 km thick low-velocity layer structure above the 410-km discontinuity in Northeast China and the northwest Sea of ??Japan. The P-wave anomaly value is -1.5%, the S-wave anomaly value is -2.5%, and the east-west spread is nearly 900
km.
This study has good coverage of North Korea and the Sea of ??Japan region, which have been less studied by previous researchers (Figure 2).
High-temperature and high-pressure experiments on minerals show that as depth increases (pressure increases), the solid-liquid dihedral angle gradually decreases, and the better the connectivity between partial melts, the more obvious the impact on seismic wave velocity.
The seismic regional tomography model shows that the Pacific slab is stagnant in the mantle transition zone, and will undergo hydration and heat exchange with the surrounding mantle minerals, forming an unstable upwelling. When it passes through the 410-km discontinuity, it will be formed by Watts
Sharp stone changes into olivine.
Due to the significant difference in the water storage capacity of the upper and lower mantle minerals, the olivine after phase change is likely to be in a saturated or supersaturated state, inducing partial melting and forming the detected low-velocity layer structure.
Because the melt fraction is small, the low-velocity layer can maintain a large thickness without being compressed, has a density similar to that of the surrounding mantle minerals, and can exist stably above the 410-km discontinuity (Fig. 3).
This study provides observational evidence for the interconnection between slab subduction and mantle material recycling.
If the dehydration mechanism associated with the subducting slab is correct, this low-velocity layer structure is likely to continue to the west.
Figure 2 Comparison between the detected low-velocity layer distribution position and previous research results Figure 3 Schematic diagram of the formation mechanism of the overlying low-velocity layer structure on the 410-km discontinuous surface.
In the picture on the left, the orange area above the mantle transition zone indicates the location of the low-velocity layer clearly detected in this study, and the light pink area to the left indicates the possible westward extension of the low-velocity layer.
The image to the right shows the tiny structures between mineral particles.
When the solid-liquid dihedral angle is small, the melt can wrap around the boundaries of mineral particles and connect with each other, significantly reducing the seismic wave velocity (Yoshino et al., 2007, EPSL). The research results were published in the authoritative international academic journal EPSL.