17:15 〜 19:15
[SIT20-P01] Inversion of antipodal PKPab waves by the adjoint method to reveal the locations of the uplifting mantle plume at the base of the mantle
キーワード：Core mantle Boundary、Antipodal observation、PKPab、Adjoint Method
The base of the Earth's lower mantle is characterized by large seismic velocity anomalies, known as large low velocity provinces (LLVPs). There are several hypotheses related to the origin of LLVPs, such as remnants of Earth's early differentiation and buried relics of proto-Earth's mantle after the Moon-forming giant impact. However, the geodynamical implications, such as the role of LLVPs as driving mechanisms of plumes or subducted slabs, are not well resolved because most observations of the polarization of seismic velocity at LLVPs are limited to the azimuthal anisotropy of shear wave splitting. Here, we combine new observations of antipodal PKPab seismic waves with the adjoint method to perform an inversion of the radially anisotropic Vp structure at the base of the lower mantle. We have carefully examined antipodal stations with high S/N ratios for both the vertical and horizontal components over the past 30 years and selected 23 source-receiver pairs with epicentral distances greater than 178.0 degrees and Mw values less than 7.0. We calculate synthetic seismograms with an accuracy of 6.9 s and perform an inversion of the radially anisotropic Vp structure at the base of the lower mantle by the adjoint method. The results of our inversion show that vertically polarized Vp is dominant within the LLVPs of the Pacific and African regions. These features are characterized by relatively small spots of high vertically polarized Vp anomalies, which may be interpreted as the locations of ascending mantle plumes inside LLVPs in the Pacific region. Our results do not agree with previous observations, which show that plumes are found at the edges of LLVPs and suggest that the plumes originate from impingement by downgoing slabs at the base of the mantle. Instead, our results suggest that LLVPs are not thermally stable for hundreds of million years.