[S-IT14] Deep Earth Sciences

[SIT14-P20] Adjoint inversion of antipodal PKPab waveforms for P wave velocity anomaly at the base of lower mantle and implications for dlnVs/dlnVp

*Seiji Tsuboi¹, Rhett Butler² (1.JAMSTEC, Center for Earth Information Science and Technology, 2.Univ. Hawaii)

Keywords:Core Mantle Boundary, Adjoint inversion, P wave heterogeneity, Antipode observation

We had analyzed seismic waveforms observed at the antipodes of earthquakes to study the structure of the inner-outer core boundary (Butler and Tsuboi, 2010,2021,2023). Here, we attempt to perform an adjoint inversion by using antipodal PKPab phases to estimate the heterogeneous Vp structure at the base of the lowermost mantle. We have carefully examined antipodal stations with high S/N ratio during the past 30 years and selected 20 source-receiver pairs with the epicentral distances greater than 179.0 degree and the Mw less than 7.0. We have used SPECFEM3D_GLOBE software on the Earth Simulator 4 of JAMSTEC to calculate the synthetic seismograms for a heterogeneous mantle Vp model with the accuracy of about 6.9 second, of which NEX=640 and number of cores is 9600. We have set up the time window to retrieve the PKPab phase from the vertical component of both observed and computed seismograms to calculate the adjoint source to obtain the sensitivity kernels of Vp in the mantle. The computed Vp sensitivity kernel for each event shows the characteristic annulus pattern in the lowermost mantle, which covers a large area of the CMB. Therefore, although the number of source-receiver pairs is not large, the summed sensitivity kernels of the PKPab phase for Vp structure at the base of the mantle may be sufficient to model heterogeneity of the Vp structure at the CMB. We summed each event kernel to set up a sensitivity kernel of Vp in the lowermost mantle and iterated the inversion to estimate a heterogeneous structure in D”. Although we have iterations that dominantly affect both South America and the South Pacific, the summary final model shows features within South of Africa, South Pacific, SE Australia, and Central & South America. Keeping the Vs model fixed, we map the CMB Vp heterogeneity measured by the parameter Rs,p = dlnVs/dlnVp and find qualitative, proximal correspondence with the degree-2 pattern of Large Low Velocity Shear Provinces observed in shear tomographic models: in the Pacific and Africa Rs,p > 2.0, whereas the surrounding edges of the edges of the Pacific show Rs,p < 2.0.