New Global P-wave Tomographic Model of the Earth's Mantle

We have inverted a very large combination of delay times for P, pP, PcP, PKPab, PKPbc from the ISC-EHB catalog, augmented with hand-picked OBS and MERMAID onset data and cross-correlation delays from broadband stations to derive a new global model, UNICA25, of P-wave velocity variations in the mantle. This paper presents the model, summarizes the data processing and inversion, and shows resolution tests at different scales. After rejecting 109,917 outliers (1% of the total), the data set comprises 10,571,152 arrival times. The misfit of the observed delays, with respect to a background model consisting of the crust of LITHO1.0 on top of model AK135, is on average 2.14 standard errors (σ), which is reduced to 0.99σ by inverting for P-wave velocity variations using both ray theory and finite frequency theory, and 5000 iterations of the iterative solver LSQR. Extensive resolution tests show a resolution at 500 km scale or better in most of the lower mantle. An upper mantle resolution of the order of 300 km is obtained under densely instrumented continents, but upper mantle resolution remains deficient under much of the oceans except where covered by MERMAIDs. Linearized tomography is at least three orders of magnitude faster than recent full waveform inversions (FWI), and reaches much higher frequencies for cross-correlated delays. Model UNICA25 has a P-velocity resolution that rivals that of FWI and shows a superior performance in fitting delay times, notably the ISC-EHB P-delays.