Wavefield distortion imaging of Earth's deep mantle

The seismic wavefield, as recorded at the surface, carries information about the seismic source and Earth's structure along the seismic path, essential for the understanding of the interior of our planet. For 40 years seismic tomography studies have resolved the 3D seismic velocity structure in growing detail using seismic traveltimes and waveforms. These studies have been driving our understanding of the dynamics and evolution of the planet, but are limited in their spatial resolution to imaging scales of a few 100 s to 1000 km due to the constraints of the tomographic inversion. Detailed studies of seismic waveforms can resolve finer scale structure but are often reliant on serendipitous source-receiver combinations and provide very uneven coverage of the planet. Therefore, we often lack an understanding of the fine scale structure of the Earth that is important to understand structures and processes such as mantle plumes or details of slab recycling. Here we show evidence that we can exploit slowness vector deviations of the seismic wavefield to extend our knowledge of Earth structure to smaller scales using large datasets. Analysing seismic array data, we show strong and measurable focussing and defocussing effects of the teleseismic P and Pdiff wavefield sampling the deep Earth. We compare the P-wave results to additional S and Sdiff data and find good agreement between both wavetypes. We can link the wavefield deviations to strong velocity variations assuming sharp boundaries are sampled along the path in the deep mantle. The dataset samples the Pacific and Gulf of Mexico well and shows strong horizontal incidence (backazimuth) deviations in the Pacific (up to 14 & DEG; westwards) and beneath the Gulf of Mexico (up to 5 to 8 & DEG; eastand west-ward). The backazimuth deviations are also reflected in slowness deviations in the range of & PLUSMN; 0.8 s/& DEG; relating to velocity variations in the range of & PLUSMN; 9 km/s. Using 3D raytracing we are able to forward model the detected backazimuth variations of the P and Pdiff dataset. The high frequencies of the P-waves, density of the ray-paths, and low computational cost of our forward calculation allow us to construct a higher resolution and more detailed model of velocity anomalies under Hawaii than was possible with previous methods. The best-fitting velocity model for the Pacific contains two lowvelocity regions located at N25 & DEG;/W155 & DEG; and N25 & DEG;/W165 & DEG; beneath the tip of the Hawaii Emperor chain. The Pacific anomalies have diameters (D) of 6 & DEG; and 2 & DEG; with velocity reductions (dVP ) of 8% and 4% with heights (H) above the CMB of 70 km and at least 200 km, respectively. We also detect a fast region of 3% velocity increase in the North Pacific rising at least 300 km above the CMB with a diameter of 12 & DEG; at N60 & DEG;/W175 & DEG;. Beneath the Gulf of Mexico we find ambiguous results with either a slow region (N25 & DEG;/W85 & DEG;, H = 200 km, dVP =-3%, D = 2 & DEG;) or a fast region (N15 & DEG;/W75 & DEG;, H = 200 km, dVP = 3%, D = 4 & DEG;) able to explain the data. We thus show that the directivity information of the seismic wavefield - largely underexploited - can be used to resolve the fine scale velocity structure of the Earth's interior with great accuracy and can deliver additional insight into deep Earth dynamics. & COPY; 2023 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons .org /licenses /by /4 .0/).

Automated summary

The seismic wavefield carries important information about the seismic source and Earth's structure, but current methods have limitations in resolving fine scale structure. In this study, we show that analyzing slowness vector deviations can help extend our knowledge to smaller scales. By analyzing seismic array data, we were able to detect strong focal and defocal effects of the teleseismic wavefield, providing more insight into the fine scale velocity structure of Earth's interior.