Multi-Scale Seismic Tomography in Rift Zones: Implications for Lithospheric Evolution in the Afar Region

Abstract

The Afar Depression, a nascent oceanic basin within the East African Rift system, offers a unique natural laboratory to investigate the interplay between mantle dynamics and continental breakup. This study employs seismic travel-time tomography to elucidate the interplay between deep mantle dynamics and surface tectonics beneath the Afar region, encompassing northern Ethiopia, Eritrea, Djibouti, northwestern Somalia, and southern Yemen. Utilising 17,476 first-arrival P-waves from 2,911 earthquakes recorded at 114 seismic stations between 1976 and 2023, a 3D P-wave velocity model is developed to a depth of 70 km. The analysis incorporated three different grid resolutions (0.25°, 0.5°, and 1.0°) to optimise the balance between detailed resolution and robust structural imaging. Checkerboard and structural reconstruction tests confirm good resolution across most of the study area. The resulting tomographic images reveal significant velocity heterogeneity throughout the region, with prominent Low-Velocity Zones (LVZs) beneath tectonically active areas including the Afar Depression, Red Sea Rift, Gulf of Aden Rift, and Main Ethiopian Rift. These LVZs extend from shallow depths to 70 km, suggesting the presence of thermally perturbed lithosphere, partial melting, and active magmatism. Conversely, High-Velocity Zones (HVZs) predominate beneath the more stable Nubian, Arabian, and Somalian plates, indicating cooler and thicker lithosphere. The spatial distribution of earthquakes along boundaries between velocity anomalies further supports the relationship between seismic activity and tectonic stress at interfaces between hot, weak mantle material and cooler, stronger lithosphere. Our findings contribute to the understanding of how mantle dynamics influence continental breakup and incipient seafloor spreading in plume-assisted rift systems, supporting models of passive upwelling beneath magmatic segments driven by far-field stresses and buoyancy-related processes.