Data Sheet 1_Seismic anisotropy reveals stress rotation caused by magmatic-tectonic interactions in the Natron rift.csv

Oldoinyo Lengai volcano is located within the Natron Rift, a young magmatic segment of the East African Rift System, where tectonic extension and magmatic processes are tightly coupled. Understanding how stress, deformation, and magma transport interact in such settings remains a key challenge. Here, we use local shear-wave splitting measurements to image seismic anisotropy and constrain the distribution of stress and deformation across the Natron Rift. We analyze ∼4,400 station–event pairs from the dense SEISVOL seismic network, identifying anisotropy with an average delay time of ∼0.12 s and a dominant fast-axis orientation of ∼21°N, broadly consistent with the regional extensional stress field. However, pronounced lateral variability reveals distinct anisotropic domains across the Natron Basin, the region east of Gelai, and the magmatically active zone in the vicinity of Oldoinyo Lengai. In all areas, anisotropy is dominated by a shallow crustal layer (∼5–6 km), attributed to stress-aligned crack systems, with evidence for additional complexity from anisotropic layering and lateral heterogeneity. In the magmatically active region, we observe significant deviations from the regional pattern, including localized rotations of fast-axis orientation and increased delay times. Depth-dependent analysis indicates an additional anisotropic contribution at mid-crustal depths (∼11–15 km), consistent with a previously inferred magma storage region. Temporal variations in splitting further correlate with periods of elevated volcanic activity, suggesting that the carbonatite and silicic plumbing systems are strongly coupled and the corresponding melt and fluid transport dynamically modifies the stress field and fracture network. We interpret these observations as the expression of a magmatically influenced transfer zone, where stress rotation, shear accommodation, and focused magma ascent interact. Our results demonstrate that seismic anisotropy captures both regional stress and its local modification by magmatic processes, highlighting the key role of magma–tectonic feedbacks in controlling deformation and segmentation in young continental rifts.