The complex plumbing system of Oldoinyo Lengai seen by 3D attenuation tomography

Oldoinyo Lengai volcano, located in the Natron Basin (Tanzania), is the only active natrocarbonatite volcano worldwide. It thus represents an essential end-member magmatic system in a young rift segment (~3 Ma) of the East African Rift System. Following a period of relative quiescence after the 2007-08 explosive eruption and dike intrusions beneath the volcano itself and neighbouring inactive shield volcano Gelai, seismicity and effusive lava flows within the crater show a heightened level of activity since 2019. Employing data from a recent seismic experiment, Reiss et al. 2021 used seismicity and focal mechanisms patterns to map the complex volcanic plumbing system and its impact on rift processes.

Here, we use the recorded waveforms of local earthquakes to employ the newly developed 3D multi-scale reasonable attenuation tomography (MuRAT) to constrain the complex volcanic plumbing system in unprecedented detail. Our attenuation analysis measures peak delay and coda wave attenuation to separately measure seismic scattering, attenuation and absorption and model those parameters in 3D. Compared to a classical travel time tomography, this allows us to map seismic interfaces such as faults, fluid reservoirs and melt batches. We use over 20 000 waveforms and perform a separate inversion for coda wave attenuation and a regionalisation for peak delay measurements in different frequencies, which are sensitive to different structures and depths.

While the lower frequencies are sensitive to larger-scale features and structures close to the surface, the higher frequencies provide better resolution on smaller features and structures at depth. Accordingly, we map different aspects of the complex 3D plumbing system of Oldoinyo Lengai and the rift itself in different frequencies. Our results show strong scattering and attenuation near fluid-filled, deep-reaching faults, producing seismic swarms. We also detect the existence of previously unknown, small magma reservoirs in the shallowest part of the crust that might have fed previous dike intrusions and clearly shows an interconnected plumbing system stretching from the border fault across a developing magmatic rift segment.