Illuminating eruptions, fluid pathways and melt storage with seismo-(acoustic) tremor at Oldoinyo Lengai volcano, Tanzania

Oldoinyo Lengai volcano is a strato-volcano in the eastern part of the East African Rift system and as such, a curious end member of a young magmatic segment: it is the only volcano worldwide that currently erupts carbonatitic lava. Known to alternate between large explosive (ash) and smaller effusive eruptions, we analyze volcanic tremor from Oldoinyo Lengai during a renewed phase of eruptive but non-explosive activity beginning in late 2018,  which we recorded with a seismo-acoustic network between March 2019 - June 2020 with the SEISVOL (Seismic and Infrasound Networks to Study the Volcano Oldoinyo Lengai) project. We focus on two different aspects of our data set, which are the very first observations of seismo-(acoustic) tremor at this peculiar volcano. First, we analyze one year of data at a co-located seismic and infrasound station about 200 m below the summit together with satellite InfraRed thermal data and reconstruct different phases of volcanic activity (varying styles of extrusive activity, in particular spattering, degassing, activity from a lava pond, intrusive activity, and the construction of hornitos) and the evolution of crater morphology. We characterize the near-constant but highly variable tremor by analyzing its seismic amplitude, duration, recurrence, dominant seismic frequency and harmonics. Frequency gliding occurs frequently and over short (minutes to hours) to long time scales (hours to days). Seismic and acoustic wavefields correlate well for stronger eruptive sequences but are only partially coherent which suggests that high-frequency seismic tremor (up to 25 Hz) may be caused by the low viscosity of the carbonatitic melt. Second, we focus on a selected number of seismic network stations to locate stronger tremors using the network-covariance matrix and a raytracing approach. While many tremors locate in the shallowest part of the edifice of Oldoinyo Lengai, supporting our previous analysis that much of the tremor is caused by eruptions, tremors are also located in the crust to depths of ~15 km. Most importantly, strong (gliding/harmonic) tremor seems to be connected to a migration of tremor depths which mostly locate in rock volumes not associated with seismicity. This suggests we may be able to use tremor to study melt and fluid pathways in the mushy part of a volcanic plumbing system. Overall, our study provides important insights into the eruption dynamics, as well as melt transport and storage of this peculiar volcano.