Seismic Signals of Crater Instability at Oldoinyo Lengai Volcano, Tanzania

Oldoinyo Lengai in the North Tanzanian Divergence is one of the few highly active volcanoes of the East African Rift System. Its eruptive cycle is characterized by effusions of carbonatite lava and severe explosions. The most recent of these occurred in 2007 and left a circular crater nearly 400 wide and approximately 100 m deep. The crater is currently being refilled with new lava which has formed several characteristic hornitos. In this study, we analyze data from a small-scale seismic network of nine short-period stations in operation for about five weeks during 2019. The recordings show tremor activity as well as more than 80 distinct events characterized by high-frequency seismic signals apparently located within the immediate vicinity of the network. However, the recordings lack clear S-wave arrivals. We apply two different localization methods and account for the shape of the volcanic edifice during the inversion: (1) a grid-search approach that includes topographic information by using a Bayesian formulation and (2) a linearized iterative approach where the regularization is controlled by the topography. The results show that most of the events are located at high altitude in the vicinity of the circular crater rim. We argue that the events are caused by sliding segments of the crater wall which have become gravitationally unstable, possibly due to magmatic undermining. The interpretation is supported by surface observations of opening cracks at the outer base of the crater rim.