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 in Africa. 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 filled with new lava which solidifies and has formed several characteristic hornitos. In 2019, we set up a temporary seismic network of 10 short-period stations, equipped with 4.5 Hz geophones, surrounding the crater area at altitudes between about 1990 and 2885 m to monitor the eruptive activity of the volcano. Seven of the stations were recovered in February 2020. The retrieval of the remaining stations was delayed due travel restrictions caused by the pandemic. However, in Sept. 2021, two of the missing stations were returned from the volcano. Due to the limited battery capacity, recordings were restricted to a period of about five weeks between 14/09 and 23/10/2019. Analysis of the data shows tremor activity and more than 80 distinct recordings of high-frequency seismic signals in the immediate vicinity of the network. However, the recordings lack clear S-wave arrivals, and the station configuration is unfavorable for the application of classical localization techniques based on iterative inversion. We, therefore, apply a grid-search approach based on a Bayesian formulation which also accounts for the topography and shape of the volcanic edifice. The results show that the events are located within or close to 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.