Identification of subsiding areas undergoing significant magmatic carbon dioxide degassing, along the northern shore of Lake Kivu, East African Rift

We processed InSAR time series of ENVISAT ASAR descending and ascending datasets and identified two new deforming areas on the northern shore of Lake Kivu, characterized by steady ground subsidence of up to ~1 cm/year over a time period of about seven years (December 2002/January 2003-March 2010). Two distinct areas can be identified: one centered on the Rumoka volcanic cone, which built up during the 1912 eruption of Nyamulagira volcano, and a broader one centered on the Bulengo area on the northern shoreline of Lake Kivu. Both areas include high density of diffuse magmatic degassing areas (“mazuku”), which are topographic depressions in which substantial amount of carbon dioxide accumulates. The steady deflation of at least two fluid reservoirs is consistent with geodetic data but the presence of weak, porous layers, possibly connected with hydrothermal and hydrogeological processes in aquifers and Lake Kivu, could also account for the observed subsidence. The subsidence could be due to pore pressure decrease in porous layers, which may also serve as preferential pathways for escaping gases percolating from the aquifers and Lake Kivu.