Abstract
The Western Branch of the East African Rift Valleys has two active volcanoes Nyiragongo and Nyamulagira, which erupted in 1977 and 1980 for the last five years. A base line network consisting of 18 benchmarks was set up in 1977 in and around the volcanoes to reveal the horizontal movements across the rift valley near the volcanoes and also those in the volcanic area. The measurements were carried out by Geodimeter 6-BL in 1977 and 1979. The maximum compressional strain observed at the southeastern flank of Nyiragongo was 5×10-5 with a NNW-SSE direction, which agrees well with nearly linear, N15°W trending surface fissures accompanying the 1977 Nyiragongo eruption. The maximum compressional strain at the Nyamulagira crater was 1.5×10-5 with a ENE-WSW direction, which is consistent with the NE-SW direction of the new fissure which resulted in the 1980 Nyamulagira flank eruption. These two short-term strain patterns can be explained by a hypothesis that there exist two domains with different, long-term stress systems in this volcanic area. They have the stress axes of the maximum horizontal compression in the NE-SW and NNW-SSE directions. The former stress is probably in control of a preferred alignment of monogenetic volcanoes at the flank of Nyamulagira. The latter may most probably be responsible for the volcanic activities along the linear zone across Nyiragongo and Nyamulagira. The measurements of horizontal distance across the rift valley could not give any fruitfull results because 8 benchmarks were destroyed by some persons during the two years. Tilt measurements at three sites in the volcanic area were also performed with the automatic level equipped with a micrometer. Each site set up in September, 1979 has four orthogonal leveling lines of about 100m length. It is found that the tiltings at the western and eastern sites of the rifting axis were 3μ radians down to a NNE direction and 10μ radians down to a nearly W direction, respectively, over the six months including the 1980 Nyamulagira flank eruption.
