RECENT HORIZONTAL CRUSTAL MOVEMENTS IN AND AROUND VOLCANO NYAMURAGIRA, ZAIRE

Two networks for measuring distances established across the rift valley and within the crater of Nyamuragira have been remeasured approximately every 2 years since 1977 to detect the horizontal crustal movements resulting from both the spreading of the rift valley and volcanic activity. Distance measurements were carried out seven times in the crater from 1977 to 1989, but only four times across the rift valley from 1982 to 1988 because of initial benchmark problems. Changes in line length were detected on the crater net for all five flank eruptions which occurred during the monitoring. Total strain accumulated at the crater for the 10 years indicated an inflation of 200 microstrain. Subsequently, during the 1989 eruption, a new fissure opened in the southeastern part of the crater. In contrast, there was general contraction of 7 microstrain across the rift valley during the 6 year period of measurements.