1985–1999 gravity field variations across the Asal Rift: insights on vertical movements and mass transfer

Repeated gravity measurements across the Asal Rift zone reveal that the central part of the northern rift might be subject to mass input in addition to vertical movements. Precise relative gravity measurements were made in 1985, 1988 and 1999. Temporal variations in the gravity field over the 11- and 14-yr periods reveal a probable relative gravity increase on the order of 30 μGal (300 nm/s2) in the central zone of the rift (active zone since the Ardoukoba rifting episode) with respect to both sides of the rift. The gravity data are compared with vertical deformation data collected by the French National Geographical Institute (IGN) in 1984 and 2000 along 50 km of the across-axis leveling profile. The comparison shows that vertical movements alone cannot explain the observed gravity variations and that, unlike in many other volcano studies, our gravity and elevation changes cannot be modeled by a single process such as subsidence, inflation or localized mass injection.