Ground deformation at Colima Volcano, Mexico, 1982 to 1991

Abstract

A ground deformation network, now comprising I 0 Km of levelling traverse and six dry tilt stations, has been measured four times since it was first set up at Colima Volcano by the author in 1982. The results show continuous subsidence roughly centred around the summit area, together with a progressive tilt towards the south or southwest. The rate of subsidence averages about 0.4 em per km each year, though higher rates (around 1 em per km per year) were measured between 1982-86, and again in 1990-91, in the build up to the 1991 increase in activity. Simple elastic models filled to the levelling data are (consistcnt with the deflation of a buried source at 2 to 4 km depth, perhaps a draining magma chamber or the top of a magmatic column. The dry tilt results confirm the subsidence and its variable rate, although parallel tilts often occur in the overall downslope direction, suggesting gravitational creep or sliding of the volcano downslope.