Constraints on absolute chamber volume from geodetic measurements:
Trapdoor faulting in the Galapagos
Magma chamber volume is critical for volcano monitoring and forecasting.
Standard geodetic methods cannot constrain the total volume, only the
change in volume. Here, we show that stress perturbations associated
with trapdoor faulting allow bounds to be placed on the total chamber
volume at Sierra Negra volcano, in the Galapagos. The deformation
response of the magma chamber to faulting depends on both the absolute
chamber volume and the compressibility of the magma. Bubble-free magma
provides the lower limit on compressibility, thus an upper bound on the
chamber volume of 13.6 to 20.6 km3, depending on fault
dip. We estimate an upper limit on compressibility using a conduit model
relating volatile content to lava fountain height, which is compared
with observations from the 2005 eruption, constrained by volatile
content of olivine melt inclusions. This yields a lower bound on chamber
volume of 0.5 times the upper bound.