Interpretation of 1992–1994 Gravity Changes around Mayon Volcano, Philippines, Using Point Sources

Significant gravity changes observed around the Mayon Volcano (Philippines) between 1992 and 1994 at 26 stations are interpreted in terms of an increase of mass and pressure changes at several point sources modelled using a fast inversion process. This inversion approach attempts to fit gravity and elevation changes by combining a random search for the positions of the sources and a linear least-squares fit for the incremental mass, pressure and possible common regional values for gravity or elevation changes. Some stabilizer terms are included in the misfit function. Models with one and two sources were tested against the observed changes at Mayon. Models with only one-source give a best fit for a shallow source with a positive mass increment, horizontally displaced far from the summit. The study using two sources gives a best fit that is similar to the one-source model, but in addition indicates anomalous behavior at stations in the SW. Neglecting the stations located southward from a local fracture, the bestfitting model suggests one central positive mass change source, which is likely to be an intrusion of about 0.5 MU with a depth of about 5 km beneath the volcano. Standard deviation for the residuals ranges from 7–8 lGal for one-source models to 6–7 lGal for models with two sources. Both of the cases are below the error value of 9.4 lGal estimated for the gravity data, so that it is not possible to discriminate between both possible interpretations without additional information.