Inversion of temporal gravity changes by the method of local corrections: A case study from Mayon volcano, Philippines

The 3D inversion method based on local corrections has been introduced by Prutkin to invert potential field data. It has been applied to gravity data inversion on local, regional, and global scales. Here we introduce the application of this method to invert temporal gravity changes. The inversion procedure is demonstrated by a case study on gravity changes observed at the Mayon stratovolcano between campaigns spanning a period of 1992–1996. Residual gravity changes are compiled from the observed ones. No significant surface deformation was observed within the accuracy of the campaign GPS measurements during the considered period. Residual gravity changes were first inverted in terms of sources represented by 3D line segments. The line segment approximation facilitates the second step of the iterative non-linear inversion based on local corrections. In the second step, the residual gravity changes were inverted in terms of 3D star-convex homogenous bodies representing sources of subsurface mass/density changes. Published geological evidence indicates a shallow magma system at Mayon. The absence of significant deformation accompanying the gravity changes indicates that this system was nearly open during 1992–1996. We hypothesize that the sources of the gravity signal represent mass transport, namely injection of magma into voids of the shallow system. Inversion results for campaign differences 3–1 and 4–1 are presented. For both epochs the inversion results in a source located at a depth of about 4.4 km (± 0.3 km) below sea level. The mass of injected magma was estimated at 0.35 (± 0.10) MU and 0.12 (± 0.04) MU for the two epochs, respectively. We also compare our results with previous interpretations of the same data.

► A new two step 3D inversion methodology applied to interpreting temporal gravity changes is introduced. ► The methodology is demonstrated on data observed at Mayon volcano in 1992–1996. ► Source of mass increase found at depth of 4.4 km is interpreted as reinjection of magma filling existing voids. ► Methodology yields location, depth, shape, strength, and temporal evolution of sources of temporal gravity changes.