The influence of mechanical stiffness on caldera deformation and implications for the 1971–1984 Rabaul uplift (Papua New Guinea)

Abstract

Numerical models provide a link between measured ground deformation and the inaccessible deformation source, and here we present a systematic set of new results from numerical forward modelling using a Finite Element Method with application to volcano geodesy. We first provide a generic case analysis and then evaluate ground deformation data from the Rabaul caldera in Papua New Guinea. The generic case simulates surface displacements in a flat-topped caldera setting due to pressure changes in a shallow (at 5 km depth) oblate reservoir overlain by host rock with variable mechanical stiffness. Our main findings are: i) the amplitude and wavelength of resultant ground deformation are dependent on the distribution of mechanically stiff and soft lithologies and their relative distribution above the reservoir, ii) for a given pressure change, surface displacement may be amplified by the presence of soft layers compared to generic simulations using a homogeneous background medium, and iii) the ratio of maximum horizontal over maximum vertical deformation ( u xx max/ u yy max) is particularly sensitive to the presence of rock heterogeneities. In assessing the influence of mechanical heterogeneities (as derived from seismic data) in caldera-fill successions on ground deformation at Rabaul we apply our model to inform on the source causing uplift between 1971 and 1984. The best-fit model involves a combination of two oblate sources at 3 and 1 km depth, respectively, beneath the centre of the caldera undergoing a reasonable pressure increment (∼ 38 MPa), compared to unrealistic pressurisation if modelled using a homogeneous background medium.