Deciphering causes of unrest at explosive collapse calderas. Recent advances and future challenges of joint time-lapse gravimetric and ground deformation studies

Unrest at explosive collapse calderas is the manifestation of complex subsurface processes. Geophysical signals recorded during unrest can be caused by the migration and emplacement of magma, or by tectonic or hydrothermal activity. Geodetic techniques represent a crucial part of a monitoring programme as they provide means to quantify volume changes in the feeder system of restless calderas. However, deformation data alone cannot discriminate between magma and aqueous fluid intrusions. Time-lapse gravity measurements can constrain the mass of the intrusion, and consequently the combination of geodesy and gravity measurements can be used to infer the density of the intrusive fluids and can better constrain the deformation source. Here, we consider the application of gravimetric and geodetic techniques to study caldera unrest with examples from Long Valley, Campi Flegrei, Las Can˜adas and Nisyros. We identify problems with current time-lapse gravimetric techniques, discuss several approaches to model the source of unrest from deformation and gravity data and provide an outlook into future challenges for integrated geodetic studies.