Dipping prism modelling of subduction plates in view of an improved GOCE global Moho: the Tonga example

The study of subduction zones, i.e. the process occurring at convergent boundaries by which one tectonic plate moves under another and sinks into the mantle, is a fundamental topic in many Earth−related sciences. Since usually important density variations occur in the correspondence of subduction zones, a proper modelling of these regions is fundamental when studying the Earth crust from gravity field observations. In the present work, we investigate the possibility to characterize a subduction zone by exploiting GOCE gravity gradients. The main objective of the work is to find a simple way to model subducting plates in view of a global inversion of the gravity field to recover the main features of the Earth crust. In particular, GOCE along−orbit filtered data are firstly reduced for the effects of the bathymetry, upper−sediments, middle−sediments, and lower−sediments. After that, the residual signal is further reduced for the effect of a “regular crust” by means of a Kriging procedure, isolating, in this way, the gravitational effect of the subducting plate. The signal is thus fitted, by means of a simulated annealing (SA) procedure, with the gravitational effect of a dipping prism, characterized by a set of parameters that define the prism position, size, density, and its strike and dipping angles. The methodology has been firstly assessed in a closed−loop experiment to test the performance of the SA algorithm in detecting the parameters used to best fit the isolated gravitational signal of the subduction plate. Then, the Tonga subduction plate has been chosen as a natural laboratory to perform some numerical experiments. The closed−loop simulations have shown the capability of the proposed approach to estimate the parameters with a relative error smaller than 10%, even in the presence of observation noise. As for the Tonga subduction, the estimated model well−fit the observed gravitational signal and its geometric parameters are highly−consistent with the values available in the literature.