Dipping prism modelling of subduction plates in view of an improved GOCE Global Moho: The Tonga example

Abstract

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 correspon− dence 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 ob− jective 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−sedi− ments, 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 sim− ulated 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 parame− ters used to best fit the isolated gravitational signal of the subduction plate. Then, the Tonga subduction plate has been chosen as a natural lab− oratory 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.