A CONTINUOUS PLATE TECTONIC MODEL USING GEOPHYSICAL-DATA TO ESTIMATE PLATE-MARGIN WIDTHS, WITH A SEISMICITY-BASED EXAMPLE

A continuous kinematic model of present-day plate motions is developed which (1) provides more realistic models of plate shapes than employe d in the original work of Bercovici & Wessel (1994), and (2) provides a means whereby geophysical data on intraplate deformation is used to estimate plate-margin widths for all plates. A given plate's shape fun ction (which is unity within the plate and zero outside the plate) can be represented by analytic functions as long as the distance from a p oint inside the plate to the plate's boundary can be expressed as a si ngle-valued function of azimuth (i.e. a single-valued polar function). To allow sufficient realism to the plate boundaries, without the exce ssive smoothing used by Bercovici & Wessel, the plates are divided alo ng pseudo-boundaries; the boundaries of plate sections are then simple enough to be modelled as single-valued polar functions. Moreover, the pseudo-boundaries have little or no effect on the final results. The plate shape function for each plate also includes a plate-margin funct ion which can be constrained by geophysical data on intraplate deforma tion. We demonstrate how this margin function can be determined by usi ng, as an example data set, the global seismicity distribution for sha llow (depths less than 29 km) earthquakes of magnitude greater than 4. Robust estimation techniques are used to determine the width of seism icity distributions along plate boundaries; these widths are then turn ed into plate-margin functions, that is analytic functions of the azim uthal polar coordinate (the same azimuth-of which the distance to the plate boundary is a single-valued function). The model is used to inve stigate the effects of 'realistic' finite-margin widths on the Earth's present-day vorticity (i.e. strike-slip shear) and divergence fields as well as the kinetic energies of the toroidal (strike-slip and spin) and poloidal (divergent and convergent) flow fields. The divergence a nd vorticity fields are far more well defined than for the standard di scontinuous plate model and distinctly show the influence of diffuse p late boundaries such as the northeast boundary of the Eurasian plate. The toroidal and poloidal kinetic energies of this model differ only s lightly from those of the standard plate model; the differences, howev er, are systematic and indicate a greater proportion of spin kinetic e nergy in the continuous plate model.