  We present a two-dimensional spring-block model of earthquakes with the full
nonlinear equations for the forces in terms of displacements. Correspondence of
our linearized version to earlier models reveals in them inherently asymmetric
elastic properties. Our model generalizes those models and allows an
investigation of the effects of internal strains and vectorial forces. The
former is found to be relevant to critical properties such as that described by
the Gutenberg-Richter law, but the latter as well as nonlinearities up to
second order in displacements are irrelevant.
