Referee 1 *************************************************************

The presentation is readable but there are many minor misuses of English
in the paper. Two or three times this results in ambiguity or confusion
about the technical content.

The paper addresses three topics: Formulation of the model, Description
of extending the parallel system GeoFEM, and discussion of some
applications. The formulation is fairly brief, just enough for an expert
to understand the mathematical model. There are many ways to obtain a
numerical model and how this is done is not discussed beyond noting that
it is a finite element model. Considerable attention is focused on the
contact of two sliding plates. Several diagrams and some discussion are
provided but one does not learn much about the actual model of this
interaction.

An overview is given of GeoFEM and then some discussion is given as how
to extend it to handle moving plates in contact. A mathematical model of
the contact is presented and then reformulated so as to be more suitable
for the parallelization and are within the GeoFEM framework. This
presentation is at an intermediate level, one gets the general idea but
not the actual numerical model. The key idea is that an agent (called a
coupler) is created with the capability of transferring information
between two moving meshes. The paper does not use the agent terminology
however. The numerical solvers for the equations are named by method but
no further details are given.

The examples discussed include (1) A 1.3 million degrees of freedom
simulation using GeoFEM unmodified. A picture and performance data for
one step are given. (2) A 1.2 million degrees of freedom simulation with
moving plates. However, the contact model is simplified so this is not a
realistic model. A picture is given without any performance data. (3) A
materials application which is to benchmark the computation against an
independent result. It is not clear to me that the experiment actually
validates the methodology; it is only stated that the results are the
same using one and two processors.

The paper ends with a high level conceptual design of how to combine
GeoFEM and LSMearth (another modeling code). This is a plausible design;
the issue is how well the implementation works.

In summary, this paper is an overview of a large project in simulation
of the earth. The parallelization issues are just starting to be
addressed, that is, using only a few processors. It is well organized
and, though the English is substandard, the presentation is well done.

Referee 2 *************************************************************

This is an interesting paper which is rather high level. More details
of the implementation -- especially important would be more discussion of
performance and scaling to larger machines of shared and distributed memory 
architecture.