We use a simple mass-spring model to systematically examine the dynamical behavior introduced by fault zone heterogeneities. The model consists of two sliding blocks coupled to each other and to a constant velocity driver by elastic springs. The state of this system can be characterized by the positions of the two blocks relative to the driver. A simple static/dynamic friction law is used. When the system is symmetric, we observe cyclic behavior. For an asymmetric system where the frictional forces for the two blocks are not equal, the solutions exhibit deterministic chaos. Chaotic windows occur repeatedly between regions of limit cycles on bifurcation diagrams. Our model behavior is similar to that of the one-dimensional logistic map. In many examples of deterministic chaos, chaotic behavior of a low order system implies chaos in similar higher order systems. Thus, our results provide substantial evidence that earthquakes are an example of deterministic chaos. ¿ American Geophysical Union 1990