The complex oscillatory behavior of a springblock model is analyzed via the Hopf bifurcation mechanism. The mathematical springblock model is generated by considering the Dieterich–Ruinas's friction law and the Stribeck's effect. The existence of self-sustained oscillations in the transition zone – where slow earthquakes are generated within the frictionally unstable region – is determined. An upper limit for this region is proposed as a function of seismic parameters and frictional coefficients which are concerned with presence of fluids in the system. The importance of the characteristic length scale <i>L</i>, the implications of fluids, and the effects of external perturbations in the complex dynamic oscillatory behavior as well as in the stationary solution, are take into consideration.