The presence of impurities between two surfaces may greatly affect their frictional properties. In this article, we investigate the role of the impurities by studying experimental and numerical models of geophysical interest. Both model and experiments are considered to be representative of the dynamics of an earthquake fault and both systems reliably produce salient traits of earthquake dynamics such as stick-slip intermittent dynamics and power-law event distributions. The impurities are granular particles enclosed in the fault, known as fault gouge. We consider the role of the granular particles in the statistics of the slip events, and in the production of acoustic emissions.