The physics of fault growth governs the evolution of faults within the Earth’s crust and ultimately, the behavior of devastating earthquakes. With carefully scaled physical experiments that use analogs for crustal materials, we can speed up and scale down crustal processes so that they occur within hours on a table-top rather than millennia within the Earth’s crust.  This experimental study examines the evolution of fault systems with particular attention to temporal variations in fault slip rate associated with interaction and reorganization of faults. If faults slip at the same rate throughout time, then we could use the record of past earthquakes to guide earthquake forecasts. However, with recent improvements to the fidelity of slip rate records, we see increasing geologic evidence for non-constant fault slip rates in regions of multiple active faults. The experiments show that with deformation, the faults can interact and evolve so that their slip rates may vary by ~30% through time even when the loading rate is constant. The observed variations in slip rate under constant loading rate in our experiments imply that earthquake forecasts along complex fault networks, such as southern California or the Marlborough area of New Zealand might not be able to rely on past records of slip.