Scientists from the United States Geological Survey's (USGS) Hawaiian Volcano Observatory have written an excellent article about the relationship between earthquakes and volcanic eruptions. Due to the fact that both volcanoes and active faulting occur on plate boundaries, these are closely related processes. Simply put, earthquakes change the stress field in the area in which they occur. These changes in the stress field can change the stress balance around magma chambers - bringing them closer to an eruption. Both the fields of seismology and volcanology are complex processes however, and therefore the interactions between these two systems are also complicated.
One reflection from reading this article, is the importance of understanding and incorporating the interaction of hazards within natural hazard risk assessments. If people have been affected by a large earthquake they will be more vulnerable to a volcanic eruption than if there had been no earthquake. Their properties may have been damaged, giving them less protection from ash, blocks of rock etc - and roads used as emergency escape routes may also have been damaged. This again highlights the importance of a holistic, multi-hazard approach to natural hazard analysis.