The highly active Piton de la Fournaise volcano is an excellent field laboratory to develop and test, in a short time span, new methods for detecting volcanic precursors. Since 1995, a network of four extensometers has been installed and has detected extensional, shear and vertical movements of fractures in relation to ground deformations of the volcano. This study describes new insights in the distribution of stresses in the volcanic edifice between eruptions and during magma intrusions. Continuous measurements show that all eruptions were preceded several months before by significant fracture movements: a constant opening associated with a dextral movement on the south flank and a sinistral movement on the north flank of the volcano. These movements can be attributed to a shallow pressure source below the summit craters.