Piton de la Fournaise (La Réunion island) is a very active basaltic volcano, with five eruptions between Nov 2009 and Dec 2010. Pre-eruptive seismic crises usually last for a few hours and mainly consist of a volcano-tectonic swarm. During the quiescent period between the volcano-tectonic swarm and the eruptive tremor, we identify another swarm of events with a very high frequency content. These events are shallow and are located close to the future eruption site. They seem associated with the opening of the path for the magma propagating laterally at shallow depth. As these events start to occur while the magma is still in a vertical propagation phase, this seismicity seems to be related with the generic response of the volcano to the stress perturbation and not directly induced by the magma pressure. This new observation brings new insights to short-term forecasting of the eruption location.