Along the East African Rift System (EARS) lie several active volcanoes characterized by different eruptive styles and magma plumbing systems. Furthermore, along the EARS, magma intrusions represent a major component in continental rifting. In this unique tectonic environment it is very important to study the magma emplacement mechanisms. Here we used InSAR data from different satellite (Envisat, Cosmo SkyMed, TerraSAR-X and RADARSAT) and GPS data from permanent stations in the KivuGNet network to measure ground displacement associated with 2011-2012 Nyamulagira eruption. Nyamulagira is an active shield volcano with a central caldera, located in the eastern part of the Democratic Republic of Congo, along the Kivu segment of the EARS. From 1948 to 2012, Nyamulagira mostly showed a particular eruptive cycle with 1) classical short-lived (i.e. 20-30 days) flank eruptions, sometimes accompanied with intracrateral activity, which occurred every 1-4 years on average, and 2) less frequent long-lived (i.e. several months) eruptions usually emitting larger volumes of lava that take place at larger distance (>8 km) from the central caldera. The 2011-2012 Nyamulagira eruption is of that second type. Both InSAR and GPS ground displacement observations suggest that a magma intrusion preceded by two days the eruption. Furthermore, seismic activity increased since November 4 2011, with long- and short-period earthquakes swarms. This intrusion corresponded to the migration of magma from a shallow reservoir (~3km) below the caldera to the two eruptive fissures located ~11 km ENE of the central edifice. Using analytical models we invert the InSAR measured ground displacements during the first co-eruptive month to evaluate the deformation source parameters and the mechanism of magma emplacement. GPS data are used to validate the modeled displacement. Moreover, GPS, InSAR and seismic datasets suggest the presence of a deep magmatic source that possibly fed the shallower magmatic system. This mechanism, involving a deep source for this large eruption, contrasts with the usual shallow plumbing system identified during the classical flank eruptions.