Thesis: The Ubaye region, located in the Alps at mid-distance between Grenoble and Nice, is one of the most seismically active areas in France. One of the strongest earthquakes recorded in mainland France occurred on April 5, 1959 (MLLDG 5.3). This region is characterized by a particular seismic activity composed of intertwined mainshock/aftershocks and seismic swarm sequences, involving the complex interaction of several driving processes (stress transfer, fluid-pressure, etc). Some swarms contained earthquakes of magnitude greater than 4, like the 2012-2015 swarm, which is characterized by two mainshocks: MLLDG 4.8 (26/02/2012) and MLLDG 5.2 (07/04/2014). This PhD aims at better understanding the processes that drive the seismicity and relate it to known structures in the region. The study of historical and instrumental seismicity allowed to highlight the most active areas than others (Le Lauzet, Larche, Barcelonnette, St-Paul-sur-Ubaye, Guillestre) since the 18th century. Unfortunately, the uncertainties of location and our lack of knowledge of deep structures make it very difficult to relate outcropping faults with seismicity. We then focused on the two months aftershocks that followed the April 7, 2014 earthquake. The aftershock locations distribute in a wide volume around the main fault plane. The complex spatial organization of the seismicity was quantitatively analysed through the study of the geometry of the families that composed it and confirmed by the computation of 100 focal mechanisms. During the different crisis periods (2003-2004 & 2012-2015), the presence of fluids was highlighted as one of the main drivers to explain the behaviour of the seismicity. We then decided to calculate the fluid-overpressures required to trigger each event by inverting the focal mechanisms and the stress-state. Three main results emerge: 1) the majority of events (65%) requires between 15 and 40 MPa of fluid-overpressure; 2) the strongest earthquakes also require fluid-overpressures; 3) the temporal evolution of overpressure is an indicator of the type of seismic sequence. From these results, a conceptual model is proposed to conciliate fluid and the complex behaviour of the seismicity based on the fault-valve behaviour (Sibson, 1990). After concentrating only on the 2014 crisis, I focused on the 2010-2019 period to link the pre-existing structures of the region to the 2012-2015 crisis chronology. A new relocation of the seismicity is then performed on this period starting from the LDG catalogue. The full Ubaye region seems affected by swarms and more particularly the Ubaye Valley. Moreover, the alignment of the seismicity allows us to hypothesize two faults intersecting in the area of the 2012-2015 swarm. The statistical study of the magnitude distribution (b-value of the Gutenberg-Richter law) allowed the reconstruction of a generic time l behaviour of the b-value evolutions during the swarm, interpreted as related to fluid processes. The variation of b-value is more complex for mainshock/aftershock sequences, with no generic evolution before and after the mainshock. The analysis of these different results allowed us to better understand the sequence of seismic events (mainshock/aftershocks and swarms) and gave us information on the organization of active structures in the Ubaye region. This improved understanding of the complex processes at play might then help improving the risk analysis in this area.