Harrat Lunayyir is one of the most volcanically active recent basaltic fields in western Saudi Arabia. A period of substantial seismic unrest, featuring more than 30,000 local events, occurred in Harrat Lunayyir in April-June 2009. Although this crisis was presumably related to ongoing magma activity, it ended without any surface volcanic activity. We create new tomographic models of P and S velocities (V_P and V_S) and use them to explain the causes of this unrest and the reasons the eruption failed. A large seismic anomaly of high V_P/V_S ratio below 7 km depth coincides with the locations of more than 50 recent cinder cones with ages of older than 100 ka, and is interpreted as a steady-state magma reservoir. We also identify another seismic anomaly at depths below 15 km, which is interpreted as a conduit for fluids and melts from deeper sources. Because the location of this conduit is slightly outside the main reservoir, some of the incoming material was dispersed. As a result, the activation of the crustal reservoir was not sufficiently strong to pierce the rigid basaltic cover and cause an actual eruption during the crisis in 2009.