Application of seismic techniques to an explosion event at Tungurahua volcano, Ecuador, provided clear images to elucidate its source process. A source location method using high-frequency seismic amplitudes with an S-wave velocity of 2000 m/s indicates that the event was triggered at a depth of 6 km below the summit, and the source ascended toward the summit at a speed of about 1600 m/s. Waveform inversion of low-frequency signals at the event onset points to an isotropic mechanism with initial deflation at a similar depth of 6 km. The ascending source suggests that a pressure wave propagated along the magma conduit, triggering fragmentation of magma at shallow depths. Rapid decompression of magma in a shock tube has been considered to be an important mechanism for explosive eruptions triggered by ruptures at the magma surface. However, our study suggests that explosive eruptions are triggered by pressure disturbances in magma at depth.