Abstract The durations of the pre-explosion low-frequency impulses of the broadband seismic records of 286 Vulcanian explosions of Volcan de Colima and 15 Vulcanian explosions of Popocatepetl volcano, Mexico were analyzed. These pre-explosion impulses are thought to be produced during the upward movement of fragmented magma in the volcano conduit before an explosion. It is shown that three different magma fragmentation surfaces acted simultaneously during the 2004 September–November episode at Volcan de Colima associated with the andesitic block lava extrusion. The optimal apparent depth for the fragmentation surface that generated the largest (with the energy ≥ 10 11 J) 2004–2005 explosions at Volcan de Colima and the 1998–2000 Popocatepetl explosions is constrained by pre-explosion low-frequency impulse durations between 3 and 7 s.