Several volcanic explosions have been recorded since April 1997 at broadband seismic stations located around the Popocatepetl volcano, Mexico. We have inverted waveforms of ten of these explosions to estimate the following source parameters: depth, duration, magnitude and direction of the single force, F. The crustal structure used in generating Green's function at nearest stations is derived from the inversion of teleseismic receiver functions at the broadband permanent station PPIG, located 5 km north of the volcano. This inversion reveals a low velocity zone at ~8 km beneath the summit with high Poisson ratio, possibly related to the magma chamber. We find that F scales with &tgr;, the duration of the source-time function, as F ∝ &tgr;2. Based on this relationship we determine an impulse magnitude scale, Mk. This magnitude is tied to the Mount Saint Helens initial explosive phase of May 18, 1980, whose magnitude is estimated as 4.6. Mk of the ten Popocatepetl explosions ranges between 1.8 and 3.2. Finally, we also propose an equivalent formula for rapid estimation of magnitude of future Popocatepetl explosions, which requires filtered amplitudes at PPIG. ¿ 2001 American Geophysical Union