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SUMMARY A methodology is proposed for the quantification of volcanic explosions based on three parameters derived from broad‐band seismic signals: the counter force of the eruption F, the power of the explosion P and the duration of the upward movement of the gas slug in the conduit to the free surface of magma, D. This methodology was applied to the 2004–2005 sequence of explosions at Volcán de Colima, Mexico. The broad‐band records of more than 100 explosive events were obtained at a distance of 4 km from the crater. We determined the counter force of the eruption by modelling the low‐frequency impulse of the seismic records of 66 volcanic explosions and estimated the power of 116 explosions from the spectra of the high‐frequency impulse. The power of Colima explosions spans five orders of magnitude; the counter force spans four orders of magnitude. We show that the power of a volcanic explosion is proportional to the counter force of the eruption. These parameters may be used for the elaboration of a scale of volcanic explosions.

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The block-lava effusion at Volcµn de Colima, MØxico began on November 20, 1998, after 12 months of seismic activity, and ended about 80 days later. Three types of seismic events were observed during the lava effusion. Volcano-tectonic earthquakes occurred mainly at the very beginning and after the termination of lava effusion. Explosion earthquakes occurred frequently during the period of the maximum rate in lava effusion. The remainder of the seismic signals were associated with pyroclastic flows and rockfalls from the lava dome. These latter signals increased sharply in number at the onset of lava effusion. The rate of occurrence remained high when the lava discharge rate decreased but gradually decreased after the termination of lava effusion. Maximum daily durations of seismic signals are proportional to the daily volumetric output of lava, indicating the dependence of the number of pyroclastic flows on the rate of lava output. A log-log plot of seismic signal duration vs. number of events with this duration displays a linear relationship. The short-period seismic signals can be divided into three categories based on duration: short events with durations less than 100 s; intermediate events with durations between 100 and 250 s; and long events with durations longer than 250 s. We infer that long events correspond to pyroclastic flows with mean deposit volume $210 5 m 3 , and intermediate events represent pyroclastic flows with mean deposit volume $110 3 m 3 .

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[1] A proposed conceptual model of Vulcanian explosions describes the volcanic explosion process as consisting of two stages (the movement of the fragmented magma to the surface and a subsequent explosion) and is constrained by four source parameters inferred from broadband seismic records (the counter force of the eruption F acting in the conduit before an explosion and governing the fragmented magma movement to the surface, the time D 1 of the movement of fragmented magma in the conduit before an explosion, the duration of the explosion in the conduit D 2 , and the energy E of the explosion). Our paper discusses, within the framework of the proposed conceptual model, the source scaling relationship inferred from the parameters of 135 Vulcanian explosions occurring at two Mexican andesitic volcanoes: Volcán de Colima and Popocatépetl during 1996-2006. Two groups of explosions are discriminated, small (E < 10 11 J) and large (E ! 10 11 J), characterized by a different type of scaling and origin. The scaling of the counter force F on the source duration t of the single force is different for large and small explosions but is characterized by the same tendency of F $ t 2 . According to the dependence E versus D 1 , large explosions are generated within the narrow depth zone in the conduit corresponding to D 1 from 3.5 to 7.5 s while small explosions are generated within a wide depth zone corresponding to D 1 from 0 to 13 s. The counter force of the eruption strongly governs the energy of explosions. The duration of the explosion at the explosion surface within the conduit, D 2 , has a tendency to negatively correlate with the energy of the explosion.