Temporal change in coda wave attenuation observed at Volcلn de Colima, México before the 1998 eruption

The last eruptive activity of Volcán de Colima (November 1998–January 1999) was preceded by the occurrence of several seismic swarms. An analysis of coda waves of local earthquakes recorded during these seismic swarms was made to estimate coda attenuation (Qc−1) in the frequency range 2–10 Hz. We used Sato’s single scattering model [Sato, Phys. Earth Planet. Inter. 25 (1977) 27–41], beginning at twice the S-wave travel time. We found an increase in the attenuation of 34% at lower frecuencies (2 Hz) to 38% at the frecuency band around 10 Hz before the eruption. We also observed an increase of Qc−1 during the month which preceded a dome explosion on July 1998. A frequency dependence of Qc was observed with values from 76 at 2 Hz to 309 at 10 Hz. Qc values obtained at Volcán de Colima are similar to those obtained in other active areas. Temporal changes of Qc can be attributed to the change of density of the open fractures in the rocks because of the pressure generated by the pushing of magma toward the surface, which is also responsible for the inflation of the volcanic edifice.