Temporal change in coda Q at Nevado Del Ruiz Volcano, Colombia

An analysis of coda Q (Qc), intrinsic Q (Qi), scattering Q (Qs) and total Q (Qt), was made at Nevado del Ruiz Volcano (NRV) for local earthquakes recorded during 1992–1994. has been calculated for the first time at Nevado del Ruiz Volcano (NRV) using the single-scattering model proposed by Aki and Chouet (1975). Estimations of Qc were made for five frequency bands centred at 1.5, 3.0, 6.0, 12.0 and 24.0 Hz. The estimated Qc varies from around 60 at 1.5 Hz to around 980 at 24 Hz, depending on the frequency with Qc = 30 ± 1.2F1.03±0.001. in general, Qc values at NRV are rather similar to those obtained in other volcanic areas in the world. A steady trend in Qc-1 was found from January 1992 to January 1994. This variation is coincident with the stability in the volcanic activity since the last phreato-magmatic eruption on 1 September 1989. In the middle of April 1994, a new activity started at NRV and a small volcanic crisis occurred. Continuous increasing was observed in Qc-1 around two months before the crisis. This suggests a possibility that the temporal variation of Qc can be useful for monitoring the volcanic activity at NRV. Qs were separated from Qt by using a multiple-scattering model called “Multiple Lapse Time Window Analysis”, MLTWA (Fehler et al., 1992). Estimations for Qi, Qs and Qt were made for four frequency bands, 3.0, 6.0, 12.0 and 24.0 Hz. Qi; values were found to be near to those of Qc for frequencies higher than 6.0 Hz. For low frequencies Qs values were near to Qc values. This can suggest that the intrinsic absorption could be the predominant attenuation phenomenon in the NRV area at frequencies higher than 6 Hz and the scattering phenomenon is predominant for low frequencies; therefore the change in Qc at high frequencies is considered to reflect variations in the inner conditions of the volcano, such as changes in temperature, movement of fluids, and so on.