A warning model based on temporal changes of coda q for volcanic activity at Nevado del Ruiz volcano, Colombia

ABSTRACT

The coda Q has been calculated for Nevado del Ruiz Volcano, Colombia (NRV) from 1985 to 1999 by using a single scattering model. During this period, the inverse of Q (Q-1 proportional to attenuation) exhibited a long-term decrease with time, as well as shorter-term variations related to the volcanic activity. Q-1 increased prior to volcanic crises and decreased afterward. Based on these observations, a warning seismic criterion has been developed. The parameters (frequency band, size of moving average window, and threshold levels) necessary to evidence clear and significant short-term changes in Q-1 have been investigated and appropiated values are proposed. We suggest a phenomenological model with three stages for the short-term temporal changes in Q-1 at NRV. Firstly, Q-1 increases before volcanic crises due to accumulation of gas and/or liquid, which decreases the aspect ratio of fluid pockets and increases the fractional volume of fluid in the rocks and the pore aspect ratio. Secondly, Q-1 starts to decrease during the crises by the discharging of fluids from the volcano such as gas, water, etc. Finally, Q-1 became more stable after the crisis at a lower value due to the degassing and/or increasing of rigidity of the medium because of the long-term crystallization and cooling processes. Q-1 seems a promising monitoring tool at NRV. It is possible that the observed temporal changes of Q-1, combined with other parameters, may help to predict with more accuracy a volcanic crisis at NRV. The model has been tested with a recent volcanic crisis occurred in June 9 2002. The results showed that the model is a good tool for forecasting volcanic activity at NRV. (AU)