CODA-Q BEFORE AND AFTER THE ERUPTIONS OF 13 NOVEMBER 1985, AND 1 SEPTEMBER 1989, AT NEVADO-DEL-RUIZ VOLCANO, COLOMBIA

Coda Q(-1) was calculated at Nevado del Ruiz Volcano (NRV) before and after two phreatomagmatic eruptions (November 1985, September 1989) an d for a period of stability (May 1987) using a functional form for cod a derived from a single scattering model (Sato 1977). Substantial chan ges were found before and after the eruptions. The highest value of Q( -1) was found during the November 1985 eruption, an intermediate value for the September 1989 eruption, and the lowest value for May 1987. I t seems that the changes in coda Q-I at NRV have a still-unknown relat ionship with the volume or magnitude of the eruption. A relatively str ong frequency dependence was found for all periods. Also Q(-1) clearly changed with time, suggesting that the scattering was strong fur the eruption of November 1985 and decreased for the eruption of September 1989, and that the intrinsic absorption probably increased. This sugge sts the possibility that crystallization is taking place in the NRV ma gma. The clear change of coda Q(-1) before and after the eruptions at NRV also suggests the possibility that coda Q(-1) is a premonitory too l of activity at this volcano.