Coda Q before and after the eruptions of 13 November 1985, and 1 September
1989, at Nevado del Ruiz volcano, Colombia.
J.M. Londoņo, J.J. Sánchez, L.E. Toro, F. Gil, and O.P. Bohorquez.
INGEOMINAS. Observatorio Vulcanológico y Sismológico de Manizales.
Abstract. Coda Q-1 was calculated at Nevado del Ruiz Volcano (NRV) before
and after two phreatomagmatic eruptions (November 1985, September 1989) and
for a period of stability (May 1987) using a functional form for coda
derived from a single scattering model (Sato, 1977). Substantial changes
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. It seems that
the changes of coda Q-1 at NRV have a still-unknown relationship with the
volume or magnitude of the eruption. A relatively strong frequency
dependence was found for all periods. Also Q-1 clearly changed with time,
suggesting that the scattering was strong for the eruption of November 1985
and decreased for the eruption of September 1989, and that the intrinsic
absorption probably increased. This suggests the possibility that
crystallization is taking place in the NRV magma. 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 tool of activity at this volcano.