ANALYSIS OF SEISMIC PATTERNS OBSERVED AT NEVADO-DEL-RUIZ VOLCANO, COLOMBIA DURING AUGUST-SEPTEMBER 1985

The seismic activity of the Nevado del Ruiz volcano was monitored during August-September 1985 using a three-component portable seismograph station placed on the upper part of the volcano. The objective was to investigate the frequency content of the seismic signals and the possible sources of the volcanic tremor. The seismicity showed a wide spectrum of signals, especially at the beginning of September. Some relevant patterns from the collected records, which have been analyzed by spectrum analysis, are presented. For the purpose of analysis, the records have been divided into several categories such as long-period events, tremor, cyclic tremor episodes, and strong seismic activity on September 8, 1985. The origin of the seismic signals must be considered in relation to the dynamical and acoustical properties of fluids and the shape and dimensions of the volcano's conduits. The main results of the present experiment and analysis show that the sources of the seismic signals are within the volcanic edifice. The signal characteristics indicate that the sources lie in fluid-phase interactions rather than in brittle fracturing of solid components. © 1990.