POSSIBLE CAUSES FOR THE SEISMIC ACTIVITY OBSERVED IN COTOPAXI VOLCANO, ECUADOR

Continuous monitoring of Cotopaxi volcano from January 1989 to Septemb er 1997 shows that it is subject to a steady shallow Long Period (LP) activity. EP events are concentrated in a column with a diameter of ab out 3 km and a height of about 12 km, between an elevation of 4 km and 8 km depth. They do not occur in swarms, nor have there been periods of complete calm. High frequency volcano-tectonic (VT) events are most ly located on the northeastern side of the LP column, with the maximum activity of both types of events coinciding in depth. LP activity can not be related to unrest of the volcano. The most likely explanation f or the continuous occurrence of LP events beneath Cotopaxi is that the y are produced by the interaction of glacier thaw water and hot materi al at shallow depths. The relative spatial distribution of both type o f events in two adjacent zones suggest that VT and LP activities are i nterconnected.