‘Tornillo’-type seismic signals at Galeras volcano, Colombia, 1992 1993

Unusual low-frequency seismic events, called 'tornillos' ('screws') at the Observatorio Vulcanológico y Sismológico de Pasto (OVSP), have been observed at Galeras volcano during 1992-1993. Of six eruptions that occurred between July 1992 and June 1993, five were preceded by episodes of tornillo signals. These signals are characterized by (1) a waveform having a homogeneous distribution of frequencies, (2) a long coda lasting up to several minutes, (3) a small amplitude compared to the duration, and (4) a slow decay of the coda. The tornillo signals have been grouped into thirteen principal forms. The 1992-1993 tornillos showed four main periods of occurrence, each of which was followed by an eruption. Episode I occurred from 11 to 16 July 1992 and consisted of nine events. Episode II lasted from 23 December 1992 to 14 January 1993, with twenty events. Episode III was observed from 13 February to 23 March 1993, with 74 events. Episode IV lasted from 10 April to 7 June 1993, with 109 events. The seismic activity at Galeras in April-May 1993 was characterized mainly by the occurrence of tornillo signals. The behavior of these signals was similar to that before previous episodes ending with eruptions; these observations permitted us to forecast the 7 June 1993 eruption several days to weeks beforehand. The occurrence of tornillos is the most important criterion for determining the probability of an eruption at Galeras in the current period of reactivation. The maximum daily number of tornillos and the longest duration of individual events occur near the end of each episode. Within an individual episode, the durations of single events increase progressively near to the point of eruption. A positive correlation is observed between the total number of pre-eruptive tornillo signals during an episode and the volume of material ejected by the eruption. These observations may suggest that the presence of tornillos is an indication of variations in the physical conditions between the fluid and the surrounding solid material within the volcano, and they constitute an important tool for forecasting future eruptions at Galeras.