L. Narvaez et al., TORNILLO-TYPE SEISMIC SIGNALS AT GALERAS-VOLCANO, COLOMBIA, 1992-1993, Journal of volcanology and geothermal research, 77(1-4), 1997, pp. 159-171
Unusual low-frequency seismic events, called 'tornillos' ('screws') at
the Observatorio Vulcanologico y Sismologico de Paste (OVSP), have be
en observed at Galeras volcano during 1992-1993. Of six eruptions that
occurred between July 1992 and June 1993, five were preceded by episo
des of tornillo signals. These signals are characterized by (1) a wave
form 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 sh
owed 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 19
93, with twenty events. Episode III was observed from 13 February to 2
3 March 1993, with 74 events. Episode IV lasted from 10 April to 7 Jun
e 1993, with 109 events. The seismic activity at Galeras in April-May
1993 was characterized mainly by the occurrence of tornillo signals. T
he behavior of these signals was similar to that before previous episo
des ending with eruptions; these observations permitted us to forecast
the 7 June 1993 eruption several days to weeks beforehand. The occurr
ence of tornillos is the most important criterion for determining the
probability of an eruption at Galeras in the current period of reactiv
ation. The maximum daily number of tornillos and the longest duration
of individual events occur near the end of each episode. Within an ind
ividual episode, the durations of single events increase progressively
near to the point of eruption. A positive correlation is observed bet
ween the total number of pre-eruptive tornillo signals during an episo
de and the volume of material ejected by the eruption. These observati
ons may suggest that the presence of tornillos is an indication of Var
iations in the physical conditions between the fluid and the surroundi
ng solid material within the volcano, and they constitute an important
tool for forecasting future eruptions at Galeras.