Tornillos are long period volcanic earthquakes. Each tornillo lasts for several minutes and the amplitude decreases producing a screw-like pattern on the seismograph.
Long period seismic events are caused by pressurised fluid movements close to the surface. Tornillos are an indicator of impending eruption at Galeras, Redoubt and other volcanoes.
The tragedy at Galeras volcano (1993) when 9 people were killed was preceded by tornillo earthquakes. Tornillo earthquakes were observed before the scientists entered the active crater but were ignored.
Short-term precursor to eruption
Galeras volcano, Colombia, 1992-1993.
Asama-yama volcano, Japan, 1983.
Nevado del Huila, Colombia 2007.
Tokachi-dake volcano, Japan, 1989.
During seismic swarms
Meakan-dake volcano, Japan, 1982.
Talang, Indonesia, 2008.
Tongariro, New Zealand, 2001.
Purace volcano, Colombia, 1994-1995.
Tarumai volcano, Japan, 1970-1971, 1975.