Unusual low-frequency volcanic seismic events with slowly decaying coda waves observed at Galeras and other volcanoes

Events with slowly decaying coda waves are observed in active andesitic volcanoes associated mainly with vulcanian-type eruptions or large gas emissions. However, these signals are also recorded in some quiescent volcanoes. These unusual signals are considered to be related to magmatic activity and generally occur beneath the active crater. The signals have been observed to be a short-term precursor (Galeras volcano, Colombia, 1992–1993; Asama-yama volcano, Japan, 1983), after eruption (Tokachi-dake volcano, Japan, 1989), during seismic swarms (Meakan-dake volcano, Japan, 1982) and during quiescence (Puracévolcano, Colombia, 1994–1995; Tarumai volcano, Japan, 1970–1971, 1975). Spectral analysis reveals common characteristics for this type of signal. The spectrum is characterized by one or several sharp frequency peaks. Fundamental frequencies are not affected by epicentral distance, azimuth or travel time, indicating a source effect. The damping coefficient for coda waves ranges from 0.002 to 0.02 and is related to large values of the Quality factor (Qc) ranging from 250 to 25, respectively. These parameters may be the result of large amounts of gas bubbles in the magma body.