UNUSUAL LOW-FREQUENCY VOLCANIC SEISMIC EVENTS WITH SLOWLY DECAYING CODA WAVES OBSERVED AT GALERAS AND OTHER VOLCANOS

Events with slowly decaying coda waves are observed in active andesiti c volcanoes associated mainly with vulcanian-type eruptions or large g as emissions. However, these signals are also recorded in some quiesce nt volcanoes. These unusual signals are considered to be related to ma gmatic activity and generally occur beneath the active crater. The sig nals 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-da ke volcano, Japan, 1982) and during quiescence (Purace volcano, Colomb ia, 1994-1995; Tarumai volcano, Japan, 1970-1971, 1975). Spectral anal ysis reveals common characteristics for this type of signal. The spect rum is characterized by one or several sharp frequency peaks. Fundamen tal frequencies are not affected by epicentral distance, azimuth or tr avel time, indicating a source effect. The damping coefficient for cod a waves ranges from 0.002 to 0.02 and is related to large values of th e Quality factor (Q(c)) ranging from 250 to 25, respectively. These pa rameters may be the result of large amounts of gas bubbles in the magm a body.