Dm. Gomez et Ra. Torres, UNUSUAL LOW-FREQUENCY VOLCANIC SEISMIC EVENTS WITH SLOWLY DECAYING CODA WAVES OBSERVED AT GALERAS AND OTHER VOLCANOS, Journal of volcanology and geothermal research, 77(1-4), 1997, pp. 173-193
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.