Long period channel waves from records of Friuli (Italy) earthquakes in 1976

We present a study of a group of long period waves excited by theearthquakes of Friuli (Italy) in 1976 and recorded between longitudinaland transverse body waves by Long-period seismographs at epicentraldistances up to about 3 000 Km. The spectral analysis has permitted usto determine periods of the main phases which make up the wave group.In regard to the longer period phase, which appears first in the recordings,the velocity obtained by the dromochrones equals 8.4 Km/s. The groupvelocity is instead 7.4 Km/s. Such phases have been considered as beinglongitudinal channel waves in the astenosphere. Such hypothesis explainsthe inconsistencies which had appeared in previous studies on channelwaves in the astenosphere.