| Summary: | Recordings of the seismic power (rate of radiation of seismic energy) and seismic power spectral density (frequency distribution of seismic power) of volcanic vibrations at volcanoes Ruapehu and White Island in New Zealand, Kilauea in Hawaii, and Stromboli in Italy are reported, and related to visible volcanic activity. At the andesite volcano Ruapehu, volcanic tremor of dominant frequency 2 Hz and seismic power < 1 kw occurs about 20% of the time. In 1961, 64, 66, 68 and 69, tremor increased ([is less than or equal to] 20 kW) and developed stoppages followed by powerful recommencements of tremor ([is less than or equal to] 230 kW) which resembled the B-type volcanic earthquake of Minakami. Brief eruptions of mud, water, ash, and pumice from the crater lake followed this activity in 1969, 68 and 69, the largest eruption (22 June 1969) having seismic power = 8 x 10 [to the power of] 6 W, kinetic power of eruption = 10 [to the power of] 12 W, total seismic energy = 7 x 10 [to the power of] 8 J, and total kinetic energy = 7 x 10 [to the power of] 12 J. The volume of the eruption was [greater than or equal to] 10 [to the power of] 6 m3. Ninety percent of the observed eruptions were forewarned within 12 days by the concurrence of volcanic tremor of [greater than or equal to] 1 kW, and volcano-seismic events of [greater than or equal to] 20 kW, but only about 50% of these warning indications were followed by eruptions. The behaviour is intermediate between that reported at Aso volcano, Japan, where eruptions occur after tremor (without powerful recommencements) has decreased, and Asama Volcano, Japan, where B-type earthquakes rather than tremor precede eruptions. The smoothed thermal power output of Ruapehu, calculated from the temperature of crater lake, increased during strong tremor, and exceeded 10 [to the power of] 9 W in 1964, 66 and 68. Between 1965 and 1970, the depth of the lake decreased from 300 to 65 m below overflow level, and the volume decreased from 10 x 10 [to the power of] 6 to 7 x 10 [to the ...
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