Abstract:
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 power of] 6 m [to the power of] 3 due to infilling with tephra or lava. At the andesite volcano white Island, volcanic tremor of dominant frequency 1.5 Hz, together with small swarms of A-type micro-earthquakes, occurred during long periods of non-violent ash eruption in 1966-67 and 1969. Tremor was most powerful ([less than or equal to] 3 kW) and variable (stoppages and powerful recommencements [less than or equal to] 10 kW) when the erupting tephra had maximum fresh magmatic content but a comparatively low temperature of 445 degreed c (maximum was 900 degrees c). Powerful recommencements of tremor were closely followed by pulses in the ash emission, and from recorded times, the depth of origin was inferred to be 0.8 km. At the basalt volcano Kilauea, the eruption and lava lake at Mauna Ulu vent on the upper east rift zone was the most powerful source of tremor during the study between January and April 1971. From mid February on, tremor with either a fairly flat spectrum between 1 and 5 Hz, or a dominant frequency of 1.3 Hz, occurred, while lava flowed steadily from Mauna Ulu through tubes to the SE. The tremor had stoppages, powerful recommencements and pulses which corresponded with cycles of quiet inflation of the lava lake and deflation by initially vigorous ebullition (fountaining). Rockfall earthquakes in the lake had spectra similar to tremor, and often caused ebullition and tremor to start or stop. Rapid decay of their coda indicates a Q value of 2 - 6 for vesicular lava, in which the P-wave velocity could reach as low as 17 m s [to the power of]-1. The onset of ebullition was often influenced by surface events, and it is suggested that ebullition is the self-exciting collapse of a layer of lava foam which is continuously being formed at the top of the lava column, and which has minimum stability at a shallow depth due to high order inverse variations of viscosity and foam life with depth. It appears that the tremor recorded by the author was the seismic noise of ebullition, modified by the response of bodies of rock, lava and gas and by absorption. The latter apparently caused peak near 1.3 Hz on recordings made at any appreciable distance. Narrow spectral peaks were sometimes present, especially when ebullition involved the regular bursting of bubbles of several metres diametre. Change of spectra with recording distance gave a Q value of very approximately 30 for the upper layers of Kilauea volcano. Between 1 and 10 km, RMS amplitude of tremor was inversely proportional to distance. At the trachybasalt volcano Stromboli, tremor of dominant frequency 1.6 - 2.9 Hz was almost continuous, and accompanied the ejection of spatter and discharge of short bursts of gas from the vents at intervals averaging 0.7 to 2 second. Seismic power of tremor varied from 0.3 6 kW, and its spectrum was not inconsistent with the pulse spectrum of audible discharge of gas from the vents. Once, tremor increased in power and changed in spectrum coincident with an explosion and partial collapse of a vent, which allowed more vigorous eruption. Similar slow changes of spectrum occurred during the 20 days recordings in July 1771. Between 6 and 13 explosion earthquakes per hour occurred, with remarkably few earthquakes of other types. Comparing the four volcanoes, stoppages and powerful recommencement of tremor occurred at all except stromboli. At all volcanoes, tremor usually correlated with the eruption of gas (steam?) and tephra or spatter rather than lava flow, and this was best observed at Kilauea. The data are consistent with gas release from vesicular lava as a common cause of tremor, B-type earthquakes, and explosion earthquakes. Rockfall earthquakes can also resemble B-type earthquakes however, and other types of tremor undoubtedly exist. The seismic power of eruption tremors during the study was greatest at Ruapehu (7.5 MW), followed by Stromboli (350 kW), Mauna Ulu (30 kW), and White Island (12 kW). The seismic energy per unit volume of eruption (noisiness) was greatest at stromboli (2 x 10 [to the power of] 5 J m [to the power of] -3) followed by white Island (10 [to the power of]5 J m [to the power of]-3), Ruapehu (3 x 10 [to the power of] 3 J m [to the power of] -3) and Mauna Ulu (10 [to the power of] 2 J m [to the power of] -3 on 10 February 1971). The seismic power spectra of eruption tremor were similar at all volcanoes and were broadly consistent with modified white noise. Some peaks in the spectrum are shown to be special to the transmission path, but others appear to be features in the generated spectrum.