Abstract
At approximately 09:36 UTC on 27 April 2016, a phreatic eruption occurred on Whakaari Island (White Island) producing an eruption sequence that contained multiple eruptive pulses determined to have occurred over the first 30 min, with a continuing tremor signal lasting ~ 2 h after the pulsing sequence. To investigate the eruption dynamics, we used a combination of cross-correlation and coherence methods with acoustic data. To estimate locations for the eruptive pulses, seismic data were collected and eruption vent locations were inferred through the use of an amplitude source location method. We also investigated volcanic acoustic–seismic ratios for comparing inferred initiation depths of each pulse. Initial results show vent locations for the eruptive pulses were found to have possibly come from two separate locations only ~ 50 m apart. These results compare favorably with acoustic lag time analysis. After error analysis, eruption sources are shown to conceivably come from a single vent, and differences in vent locations may not be constrained. Both vent location scenarios show that the eruption pulses gradually increase in strength with time, and that pulses 1, 3, 4, and 5 possibly came from a deeper source than pulses 2 and 6. We show herein that the characteristics and locations of volcanic eruptions can be better understood through joint analysis combining data from several data sources.
Highlights
Whakaari (Fig. 1) is an active volcanic composite cone that lies at the northern end of the Taupō Volcanic Zone in the Bay of Plenty, New Zealand (Houghton and Nairn 1991; Mayer et al 2015)
The second theory uses multiple gas slugs of different sizes moving up the preexisting crack that erupt at regular intervals, which creates the variations in the frequency content of each pulse (Ripepe et al 1993)
We favor the theory of depth differences in terms of volcanic acoustic–seismic ratio (VASR) difference, because the seismic signals (Fig. 2) show pulses 1, 3, 4, and 5 with distinct heightened traces compared to the signals of the other eruptive pulses, and due to the fact that the difference between the seismic and acoustic energies from eruptions commonly correlates to the depth of the source (Ichihara 2016)
Summary
Whakaari (Fig. 1) is an active volcanic composite cone that lies at the northern end of the Taupō Volcanic Zone in the Bay of Plenty, New Zealand (Houghton and Nairn 1991; Mayer et al 2015). After calibrating the ASL method from active source events on Whakaari, location estimates for eruptive pulses (Fig. 8, green stars) locate on or close to the inferred vent location. Location methods such as back projection (e.g., Jolly et al 2014b) cannot provide reliable results either, due to the insignificant travel-time variations and poor station distribution With this instrumental set up, we only use the Setra acoustic sensors from each station location, and estimate lag times, correlation, and coherence (Fig. 9) for the eruption (e.g., Ichihara et al 2012; Matoza and Fee 2014). Matoza et al (2013) noted that high-temperature jet-like gasses that are expelled out of the vents at high velocities, that create jet noise mixing, are highly directional, do not radiate acoustic energy uniformly, and cause an increase in turbulence within the air These jet noise effects could very well cause the frequency differences within each eruption pulse on Whakaari. The significance of this, as well as the whole eruption setting and process in acoustic and seismic
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