Abstract
In the present study, we analyze the seismic signals from a continuous volcanic tremor that occurred during a small phreatic eruption of the Hakone volcano, in the Owakudani geothermal region of central Japan, on June 29, 2015. The signals were detected for 2 days, from June 29 to July 1, at stations near the vents. The frequency component of the volcanic tremors showed a broad peak within 1–6 Hz. The characteristics of the frequency component did not vary with time and were independent of the amplitude of the tremor. The largest amplitude was observed at the end of the tremor activity, 2 days after the onset of the eruption. We estimated the location of the source using a cross-correlation analysis of waveform envelopes. The locations of volcanic tremors are determined near the vents of eruption and the surface, with the area of the upper extent of an open crack estimated using changes in the tilt. The duration-amplitude distribution of the volcanic tremor was consistent with the exponential scaling law rather than the power law, suggesting a scale-bound source process. This result suggests that the volcanic tremor originated from a similar physical process occurring practically in the same place. The increment of the tremor amplitude was coincident with the occurrence of impulsive infrasonic waves and vent formations. High-amplitude seismic phases were observed prior to the infrasonic onsets. The time difference between the seismic and infrasonic onsets can be explained assuming a common source located at the vent. This result suggests that both seismic and infrasonic waves are generated when a gas slug bursts at that location. The frequency components of the seismic phases observed just before the infrasonic onset were generally consistent with those of the tremor signals without infrasonic waves. The burst of a gas slug at the surface vent may be a reasonable model for the generation mechanism of the volcanic tremor and the occurrence of impulsive infrasonic signals.
Highlights
A volcanic tremor is a continuous seismic signal that lasts minutes to days in duration and is observed during volcanic eruptions or sometimes independently
It is reasonable to consider that the impulsive infrasonic wave and large amplitude seismic signal observed before the infrasonic onset (Fig. 12) were generated by a gas slug bursting at the surface of the vent
Given the result of the envelope correlation analysis (Fig. 9), which determined that the locations of the sources of the volcanic tremors were near the vents and the surface, the bursting of a gas slug at the surface vent could be related to the generation of the tremor signal without infrasonic waves, as well as to the seismic signals prior to the infrasonic onsets
Summary
A volcanic tremor is a continuous seismic signal that lasts minutes to days in duration and is observed during volcanic eruptions or sometimes independently. We conducted a detailed investigation of the characteristics of the continuous volcanic tremor observed from around 11:00 on June 29 until the early morning of July 1, 2015, including its frequency content, temporal variations in its amplitude, durationamplitude distribution, and source locations By comparing these results with other observations, such as the infrasonic waves and vent formation timings, we discuss the generation mechanism of volcanic tremors. Considering the location error for the source of the tremor, this difference in depth between the VT earthquakes and the volcanic tremors is significant The latter occurred around an upper extension of the open crack estimated by the tilt changes (Honda et al 2015)
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