Volcanic sound in the form of acoustic energy in the air is occasionally observed from active volcanoes, due to air turbulence during or resonance before eruptions. Although a variety of volcanic sounds have been described, it is unusual to find similar signatures from different sources. Here, we discuss volcanic sounds with extremely similar signatures produced repeatedly and alternately from two fumaroles with 7  km separation at the Tatun volcano group (TVG) in Taiwan. Those volcanic sounds were well recorded by two infrasonic sensors and tens of seismic stations, due to ground shaking converted from the acoustic waves. Because all acoustic signals had extremely similar frequency content, they were probably created by the same subsurface resonator. Also, a gradual decrease (glide) in dominant frequencies from 31 to 22  Hz suggests that resonator size may have increased by up to two times, due to strong infiltration of volcanic gases from a deeper reservoir. The observed alternative and repeated acoustic signals, with a significant regularity of time delays between and among the pairs, probably indicate the existence of a deeper volcanic reservoir beneath the TVG, providing a persistent heat source that generates volcanic sounds and the earthquakes and tremors previously observed in the area. Thus, some phreatic eruptions might take place at the TVG in the future.

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