Harmonic tremor from the deep part of Hakone volcano

The feeding system of magmatic fluid from the volcanic root to a shallow magma reservoir remains a poorly understood issue. Seismic events, including volcanic tremors and low-frequency earthquakes, in a deep part beneath volcanos are key observations for understanding the feeding system at the depth. Although deep low-frequency (DLF) earthquakes beneath volcanos have been recognized universally through dense seismic observations, volcanic tremors with harmonic frequency components originating at volcanic roots have rarely been observed. Here, we report the observation of a harmonic volcanic tremor event that occurred beneath the Hakone volcano on May 26, 2019. The tremor signal continued for approximately 10 min and was recognized at seismic stations 90 km away from the Hakone volcano. The apparent velocity of the tremor wave train is 5 km/s, corresponding to the S-wave velocity of the lower crust beneath the Hakone volcano. The frequency components varied with time. In the initial part of the tremor signal, a spectrum had a broad peak of around 1.2 Hz, whereas the tremor became harmonic with a sharp fundamental peak at 0.98 Hz in the latter part, increasing its amplitude. We estimated the source location of the volcanic tremor using the relative arrival times of the waveform envelope. The optimal source locations were estimated at a deep extension of the hypocenter distribution of the DLF earthquakes beneath the Hakone volcano, around the depth level of Moho discontinuity. The DLF earthquakes were activated immediately before the onset time of the volcanic tremor and continued for several months. The harmonic volcanic tremor may have been generated by the migration of magmatic fluid in the volcano's deep region.