A volcanic tremor is defined as a continuous seismic signal observed near active volcanos, lasting from several minutes to days. The seismic signal is observed before and during volcanic eruptions or sometimes independently. Although its driving mechanism is unclear, several physical models, involving the interactions of magmatic fluid, have been proposed (Chouet 1988; Julian 1994; Leet 1988; Rust et al. 2008; Takeo 2020). The location of a volcanic tremor is a key indicator to trace magmatic fluid (Ichihara and Matsumoto 2017) and at times reflects the eruption style (Battaglia et al. 2005). Volcanic tremor often shows harmonic characteristics in its spectrogram, with a fundamental frequency peak and its overtones. Numerous examples of harmonic tremors that occurred in shallow regions immediately below the vents of volcanos have been reported in previous studies (e.g., Ichihara et al. 2013; Kamo et al. 1977; Konstantinou and Schlindwein 2003; Maryanto et al. 2008; Ripepe et al. 2009).
Only few studies on volcanic tremors that occurred in the deep part of volcanos have been reported in the literature. For example, Aki and Koyanagi (1981) reported a deep volcanic tremor originating around a 40-km depth beneath Kilauea, Hawaii. Ukawa and Ohtake (1987) also showed a continuous seismic signal characterized by a nearly monochromatic sinusoidal wave train radiated from a 30 km depth beneath Izu-Ohsima 1 year before the 1986 eruption. The characteristics of observed waveforms reported in these studies are similar to those of volcanic tremors that occurred at shallow volcanos, such as just beneath conduits (e.g., Takeo 2020). Ukawa and Ohtake (1987) interpreted that tremor signals were also induced by the movement of magmatic fluid in the deep part of volcanos.
Around a deep part beneath volcanos, deep low-frequency (DLF) earthquakes that have low-frequency components compared to regular tectonic earthquakes with the same magnitude have also been observed. DLF earthquakes have been usually identified as isolated waveforms representing a P- or S-wave, although their onset is often unclear. Within the depth range of the middle crust to the upper mantle on Japanese Island, DLF earthquakes were recognized using a dense seismic observation network (Fig. 1a). DLF earthquakes beneath volcanos have also been detected in various regions (e.g., Nichols et al. 2011; Shapiro et al. 2017; Wech et al. 2020). Several studies have shown the physical model of DLF earthquakes related to the movement of magmatic fluid (Nakamichi et al. 2003) or the cooling of magma bodies (Aso and Tsai 2014; Wech et al. 2020). However, the generation mechanisms of DLF earthquakes, particularly mechanisms for seismic wave excitation with a long duration time, and their relation to deep volcanic tremors are unclear.
In the present study, we report a harmonic volcanic tremor that originated from a deep part beneath Hakone volcano, Central Japan, in the early morning of May 26, 2019. To the best of our knowledge, this constitutes the first report of a harmonic volcanic tremor that occurred in a deep region beneath a volcano, except for that in Hawaii reported by Aki and Koyanagi (1981), while there are numerous reports of shallow harmonic tremors in volcanos, as mentioned previously. This observation is essential for understanding the feeding process of magmatic fluid from volcanic roots. Here, we report the characteristics of the harmonic volcanic tremor as waveform records, spectrograms, and particle motion and estimate its source location. Moreover, we discuss the relationship between the deep harmonic tremor and DLF earthquakes in Hakone.
The Hakone volcano is a volcanic complex formed by several magma eruptions since 60 Ma (Nagai and Takahashi 2008) and a caldera volcano surrounded by the caldera rim with a diameter of approximately 10 km (Fig. 1b). Remarkable earthquake swarms have often been observed within a shallower depth of 7 km (Fig. 1b, c), accompanied by a crustal expansion detected by a Global Navigation Satellite System (Harada et al. 2018). Active fumarolic activity persists in Owakudani on the north flank of the central crone, where a small phreatic eruption occurred at the end of June 2015 (Mannen et al. 2018). DLF earthquakes have also occurred within the depth range of 20–30 km (Fig. 1c). The DLF earthquakes were activated before volcanic activities, such as earthquake swarms, crust expansion due to the inflation of pressure source around the depth of 7 km, and phreatic eruption (Yukutake et al. 2019), suggesting that the activation of DLF earthquakes reflects the increment of the supply of magmatic fluid at the depth. The locations of shallow magma reservoir and dehydrated hydrothermal fluid were estimated to be 9 and 6 km, respectively, using the seismic tomography method (Yukutake et al. 2015, 2021).