9:30 AM - 9:45 AM
[SVC26-03] Spectral structure and nonlinear structure of harmonic tremors observed at Azuma volcano
Keywords:Volcanic earthquake, Harmonic tremor, Nonlinear oscillation
Azuma volcano is a series of volcanoes located on the border of Fukushima and Yamagata prefectures, and its eruptive activities throughout recorded history have been observed mainly around Oana crater on the slope of Mt. Issaikyo. Recent seismic and geodetic observations have revealed a synchronous rise in seismic activity and inflation both at deep and shallow depths just beneath Oana crater during its activation phase. Furthermore, the migration of hypocenters of volcanic earthquakes from deep to shallow depths and the increase of BL- and N-type earthquakes, of which generation process are thought to be closely related with volcanic fluids, have also been observed. Therefore, it is important to quantify fluid movement to comprehend volcanic activity at Azuma volcano. In this study, we focus on harmonic tremors observed in 2022-2023 and examine the characteristics of waveforms such as spectral and nonlinear structures aiming to model fluid movement in the shallow hydrothermal system.
We use stations from Tohoku University and the Japan Meteorological Agency located near Oana crater. In spectral analysis, we compute Fourier spectrum of harmonic tremors and identify common spectral peaks across stations and components. In the analysis of nonlinear structures, we study velocity-displacement phase portraits to validate the intrinsic nonlinearity present in observed waveforms, and then measure the correlation dimension and the embedding dimension using the Grassberger-Procaccia algorithm to quantify that nonlinearity.
Our result shows that all analyzed harmonic tremors exhibit the fundamental spectral peak at 1-2 Hz and some additional spectral peaks. The obtained spectral peaks for each harmonic tremor consistently align at approximately integer multiples of the fundamental frequency. This characteristic contrasts with the unevenly spaced spectral peaks typically observed in N-type earthquakes observed at Azuma volcano. In the velocity-displacement phase portraits, we note that the periodic trajectories sometimes intersect on a two-dimensional plane, and some phase portraits show a transition between a simple circular trajectory and an intersecting one within a single event. Our analysis using the G-P algorithm further reveals that the correlation dimensions of the examined harmonic tremors levels off at around 2 to 3 for high embedding dimensions.
Obtained spectrum structure suggests that harmonic tremors result from self-sustained oscillations caused by fluid flow rather than the resonance of a fluid-filled cavity. Our findings in the phase portraits further indicate that the flow-induced oscillations are nonlinear dynamics in a high dimensional system and the temporal variation of the driving pressure gradient may cause the transition of phase portraits. The estimated correlation dimensions are comparable to those observed in flow-induced oscillation reported in previous studies at other volcanoes. They also show similar feature with those studies in terms of leveling off at a constant value for high embedding dimensions. We thus conclude that the harmonic tremors at Azuma volcano are a manifestation of nonlinear and chaotic fluid flow in a shallow hydrothermal system just beneath Oana crater.
Our findings indicate the intrinsic characteristics of harmonic tremors and provide a clue for the further investigation on the physical processes of volcanic fluid flow, which is indispensable to deepen our understanding of volcanic activity and its transition at Azuma volcano.
We use stations from Tohoku University and the Japan Meteorological Agency located near Oana crater. In spectral analysis, we compute Fourier spectrum of harmonic tremors and identify common spectral peaks across stations and components. In the analysis of nonlinear structures, we study velocity-displacement phase portraits to validate the intrinsic nonlinearity present in observed waveforms, and then measure the correlation dimension and the embedding dimension using the Grassberger-Procaccia algorithm to quantify that nonlinearity.
Our result shows that all analyzed harmonic tremors exhibit the fundamental spectral peak at 1-2 Hz and some additional spectral peaks. The obtained spectral peaks for each harmonic tremor consistently align at approximately integer multiples of the fundamental frequency. This characteristic contrasts with the unevenly spaced spectral peaks typically observed in N-type earthquakes observed at Azuma volcano. In the velocity-displacement phase portraits, we note that the periodic trajectories sometimes intersect on a two-dimensional plane, and some phase portraits show a transition between a simple circular trajectory and an intersecting one within a single event. Our analysis using the G-P algorithm further reveals that the correlation dimensions of the examined harmonic tremors levels off at around 2 to 3 for high embedding dimensions.
Obtained spectrum structure suggests that harmonic tremors result from self-sustained oscillations caused by fluid flow rather than the resonance of a fluid-filled cavity. Our findings in the phase portraits further indicate that the flow-induced oscillations are nonlinear dynamics in a high dimensional system and the temporal variation of the driving pressure gradient may cause the transition of phase portraits. The estimated correlation dimensions are comparable to those observed in flow-induced oscillation reported in previous studies at other volcanoes. They also show similar feature with those studies in terms of leveling off at a constant value for high embedding dimensions. We thus conclude that the harmonic tremors at Azuma volcano are a manifestation of nonlinear and chaotic fluid flow in a shallow hydrothermal system just beneath Oana crater.
Our findings indicate the intrinsic characteristics of harmonic tremors and provide a clue for the further investigation on the physical processes of volcanic fluid flow, which is indispensable to deepen our understanding of volcanic activity and its transition at Azuma volcano.