5:15 PM - 6:30 PM
[SVC28-P14] Estimation of source location of high-frequency tremor at Aso volcano by array analysis
Keywords:Aso volcano, volcanic tremor, array analysis
1.Introduction
Volcanic microtremors are closely related to the movement of fluids such as magma, hydrothermal fluids and volcanic gases, and it is important to understand the generation process of volcanic microtremors in order to understand volcanic phenomena and predict eruptions. In Aso volcano, microtremors such as long-period tremor, isolated tremor, and continuous tremor have been observed. The new high-frequency tremor precedes the long-period tremor, suggesting that this tremor has a mechanism to excite the crack oscillation, which is the source of the long-period tremor. Furthermore, the amplitude of the high-frequency tremor and the time difference from the long-period tremor are time-varying and that is expected to be closely related to the changes in volcanic activity.
In this study, in order to investigate the source mechanism of the high-frequency tremor, we investigated the temporal variation of the tremor generation pattern and estimated the source location using the data from several seismometer arrays including a 3D array installed near the first crater of Nakadake and broadband seismographs from 23 February to 31 July 2020.
2.Observation and data analysis
In this study, several arrays were installed near the first crater of Nakadake to observe the high-frequency tremors. The arrays are located about 1 km away from the first crater and consist of four stations: South (SUN), Northwest (WES), North (UMA) and Southwest (HND). At SUN, WES and UMA, a 2D array consisting of six vertical seismometers with a spacing of 10-30 m and a natural frequency of 2 Hz was installed. At HND, a 3D array consisting of 13 three-component seismometers with a horizontal spacing of 10-15 m and a vertical spacing of about 30 m and a natural frequency of 2 Hz was installed above the ground and in an observation tunnel located about 30 m below the surface.
In this study, we first tried to extract the high-frequency tremors from the band-pass filtered waveforms of 2Hz to 6Hz by the correlation coefficient with the reference waveform. The amplitude of the extracted tremor and the time difference from the long-period tremors were investigated. We also analyzed the source location using the semblance method assuming plane waves, and estimated the epicenter using the 2D arrays at SUN, WES and UMA, and then estimated the epicenter depth using the 3D array at HND.
3.Result and consideration
The epicenters of the high-frequency tremors are estimated to be the west wall of the first crater of Nakadake and do not change with time estimated from the results of the apparent velocity and the direction of arrival of the tremors from the semblance analysis of 2D arrays. The depth of the source is estimated to be about 200 m to 500 m from the semblance analysis of the 3D array. Furthermore, it is found that the high-frequency tremors have the large amplitude when the time difference from the long-period tremors is short, and that the frequency of high-frequency tremors increase or decrease due to the temperature change at the source or the heavy rainfall.
The estimated region of the source of the high-frequency tremors coincides with a highly conductive region estimated from electromagnetics observations (Kanda et al., 2018). In this region, it is considered that continuous tremors are generated by the inflow of volcanic fluid from deep underground, which destroys the sulfide precipitated to seal the fractures (Kanda et al., 2018). The high-frequency tremors analyzed in this study occur in the same region as the continuous tremors and the spectra of the high-frequency tremors are well correlated with the spectra of the continuous tremors, suggesting that they are generated by the same process. On the other hand, Yamamoto (2005) reported that volcanic fluid flows constantly into the cracks-like conduit from the deep underground, and that the inflow of volcanic fluid causes a disturbance near the top of the cracks, which in turn vibrates the cracks and generates long-period tremors. In addition, it was found that the inflow of volcanic fluid into the shallower part of the cracks cause cylindrical deformation at the upper end of the cracks, and isolated tremors is generated.
These suggest that the high-frequency tremors are caused by the breakdown of sulfides blocking the fluid path in the upper part of the crack-like conduit, which perturbs the outflow of volcanic fluid from the inside to the upper part of the crack, causing the crack to vibrate and generating long-period tremors.
Volcanic microtremors are closely related to the movement of fluids such as magma, hydrothermal fluids and volcanic gases, and it is important to understand the generation process of volcanic microtremors in order to understand volcanic phenomena and predict eruptions. In Aso volcano, microtremors such as long-period tremor, isolated tremor, and continuous tremor have been observed. The new high-frequency tremor precedes the long-period tremor, suggesting that this tremor has a mechanism to excite the crack oscillation, which is the source of the long-period tremor. Furthermore, the amplitude of the high-frequency tremor and the time difference from the long-period tremor are time-varying and that is expected to be closely related to the changes in volcanic activity.
In this study, in order to investigate the source mechanism of the high-frequency tremor, we investigated the temporal variation of the tremor generation pattern and estimated the source location using the data from several seismometer arrays including a 3D array installed near the first crater of Nakadake and broadband seismographs from 23 February to 31 July 2020.
2.Observation and data analysis
In this study, several arrays were installed near the first crater of Nakadake to observe the high-frequency tremors. The arrays are located about 1 km away from the first crater and consist of four stations: South (SUN), Northwest (WES), North (UMA) and Southwest (HND). At SUN, WES and UMA, a 2D array consisting of six vertical seismometers with a spacing of 10-30 m and a natural frequency of 2 Hz was installed. At HND, a 3D array consisting of 13 three-component seismometers with a horizontal spacing of 10-15 m and a vertical spacing of about 30 m and a natural frequency of 2 Hz was installed above the ground and in an observation tunnel located about 30 m below the surface.
In this study, we first tried to extract the high-frequency tremors from the band-pass filtered waveforms of 2Hz to 6Hz by the correlation coefficient with the reference waveform. The amplitude of the extracted tremor and the time difference from the long-period tremors were investigated. We also analyzed the source location using the semblance method assuming plane waves, and estimated the epicenter using the 2D arrays at SUN, WES and UMA, and then estimated the epicenter depth using the 3D array at HND.
3.Result and consideration
The epicenters of the high-frequency tremors are estimated to be the west wall of the first crater of Nakadake and do not change with time estimated from the results of the apparent velocity and the direction of arrival of the tremors from the semblance analysis of 2D arrays. The depth of the source is estimated to be about 200 m to 500 m from the semblance analysis of the 3D array. Furthermore, it is found that the high-frequency tremors have the large amplitude when the time difference from the long-period tremors is short, and that the frequency of high-frequency tremors increase or decrease due to the temperature change at the source or the heavy rainfall.
The estimated region of the source of the high-frequency tremors coincides with a highly conductive region estimated from electromagnetics observations (Kanda et al., 2018). In this region, it is considered that continuous tremors are generated by the inflow of volcanic fluid from deep underground, which destroys the sulfide precipitated to seal the fractures (Kanda et al., 2018). The high-frequency tremors analyzed in this study occur in the same region as the continuous tremors and the spectra of the high-frequency tremors are well correlated with the spectra of the continuous tremors, suggesting that they are generated by the same process. On the other hand, Yamamoto (2005) reported that volcanic fluid flows constantly into the cracks-like conduit from the deep underground, and that the inflow of volcanic fluid causes a disturbance near the top of the cracks, which in turn vibrates the cracks and generates long-period tremors. In addition, it was found that the inflow of volcanic fluid into the shallower part of the cracks cause cylindrical deformation at the upper end of the cracks, and isolated tremors is generated.
These suggest that the high-frequency tremors are caused by the breakdown of sulfides blocking the fluid path in the upper part of the crack-like conduit, which perturbs the outflow of volcanic fluid from the inside to the upper part of the crack, causing the crack to vibrate and generating long-period tremors.