17:15 〜 18:30
[SVC28-P21] Temporal changes of seismic velocities and scattering properties at Suwanosejima volcano inferred from seismic interferometry
キーワード：地震波干渉法、散乱特性変化、諏訪之瀬島
In recent years, seismic interferometry [e.g., Curtis et al., 2006; Shapiro, 2004] has been widely used to detect seismic velocities and/or scattering properties related to large earthquakes and volcanic activities. Some previous studies succeeded in detecting precursory changes in volcanic regions 10 days - a few days before eruption/increase in activity [e.g., Budi-Santoso and Lesage, 2016; Obermann et al., 2013a; Olivier et al., 2018]. Increasing the number of applications will be important to improve volcano monitoring with seismic interferometry. In this study, we conduct seismic interferometry analyses at Suwanosejima volcano, where the number of eruptions increases in the last few months, and discuss temporal changes of seismic velocities and scattering properties.
We used seismic ambient noise data of three JMA stations during Jan. 2018 - Feb. 2021 at Suwanosejima volcano and estimated temporal changes of seismic velocities and scattering properties during that period. A reduction of waveform correlation (decorrelation) will indicate a seismic scattering property change. We firstly divided continuous seismic records on vertical component into 10-minutes-long segments and applied spectral whitening [e.g., Shapiro et al., 2006; Bensen et al., 2007] and one-bit normalization [Larose et al., 2004]. Daily CCFs (DCCFs) were obtained by stacking CCFs every 10 minutes. Finally, we measured relative velocity change and waveform correlation (CC) between a reference CCF and 3-days-stacked CCF at 0.5-1 Hz, 1-2 Hz, and 2-4 Hz bands by using stretching method [Lobkis and Weaver, 2003]. Where the reference CCF is computed by stacking DCCFs between Jun. 10, 2020, and Jan. 31, 2021.
At the 0.5-1 Hz band, seismic velocity decreased by 2.6-2.9 % from 12 days before the explosion on Dec. 28, and significant decorrelations were detected 5-15 days before this explosion. Seismic velocity decreased by 2.1 % at the maximum of 9 days before the eruption on Jan. 20. Significant decorrelations were detected from 7 days before to 3 days after this eruption. Estimated seismic velocity changes just before those eruptions have large errors caused by significant decorrelations. Therefore, it is difficult to discuss seismic velocity changes related to those explosions. Although the seismic velocity changes and the decorrelations were also detected at 1-2 Hz and 2-4 Hz bands, further investigation is required because CCs at these frequency bands show large fluctuations. To investigate temporal changes of CC values in more detail, we directly computed CCs between each DCCF. CC values between DCCFs around those eruptions have relatively high, suggesting that scattering properties at the shallow region of the volcano in those periods of time are similar to each other.
To discuss possible causes of significant decorrelations, we compared temporal changes of CC with other observations. No heavy precipitations were observed at AMeDAS station in Suwanosejima during Dec. 2020 - Feb. 2021, indicating the significant decorrelations should not have been caused by heavy rains. No geodetic data showed precursory changes related to the explosions. Finally, we checked temporal changes of SO2 flux. Daily SO2 flux increased to 2,500 tons on Dec. 26, 2020 [JMA, 2021], suggesting that volcanic gas moved to the shallow region of Suwanosejima. The inflow of volcanic gas to cracks at the shallow part of Suwanosejima might have caused significant decorrelations.
Acknowledgments: We used seismograms recorded by Japan Meteorological Agency (JMA) and AMeDAS data provided by JMA.
We used seismic ambient noise data of three JMA stations during Jan. 2018 - Feb. 2021 at Suwanosejima volcano and estimated temporal changes of seismic velocities and scattering properties during that period. A reduction of waveform correlation (decorrelation) will indicate a seismic scattering property change. We firstly divided continuous seismic records on vertical component into 10-minutes-long segments and applied spectral whitening [e.g., Shapiro et al., 2006; Bensen et al., 2007] and one-bit normalization [Larose et al., 2004]. Daily CCFs (DCCFs) were obtained by stacking CCFs every 10 minutes. Finally, we measured relative velocity change and waveform correlation (CC) between a reference CCF and 3-days-stacked CCF at 0.5-1 Hz, 1-2 Hz, and 2-4 Hz bands by using stretching method [Lobkis and Weaver, 2003]. Where the reference CCF is computed by stacking DCCFs between Jun. 10, 2020, and Jan. 31, 2021.
At the 0.5-1 Hz band, seismic velocity decreased by 2.6-2.9 % from 12 days before the explosion on Dec. 28, and significant decorrelations were detected 5-15 days before this explosion. Seismic velocity decreased by 2.1 % at the maximum of 9 days before the eruption on Jan. 20. Significant decorrelations were detected from 7 days before to 3 days after this eruption. Estimated seismic velocity changes just before those eruptions have large errors caused by significant decorrelations. Therefore, it is difficult to discuss seismic velocity changes related to those explosions. Although the seismic velocity changes and the decorrelations were also detected at 1-2 Hz and 2-4 Hz bands, further investigation is required because CCs at these frequency bands show large fluctuations. To investigate temporal changes of CC values in more detail, we directly computed CCs between each DCCF. CC values between DCCFs around those eruptions have relatively high, suggesting that scattering properties at the shallow region of the volcano in those periods of time are similar to each other.
To discuss possible causes of significant decorrelations, we compared temporal changes of CC with other observations. No heavy precipitations were observed at AMeDAS station in Suwanosejima during Dec. 2020 - Feb. 2021, indicating the significant decorrelations should not have been caused by heavy rains. No geodetic data showed precursory changes related to the explosions. Finally, we checked temporal changes of SO2 flux. Daily SO2 flux increased to 2,500 tons on Dec. 26, 2020 [JMA, 2021], suggesting that volcanic gas moved to the shallow region of Suwanosejima. The inflow of volcanic gas to cracks at the shallow part of Suwanosejima might have caused significant decorrelations.
Acknowledgments: We used seismograms recorded by Japan Meteorological Agency (JMA) and AMeDAS data provided by JMA.