11:00 〜 13:00
[SVC34-P09] 地震波干渉法から推定された2021年10月阿蘇山噴火に伴う地震波速度変化
キーワード：地震波干渉法、地震波速度変化、阿蘇山
The recent development of the seismic interferometry technique [e.g., Curtis et al., 2006; Shapiro, 2004] has allowed us to detect precursory changes in seismic velocities and/or seismic scattering properties 10 days-a few days before eruption/increase in volcanic activity [e.g., Budi-Santoso and Lesage, 2016; Obermann et al., 2013a; Olivier et al., 2018]. To improve the utility of seismic interferometry in volcano monitoring, it is important to apply this technique to more eruptions and to interpret the correspondence with volcanic phenomena. At Aso volcano, Japan, volcanic eruptions occurred on October 14 and 20, 2021. We report temporal changes in seismic velocities before and after these eruptions.
We used seismic ambient noise data at 4 permanent NIED (V-net) stations, 8 permanent JMA stations, and 19 temporal stations maintained by NIED during January 2020-December 2021 at Aso volcano. We first divided continuous seismic records on the vertical component into 10-minute segments and applied spectral whitening [e.g., Shapiro et al., 2006; Bensen et al., 2007] and one-bit normalization [Larose et al., 2004]. We then calculated seismic ambient noise cross-correlation functions (CCFs) every 10 minutes. Daily cross-correlation functions (DCCFs) were obtained by stacking these 10-minute CCFs. Finally, we measured relative seismic velocity changes (dv/v) between a reference CCF and a 5-day stacked CCF (SCCF) in the 0.5-1 Hz band using the stretching method [Lobkis and Weaver, 2003], where the reference CCF is computed by stacking DCCFs between January and December 2020. Daily dv/v values were estimated by sliding the time window to calculate the SCCF.
Significant seismic velocity changes from early October 2021 were estimated at many station pairs. Station pairs from seismic stations near the crater (within 4 km from the crater) had more significant seismic velocity changes. About 0.5 % seismic velocity increases were estimated for these station pairs. On the other hand, in the case of station pairs from seismic stations far from the crater (more than 9 km from the crater), only 0.1 % seismic velocity changes were observed. These results suggest that seismic velocity structures in the vicinity of the crater changed significantly associated with eruptions.
From about a week before the eruption on October 14, 2021, dilatation in the shallow region beneath the crater was detected by an extensometer meter, which is installed about 1 km southwest of the crater [JMA, 2022]. A hydrothermal reservoir was estimated around a few hundred meters beneath the crater by magnetotelluric surveys [Kanda et al., 2008; Kanda et al., 2019]. Assuming the expansion of a spherical pressure source several hundred meters underground, an increase in velocity is expected around the observation points near the crater. The significant increase in seismic velocity at the pair of stations near the crater may have been caused by crack closure due to the expansion of the shallow hydrothermal reservoir.
Acknowledgments: We would like to thank the Japan Meteorological Agency for providing continuous seismograms.
We used seismic ambient noise data at 4 permanent NIED (V-net) stations, 8 permanent JMA stations, and 19 temporal stations maintained by NIED during January 2020-December 2021 at Aso volcano. We first divided continuous seismic records on the vertical component into 10-minute segments and applied spectral whitening [e.g., Shapiro et al., 2006; Bensen et al., 2007] and one-bit normalization [Larose et al., 2004]. We then calculated seismic ambient noise cross-correlation functions (CCFs) every 10 minutes. Daily cross-correlation functions (DCCFs) were obtained by stacking these 10-minute CCFs. Finally, we measured relative seismic velocity changes (dv/v) between a reference CCF and a 5-day stacked CCF (SCCF) in the 0.5-1 Hz band using the stretching method [Lobkis and Weaver, 2003], where the reference CCF is computed by stacking DCCFs between January and December 2020. Daily dv/v values were estimated by sliding the time window to calculate the SCCF.
Significant seismic velocity changes from early October 2021 were estimated at many station pairs. Station pairs from seismic stations near the crater (within 4 km from the crater) had more significant seismic velocity changes. About 0.5 % seismic velocity increases were estimated for these station pairs. On the other hand, in the case of station pairs from seismic stations far from the crater (more than 9 km from the crater), only 0.1 % seismic velocity changes were observed. These results suggest that seismic velocity structures in the vicinity of the crater changed significantly associated with eruptions.
From about a week before the eruption on October 14, 2021, dilatation in the shallow region beneath the crater was detected by an extensometer meter, which is installed about 1 km southwest of the crater [JMA, 2022]. A hydrothermal reservoir was estimated around a few hundred meters beneath the crater by magnetotelluric surveys [Kanda et al., 2008; Kanda et al., 2019]. Assuming the expansion of a spherical pressure source several hundred meters underground, an increase in velocity is expected around the observation points near the crater. The significant increase in seismic velocity at the pair of stations near the crater may have been caused by crack closure due to the expansion of the shallow hydrothermal reservoir.
Acknowledgments: We would like to thank the Japan Meteorological Agency for providing continuous seismograms.