2:15 PM - 2:30 PM
[SVC28-09] Attenuation characteristics of Hachijojima Island from twofold spectral ratio method using dense seismic observation
Keywords:hachijojima Island, attenuation, twofold spectral ratio method
In order to interpret volcanic phenomena correctly, it is necessary to understand subsurface structure. Hachijojima Island, the target of this study, is an active volcano being monitored 24 hours a day by JMA. However, the subsurface structure of Hachijojima Island has not been clarified with sufficient resolution. The attenuation characteristics are known to be highly correlated with the properties of magma. In this study, we aim to estimate the attenuation characteristics of Hachijojima Island by using twofold spectral ratio method.
Twofold spectral ratio method removes the source and site amplification characteristics by using multiple earthquake records observed at multiple stations, and extracts path characteristics, attenuation characteristics. The attenuation characteristics estimated by twofold spectral ratio method are average over the entire ray-path. In this study, we estimate the attenuation characteristics between seismic stations by selecting distant earthquakes whose ray-paths are almost same. Earthquake data observed by a dense seismic network of 46 stations in Hachijojima Island during the period 2019 September to 2020 March are used. In this study, we calculated the twofold spectral ratios for 13 station pairs in Nishiyama volcano and 3 pairs of stations in Higashiyama volcano.
The figure represents average of twofold spectral rations (ATSR) from 3Hz to 10Hz. The numbers in the figure indicate station numbers. As shown in the figure, the ATSR of Mt.Nishiyama is relatively smaller than that of Higashiyama volcano, i.e., the attenuation is larger. This result is consistent with the fact that Nishiyama volcano is more active than Higashiyama volcano in the past 10,000 years.
The low seismic attenuation area is located in the same area as dike intrusion model (Kimata, et al., 2004) and the high seismic attenuation area is situated in the area of tuff cones and tuff rings formed by phreatomagmatic explosions (Sugihara and Shimada, 1998).
The high seismic attenuation regions in Nishiyama and Higashiyama volcanos roughly correspond to the low-velocity region of the velocity structure of Hagiwara and Watanabe (2019).
Twofold spectral ratio method removes the source and site amplification characteristics by using multiple earthquake records observed at multiple stations, and extracts path characteristics, attenuation characteristics. The attenuation characteristics estimated by twofold spectral ratio method are average over the entire ray-path. In this study, we estimate the attenuation characteristics between seismic stations by selecting distant earthquakes whose ray-paths are almost same. Earthquake data observed by a dense seismic network of 46 stations in Hachijojima Island during the period 2019 September to 2020 March are used. In this study, we calculated the twofold spectral ratios for 13 station pairs in Nishiyama volcano and 3 pairs of stations in Higashiyama volcano.
The figure represents average of twofold spectral rations (ATSR) from 3Hz to 10Hz. The numbers in the figure indicate station numbers. As shown in the figure, the ATSR of Mt.Nishiyama is relatively smaller than that of Higashiyama volcano, i.e., the attenuation is larger. This result is consistent with the fact that Nishiyama volcano is more active than Higashiyama volcano in the past 10,000 years.
The low seismic attenuation area is located in the same area as dike intrusion model (Kimata, et al., 2004) and the high seismic attenuation area is situated in the area of tuff cones and tuff rings formed by phreatomagmatic explosions (Sugihara and Shimada, 1998).
The high seismic attenuation regions in Nishiyama and Higashiyama volcanos roughly correspond to the low-velocity region of the velocity structure of Hagiwara and Watanabe (2019).