10:45 AM - 12:15 PM
[SVC31-P10] Rayleigh wave phase velocities estimated from dense seismic observation data on Kozushima Island
Keywords:Kozushima Island, dense seismic observation, ambient noise, zero-crossing method, phase velocity
Understanding the subsurface structure is important to correctly understand volcanic activity. Kozushima Island, the subject of this study, is a volcanic island belonging to the Izu Islands and is one of the volcanoes under constant observation. However, only a few studies have been conducted on the subsurface structure of Kozushima Island, and the subsurface structure is not well understood.
In this study, as a first step of S-wave velocity structure estimation in Kozushima Island, we used ambient noise observed at 31 stations for about 6 months from October 2020 to March 2021 by the dense seismic observation and estimated the Rayleigh wave phase velocity in the frequency domain for all 465 station pairs.
First, pre-processing such as a band-pass filter was applied. Then, the cross spectra of each station pair were calculated and stacked for the entire period. Then the zero-crossing method (Ekström et al.,2009) was used to estimate the phase velocity of the Rayleigh wave between each observation pair. Since the zero-crossing method calculates candidate phase velocities considering the effect of noise, a reference value (reference phase velocity) is needed to determine a single-phase velocity. In this study, we have applied the eSPAC method (Ling and Okada,1993) to all stations to obtain the average phase velocity at Kozushima Island, which was used as the reference phase velocity for determining a single-phase velocity. Based on the reference phase velocity, the appropriate phase velocity was selected in terms of continuity. Finally, an approximate curve was obtained from the selected phase velocities as the dispersion curve for each observation point pair. Dispersion curves were not estimated for pairs for which there was no continuity, and the appropriate phase velocity could not be determined. Finally, we have obtained 402 dispersion curves from the 465 pairs.
From the 402 dispersion curves obtained, phase velocity maps were created for each 0.1Hz interval from 0.5to 0.8 Hz, and the distribution trend of Rayleigh wave phase velocity on Kozushima Island was examined. As a result, it was found that the phase velocity tended to increase from southeast to northwest of Kozushima Island.
In this study, as a first step of S-wave velocity structure estimation in Kozushima Island, we used ambient noise observed at 31 stations for about 6 months from October 2020 to March 2021 by the dense seismic observation and estimated the Rayleigh wave phase velocity in the frequency domain for all 465 station pairs.
First, pre-processing such as a band-pass filter was applied. Then, the cross spectra of each station pair were calculated and stacked for the entire period. Then the zero-crossing method (Ekström et al.,2009) was used to estimate the phase velocity of the Rayleigh wave between each observation pair. Since the zero-crossing method calculates candidate phase velocities considering the effect of noise, a reference value (reference phase velocity) is needed to determine a single-phase velocity. In this study, we have applied the eSPAC method (Ling and Okada,1993) to all stations to obtain the average phase velocity at Kozushima Island, which was used as the reference phase velocity for determining a single-phase velocity. Based on the reference phase velocity, the appropriate phase velocity was selected in terms of continuity. Finally, an approximate curve was obtained from the selected phase velocities as the dispersion curve for each observation point pair. Dispersion curves were not estimated for pairs for which there was no continuity, and the appropriate phase velocity could not be determined. Finally, we have obtained 402 dispersion curves from the 465 pairs.
From the 402 dispersion curves obtained, phase velocity maps were created for each 0.1Hz interval from 0.5to 0.8 Hz, and the distribution trend of Rayleigh wave phase velocity on Kozushima Island was examined. As a result, it was found that the phase velocity tended to increase from southeast to northwest of Kozushima Island.