5:15 PM - 6:45 PM
[SVC29-P08] Seismic Attenuation Structure of Hachijojima Island from Twofold Spectral Ratio Method Using Dense Seismic Observation Data
Keywords:Twofold Spectral Ratio Method, Attenuation Structure
1. Introduction
Japan is one of the most volcanically active countries in the world, with 111 active volcanoes. Hachijojima Island, located 287 km south of Tokyo, is composed of two joined volcanoes, Higashiyama and Nishiyama, and is one of the continuously monitored volcano by JMA. In order to prevent and mitigate volcanic disasters, it is essential not only to detect volcanic phenomena that may be signs of an eruption through observation and monitoring, but also to elucidate the underground structure to interpret the observed data correctly. For this reason, Tokyo Metropolitan University has conducted temporary dense seismic observations at Hachijojima Island in fiscal year 2019 and fiscal year 2021 to estimate the underground structure. As part of this project, a study to estimate the seismic attention of Hachijojima Island by applying the Twofold Spectral Ratio Method (TSR) to the observation data in fiscal year 2019 has been conducted. However, the number of data is limited and the results are not stable. Therefore, the purpose of this study is to estimate the seismic attenuation structure of Hachijojima Island by adding the observation data of fiscal year 2021.
2. Dense seismic observation data
The observations were made at 46 points in Hachijojima Island (35 points around Nishiyama, 10 points around Higashiyama, and 1 point at Hachijokojima Island) for 7 months from September 2, 2019 to March 27, 2020, and for 6 months from September 14, 2021 to March 18, 2022. The three components were continuously observed by a 1Hz velocity meter at a sampling frequency of 250 Hz. For the observed data, The P waves initial motion was manually measured using the WIN system (Urabe and Tsukada, 1992) based on the hypocenter information of the seismological bulletin of Japan Meteorological Agency.
3. Analysis method
The TSR Method was used in the analysis. First, the TSR Method was applied to the data taken from 0.2 seconds before the initial motion of the P-wave to 0.7 seconds, and the Average TSR (ATSR) between each two stations was calculated. Next, the Qp structure was estimated from the ATSR using an inverse analysis. In the inverse analysis, an initial model was created by BPT using the ATSR, and the Qp structure was estimated by iteratively modifying the model to minimize the difference between the theoretical ATSR and the observed ATSR by SIRT. In addition, in order to confirm the depth at which the observed seismic waves passed through, the depth of propagation of the seismic waves was confirmed by ray-tracing.
4. Analysis Results
To obtain a stable Twofold Spectral Ratio, the ATSR was calculated only for pairs with at least three earthquakes on each side. The Qp structure was estimated by conducting an inverse analysis using the ATSR obtained from the calculations. From the estimated Qp structure, we were able to evaluate the degree of attenuation in each region.
5. Discussion
The relationship between the Qp structure estimated in this study and the results of Watanabe (2021), who estimated the attenuation characteristics of Hachijojima Island from the observation data in 2019, the dyke Intrusion position in 2002 estimated by Kimata et al. (2004), and the velocity structure beneath Hachijojima Island estimated by Hagiwara and Watanabe (2019) are discussed.
6. Conclusion
In this study, we applied TSR Method to dense seismic observation data at Hachijojima Island and used the results to estimate the Qp structure of Hachijojima Island by inverse analysis.
The relationship between the Qp structure of Hachijojima Island and volcanic activities and velocity structure of Hachijojima Island are discussed.
References
Fumiaki KIMATA et al. (2004):Volcano, 49, 13-22
Hiroko Hagiwara, Hidefumi Watanabe(2019):Kozushima and Hachijojima volcanoes, Japan Geoscience Union Meeting 2019, 23
URABE Taku, TSUKADA Shin'ya(1992):Programme and Abstracts, the Seismological Society of Japan, 2,41
Utako WATANABE (2021):Proceeding of the SEGJ Conference, 144, 83-86
Japan is one of the most volcanically active countries in the world, with 111 active volcanoes. Hachijojima Island, located 287 km south of Tokyo, is composed of two joined volcanoes, Higashiyama and Nishiyama, and is one of the continuously monitored volcano by JMA. In order to prevent and mitigate volcanic disasters, it is essential not only to detect volcanic phenomena that may be signs of an eruption through observation and monitoring, but also to elucidate the underground structure to interpret the observed data correctly. For this reason, Tokyo Metropolitan University has conducted temporary dense seismic observations at Hachijojima Island in fiscal year 2019 and fiscal year 2021 to estimate the underground structure. As part of this project, a study to estimate the seismic attention of Hachijojima Island by applying the Twofold Spectral Ratio Method (TSR) to the observation data in fiscal year 2019 has been conducted. However, the number of data is limited and the results are not stable. Therefore, the purpose of this study is to estimate the seismic attenuation structure of Hachijojima Island by adding the observation data of fiscal year 2021.
2. Dense seismic observation data
The observations were made at 46 points in Hachijojima Island (35 points around Nishiyama, 10 points around Higashiyama, and 1 point at Hachijokojima Island) for 7 months from September 2, 2019 to March 27, 2020, and for 6 months from September 14, 2021 to March 18, 2022. The three components were continuously observed by a 1Hz velocity meter at a sampling frequency of 250 Hz. For the observed data, The P waves initial motion was manually measured using the WIN system (Urabe and Tsukada, 1992) based on the hypocenter information of the seismological bulletin of Japan Meteorological Agency.
3. Analysis method
The TSR Method was used in the analysis. First, the TSR Method was applied to the data taken from 0.2 seconds before the initial motion of the P-wave to 0.7 seconds, and the Average TSR (ATSR) between each two stations was calculated. Next, the Qp structure was estimated from the ATSR using an inverse analysis. In the inverse analysis, an initial model was created by BPT using the ATSR, and the Qp structure was estimated by iteratively modifying the model to minimize the difference between the theoretical ATSR and the observed ATSR by SIRT. In addition, in order to confirm the depth at which the observed seismic waves passed through, the depth of propagation of the seismic waves was confirmed by ray-tracing.
4. Analysis Results
To obtain a stable Twofold Spectral Ratio, the ATSR was calculated only for pairs with at least three earthquakes on each side. The Qp structure was estimated by conducting an inverse analysis using the ATSR obtained from the calculations. From the estimated Qp structure, we were able to evaluate the degree of attenuation in each region.
5. Discussion
The relationship between the Qp structure estimated in this study and the results of Watanabe (2021), who estimated the attenuation characteristics of Hachijojima Island from the observation data in 2019, the dyke Intrusion position in 2002 estimated by Kimata et al. (2004), and the velocity structure beneath Hachijojima Island estimated by Hagiwara and Watanabe (2019) are discussed.
6. Conclusion
In this study, we applied TSR Method to dense seismic observation data at Hachijojima Island and used the results to estimate the Qp structure of Hachijojima Island by inverse analysis.
The relationship between the Qp structure of Hachijojima Island and volcanic activities and velocity structure of Hachijojima Island are discussed.
References
Fumiaki KIMATA et al. (2004):Volcano, 49, 13-22
Hiroko Hagiwara, Hidefumi Watanabe(2019):Kozushima and Hachijojima volcanoes, Japan Geoscience Union Meeting 2019, 23
URABE Taku, TSUKADA Shin'ya(1992):Programme and Abstracts, the Seismological Society of Japan, 2,41
Utako WATANABE (2021):Proceeding of the SEGJ Conference, 144, 83-86