09:30 〜 09:45
[SVC33-03] A preliminary analysis of seismic activity off the coast of Ito, Shizuoka
キーワード：火山、地震活動、低周波地震
Before the 1989 submarine eruption that formed the Teishi Knoll, there was an earthquake swarm in the coastal to offshore area of Ito City, Shizuoka Prefecture. This area is referred as the Ito area. Therefore, it is thought that there is a relationship between earthquakes and magmatic activity. The purpose of this study was to clarify the characteristics of seismic activity in the Ito area as a first step to suggest its state of magmatic activity.
The earthquake catalog maintained by the Japan Meteorological Agency (JMA) was used as the source of earthquake data for the Ito area. This catalog includes ordinary earthquakes and low-frequency earthquakes (LFEs). In the Ito area, the former earthquakes occur mainly at depths shallower than 15 km, while the latter ones occur at depths of 30 km or deeper, almost immediately below the former. In addition, more than 10,000 ordinary earthquakes occurred between 2005 and 2020, and there were swarms in 2006, 2009 and 2011. On the other hand, LFEs were not active and occurred 48 times during the same period. Given that this number of LFEs is small, it is possible that the characteristics of seismic activity cannot be fully understood. Moreover, since LFEs in the Ito area are deeper than normal earthquakes, it is expected that it will be difficult for conventional event-detection methods employed by the JMA to detect LFEs that are easily buried in noise due to their low signal-to-noise ratios.
To resolve the difficulty in detecting LFEs by conventional methods, we produced a LFE catalog using the matched-filter (MF) method, which cross-correlated a template to continuous seismic signals. In this method, which considers a continuum of seismic signals, a LFE is identified when the timing of a seismic signal and a template signal overlap. In this study, the MF system used for detecting LFEs beneath the Hakone volcano4 was modified so that it was applicable to the Ito area. Our resultant catalog included 405 LFEs during the period from 2005 to 2020. This is about 8.4 times more than the number of earthquakes (48) reported by JMA during the same period. This means that LFEs occurred more frequently than were previously thought. We also found that a swarm of LFEs occurred synchronously with the swarm of ordinary earthquakes in 2006. To our knowledge, this synchronous phenomenon was clarified for the first time in this study, while the swarms of LFEs were not clear in the JMA data due to their small number. Ordinary earthquakes and LFEs in the Ito area suggest that magmatic activity occurs on a daily basis. In addition, the two types of earthquakes may or may not be activated at the same time, suggesting the complexity of magma activity.
We are now reprocessing a LFE catalog that uses the MF method to increase the number of detected LFEs. Moreover, to capture essential aspects of the more-detailed characteristics of seismic activity in the Ito area, ordinary earthquakes and LFEs are being analyzed by using statistical-seismology methods that our team recently developed using the Epidemic Type Aftershock Sequence (ETAS) and the b-value of the Gutenberg-Richter frequency-magnitude distributions of earthquakes1,2,3. In this presentation, a preliminary report on the most recent results will be provided.
References: 1. Kumazawa, T., et al., Earth Planets Space 71, 130 (2019); 2. Nanjo, K. Z., Nature Commun. 11, 3082 (2020); 3. Nanjo, K. Z. & Yoshida, A., Nature Commun. 9(1), 1117 (2018); 4. Yukutake, Y., et al., Geophys. Res. Lett. 46(20), 11035-11043 (2019).
Acknowledgement: We used the waveform records obtained from the permanent stations of the National Research Institute for Earth Science and Disaster Resilience, the Earthquake Research Institute at the University of Tokyo, JMA, and the Hot Springs Research Institute of Kanagawa Prefectural Government. We also used the JMA earthquake catalog. This study was partially supported by JSPS KAKENHI Grant Numbers JP 20K05050 (K.Z.N.), 22K03752 (Y.Y.), 20K11704 (T.K.), a Research Grant of the Izu Peninsula UNESCO Global Geopark (K.Z.N. and Y.Y.), and Chubu Electric Power's research based on selected proposals (K.Z.N.). The authors thank Y. Noda for help with implementing the MF method.
The earthquake catalog maintained by the Japan Meteorological Agency (JMA) was used as the source of earthquake data for the Ito area. This catalog includes ordinary earthquakes and low-frequency earthquakes (LFEs). In the Ito area, the former earthquakes occur mainly at depths shallower than 15 km, while the latter ones occur at depths of 30 km or deeper, almost immediately below the former. In addition, more than 10,000 ordinary earthquakes occurred between 2005 and 2020, and there were swarms in 2006, 2009 and 2011. On the other hand, LFEs were not active and occurred 48 times during the same period. Given that this number of LFEs is small, it is possible that the characteristics of seismic activity cannot be fully understood. Moreover, since LFEs in the Ito area are deeper than normal earthquakes, it is expected that it will be difficult for conventional event-detection methods employed by the JMA to detect LFEs that are easily buried in noise due to their low signal-to-noise ratios.
To resolve the difficulty in detecting LFEs by conventional methods, we produced a LFE catalog using the matched-filter (MF) method, which cross-correlated a template to continuous seismic signals. In this method, which considers a continuum of seismic signals, a LFE is identified when the timing of a seismic signal and a template signal overlap. In this study, the MF system used for detecting LFEs beneath the Hakone volcano4 was modified so that it was applicable to the Ito area. Our resultant catalog included 405 LFEs during the period from 2005 to 2020. This is about 8.4 times more than the number of earthquakes (48) reported by JMA during the same period. This means that LFEs occurred more frequently than were previously thought. We also found that a swarm of LFEs occurred synchronously with the swarm of ordinary earthquakes in 2006. To our knowledge, this synchronous phenomenon was clarified for the first time in this study, while the swarms of LFEs were not clear in the JMA data due to their small number. Ordinary earthquakes and LFEs in the Ito area suggest that magmatic activity occurs on a daily basis. In addition, the two types of earthquakes may or may not be activated at the same time, suggesting the complexity of magma activity.
We are now reprocessing a LFE catalog that uses the MF method to increase the number of detected LFEs. Moreover, to capture essential aspects of the more-detailed characteristics of seismic activity in the Ito area, ordinary earthquakes and LFEs are being analyzed by using statistical-seismology methods that our team recently developed using the Epidemic Type Aftershock Sequence (ETAS) and the b-value of the Gutenberg-Richter frequency-magnitude distributions of earthquakes1,2,3. In this presentation, a preliminary report on the most recent results will be provided.
References: 1. Kumazawa, T., et al., Earth Planets Space 71, 130 (2019); 2. Nanjo, K. Z., Nature Commun. 11, 3082 (2020); 3. Nanjo, K. Z. & Yoshida, A., Nature Commun. 9(1), 1117 (2018); 4. Yukutake, Y., et al., Geophys. Res. Lett. 46(20), 11035-11043 (2019).
Acknowledgement: We used the waveform records obtained from the permanent stations of the National Research Institute for Earth Science and Disaster Resilience, the Earthquake Research Institute at the University of Tokyo, JMA, and the Hot Springs Research Institute of Kanagawa Prefectural Government. We also used the JMA earthquake catalog. This study was partially supported by JSPS KAKENHI Grant Numbers JP 20K05050 (K.Z.N.), 22K03752 (Y.Y.), 20K11704 (T.K.), a Research Grant of the Izu Peninsula UNESCO Global Geopark (K.Z.N. and Y.Y.), and Chubu Electric Power's research based on selected proposals (K.Z.N.). The authors thank Y. Noda for help with implementing the MF method.