11:30 AM - 11:45 AM
[S09-17] Analysis of seismic activity off the coast of Ito, Shizuoka
Before the 1989 submarine eruption that formed the Teishi Knoll (a volcano belonging to the Izu-Tobu Volcano Group), there was an earthquake swarm off the coast of Ito, located in Shizuoka Prefecture of Japan. This area is referred as the Ito area. 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 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). More than 10,000 ordinary earthquakes occurred between 2005 and 2020, and there were swarms in 2006, 2009 and 2011. 48 LFEs occurred 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. It may be due to difficulty in detecting LFEs that are easily buried in noise due to their low signal-to-noise ratios for conventional event-detection methods employed by the JMA. To resolve this difficulty, we produced a LFE catalog using the matched-filter (MF) method1. Our resultant catalog included about 900 LFEs during the period from 2005 to 2020. This is about 17.8 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.
To capture essential aspects of seismic activity in the Ito area, the two types of earthquakes were analyzed by using the Epidemic-Type Aftershock Sequence (ETAS) model2 and assuming that this model, which was originally developed for ordinary earthquakes, was applicable to LFEs (Nanjo et al., 2023). Our results indicated that seismic quiescence started at 2010-2011 for ordinary earthquakes and at 2011-2014 for LFEs. These timings of start of seismic quiescence are consistent with each other. We compared our results with timeseries of crustal movement conducted by Geospatial Information Authority of Japan4. Based on a leveling survey along the eastern coast of Izu peninsula since 1904, crustal movement that showed uplift before 2010 showed stagnant or subsidence after it. The timings of start of seismic quiescence roughly coincided with the timing of start of stagnant or subsidence. This coincidence suggests the changes in magmatic activity in the Ito area. Actually, no seismicity was reportedly activated since the 2011 swarm.
In this presentation, a report on the most recent results will be provided.
References: 1Yukutake et al., Geophys. Res. Lett. 46(20), 11035-11043 (2019); 2Kumazawa et al., Earth Planets Space 71, 130 (2019); 3Nanjo et al., Sci. Rep. 13, 10562 (2023); 4Geospatial Information Authority of Japan, https://cais.gsi.go.jp/YOCHIREN/report/kaihou96/05_01.pdf (2016)
Acknowledgments: 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 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). More than 10,000 ordinary earthquakes occurred between 2005 and 2020, and there were swarms in 2006, 2009 and 2011. 48 LFEs occurred 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. It may be due to difficulty in detecting LFEs that are easily buried in noise due to their low signal-to-noise ratios for conventional event-detection methods employed by the JMA. To resolve this difficulty, we produced a LFE catalog using the matched-filter (MF) method1. Our resultant catalog included about 900 LFEs during the period from 2005 to 2020. This is about 17.8 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.
To capture essential aspects of seismic activity in the Ito area, the two types of earthquakes were analyzed by using the Epidemic-Type Aftershock Sequence (ETAS) model2 and assuming that this model, which was originally developed for ordinary earthquakes, was applicable to LFEs (Nanjo et al., 2023). Our results indicated that seismic quiescence started at 2010-2011 for ordinary earthquakes and at 2011-2014 for LFEs. These timings of start of seismic quiescence are consistent with each other. We compared our results with timeseries of crustal movement conducted by Geospatial Information Authority of Japan4. Based on a leveling survey along the eastern coast of Izu peninsula since 1904, crustal movement that showed uplift before 2010 showed stagnant or subsidence after it. The timings of start of seismic quiescence roughly coincided with the timing of start of stagnant or subsidence. This coincidence suggests the changes in magmatic activity in the Ito area. Actually, no seismicity was reportedly activated since the 2011 swarm.
In this presentation, a report on the most recent results will be provided.
References: 1Yukutake et al., Geophys. Res. Lett. 46(20), 11035-11043 (2019); 2Kumazawa et al., Earth Planets Space 71, 130 (2019); 3Nanjo et al., Sci. Rep. 13, 10562 (2023); 4Geospatial Information Authority of Japan, https://cais.gsi.go.jp/YOCHIREN/report/kaihou96/05_01.pdf (2016)
Acknowledgments: 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 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.