5:15 PM - 7:15 PM
[SCG45-P21] Tidal sensitivity of shallow very low-frequency earthquakes in the Hyuganada region
Keywords:Nankai subduction zone, Hyuganada, slow earthquake, very low-frequency earthquake, earth tide
Analyses of deep low-frequency tremors in the Nankai Trough subduction zone and shallow very low-frequency earthquakes (SVLFE) in the Ryukyu Trench subduction zone have shown the existence of source regions that show high sensitivity to tidal stress (Ide 2010; 2012, Nakamura & Kakazu 2017). The tidal response of slow earthquakes is thought to reflect the state of the transition zone at the subducting plate boundary, and it is important to investigate their spatiotemporal characteristics. In this study, we determined the epicenters of SVLFE in the Hyuganada region using the correlation method and investigated the tidal response of SVLFE.
The data used were the vertical component of the F-net broadband seismograph records from 27 stations in the Kii Peninsula, Chugoku and Shikoku, and Kyushu from January 1, 2010 to August 31, 2024. The epicenters were determined by applying the correlation method to the waveforms that were resampled every second after being filtered with the passband of 20-50 second. The analysis was carried out continuously with a time width of 300 seconds and a time step of 150 seconds. Among the epicenters detected, those with an error of 7.5 km or less using the bootstrap method were adopted as the results.
The SVLFE epicenters were distributed along the edge of the large-scale sea mount shown as a geomagnetic anomaly, and the northeastern end reached as far south as Cape Ashizuri, while the southeastern end reached as far east as Tanegashima Island. During the period of analysis, various-scale SVLFE activity occurred, but in this study, we defined large-scale activity as activity that lasted for more than 10 days and expanded to an area of more than 150 km and 100 km in the direction of latitude and longitude . Such large-scale activities occurred a total of nine times in 2010, 2013, 2015, 2016, 2017-2018, 2019, 2021, 2023, and 2024.
We calculated tidal sensitivities for the entire analysis period and for the large-scale activities. The tidal sensitivity was quantified as the amplitude of the normalized Fourier spectrum, as in Ide (2010; 2012). In this study, grids were placed at 5 km intervals in the SVLFE activity area, and the amplitude of the Fourier spectrum at the M2 period(12.420601 h) was calculated for each grid, if the radius of the circle containing 50 data points centered on the grid is 10 km or less.
As a result, the tidal sensitivity was low in the central to southeastern part of the active region and high in the northeastern part of the active region for the entire analysis period. On the other hand, the tidal sensitivity for each large-scale activity was different: the large-scale activities in 2017-2018, 2019, 2021, and 2023, in the latter half of the entire analysis period, showed high sensitivity in the central to southeastern part of the active region, which showed low tidal sensitivity for the entire analysis period. Such a temporal variation in tidal sensitivity is thought to correspond to the temporal variation in the state of the fault plane, which is the source region of the SVLFE. In the presentation, duration and rapid migration will be compared with the tidal sensitivity.
Acknowledgements: We would like to thank the staff of the National Research Institute for Earth Science and Disaster Prevention for providing us with data from the F-net broadband seismograph network.
The data used were the vertical component of the F-net broadband seismograph records from 27 stations in the Kii Peninsula, Chugoku and Shikoku, and Kyushu from January 1, 2010 to August 31, 2024. The epicenters were determined by applying the correlation method to the waveforms that were resampled every second after being filtered with the passband of 20-50 second. The analysis was carried out continuously with a time width of 300 seconds and a time step of 150 seconds. Among the epicenters detected, those with an error of 7.5 km or less using the bootstrap method were adopted as the results.
The SVLFE epicenters were distributed along the edge of the large-scale sea mount shown as a geomagnetic anomaly, and the northeastern end reached as far south as Cape Ashizuri, while the southeastern end reached as far east as Tanegashima Island. During the period of analysis, various-scale SVLFE activity occurred, but in this study, we defined large-scale activity as activity that lasted for more than 10 days and expanded to an area of more than 150 km and 100 km in the direction of latitude and longitude . Such large-scale activities occurred a total of nine times in 2010, 2013, 2015, 2016, 2017-2018, 2019, 2021, 2023, and 2024.
We calculated tidal sensitivities for the entire analysis period and for the large-scale activities. The tidal sensitivity was quantified as the amplitude of the normalized Fourier spectrum, as in Ide (2010; 2012). In this study, grids were placed at 5 km intervals in the SVLFE activity area, and the amplitude of the Fourier spectrum at the M2 period(12.420601 h) was calculated for each grid, if the radius of the circle containing 50 data points centered on the grid is 10 km or less.
As a result, the tidal sensitivity was low in the central to southeastern part of the active region and high in the northeastern part of the active region for the entire analysis period. On the other hand, the tidal sensitivity for each large-scale activity was different: the large-scale activities in 2017-2018, 2019, 2021, and 2023, in the latter half of the entire analysis period, showed high sensitivity in the central to southeastern part of the active region, which showed low tidal sensitivity for the entire analysis period. Such a temporal variation in tidal sensitivity is thought to correspond to the temporal variation in the state of the fault plane, which is the source region of the SVLFE. In the presentation, duration and rapid migration will be compared with the tidal sensitivity.
Acknowledgements: We would like to thank the staff of the National Research Institute for Earth Science and Disaster Prevention for providing us with data from the F-net broadband seismograph network.