4:00 PM - 4:15 PM
[SSS11-03] Comparison of the records of the three Hyuganada events observed in the Tokyo Bay area
Keywords:Long-period strong ground motion, Velocity response spectrum, Earthquakes in Nankai Trough, Surface wavs
1. Introduction
Since 2006, we have been conducting observations using broadband velocity seismometers at thermal power plants in the Tokyo Bay area to observe the long-period ground motions from the earthquakes in the Nankai Trough. In recent years, large-magnitude earthquakes have occurred around the Hyuga-nada area. An Mj7.1 inter-plate earthquake occurred on August 8, 2024, and an Mj6.6 inter-plate earthquake occurred on January 13, 2025. Additionally, on January 22, 2022, an Mj6.6 intra-slab earthquake occurred. We will conduct a comparative analysis of records obtained in the Tokyo Bay area from these earthquakes. We will also compare the waveforms of the Bungo Channel intra-slab earthquake Mj6.6 on April 17, 2024, and the 2016 Kumamoto earthquake Mj7.3.
2. Velocity waveforms observed in the Tokyo Bay area
The figure shows the waveforms and velocity response spectra (h=5%) obtained at Kawasaki Thermal Power Station (KWS) in Kanagawa Prefecture for the three earthquakes in the Hyuga-nada area. Earthquake observation uses a velocity sensor (VSE355-G3) and a digital recording device (CV570) manufactured by Tokyo Sokushin. Due to trigger observation, the onset of the seismic waves is missing, but a long-time record has been obtained.
First, we will compare the waveforms of the earthquake on August 8, 2024 (Mj7.1, depth (H=)31 km, epicenter distance(R=) 855 km) and the earthquake on January 13, 2025 (Mj6.6, H=36 km, R=865 km). These two earthquakes are reverse fault earthquakes at plate boundaries, and their epicenters are close. The waveforms of the two earthquakes have long periods for all three components. The amplitude of the August 8 event is larger than the January 13 event, but the shape and timing of the appearance of the long-period phases are almost the same. On the other hand, the waveform of the intra-slab earthquake on January 22, 2022 (Mj6.6, H=45 km, R=774 km) has a relatively short period, and no noticeable wave groups are found. Multi-filter analysis shows that the frequencies below 0.1 Hz were predominant in the large amplitude wave packets of the August 2024 and January 2025 earthquakes, and the dispersion is clear. No predominance of components below 0.1 Hz is observed in the January 2022 earthquake waveform.
3. Velocity response spectrum
The velocity response spectra (h= 5 %) of three earthquakes at KWS are shown on the right side of Figure. For the earthquakes in August 2024 and January 2025, a clear peak can be seen at about 15 seconds. In the 2 to 10 seconds period, the August 2024 earthquake is larger than the January 2025 earthquake, but in less than 2 seconds, both earthquakes are almost the same amplitude. On the other hand, the spectrum of the January 2022 earthquake does not show a peak for about 15 seconds. However, the spectral amplitude of 4 seconds or less is about the same for the April 2024 and August 2024 earthquakes.
4. Comparison with records from nearby earthquakes
A comparison was made with earthquake records in the surrounding area of Hyuga-nada. The earthquake in Bungo Channel on April 17, 2024 (Mj6.6, H=39 km, R=723 km) was an intra-slab earthquake. No significant long-period wave groups were observed in the waveforms, and the shape and amplitude of the velocity response spectra were the same as the January 2022 earthquake. The waveform of the 2016 Kumamoto Earthquake (Mj7.3, H=12 km, R=884 km) that occurred in inland Kyushu has a larger velocity amplitude than the August 2024 earthquake (Mj7.1). It is about 10 times the horizontal components and about 3 times the vertical component. However, the duration of later phases is short, and the peak period of the response spectrum is about 10 seconds. The characteristics of the seismic source and the propagation path of the seismic waves influence the shape of the observed seismic waves.
Since 2006, we have been conducting observations using broadband velocity seismometers at thermal power plants in the Tokyo Bay area to observe the long-period ground motions from the earthquakes in the Nankai Trough. In recent years, large-magnitude earthquakes have occurred around the Hyuga-nada area. An Mj7.1 inter-plate earthquake occurred on August 8, 2024, and an Mj6.6 inter-plate earthquake occurred on January 13, 2025. Additionally, on January 22, 2022, an Mj6.6 intra-slab earthquake occurred. We will conduct a comparative analysis of records obtained in the Tokyo Bay area from these earthquakes. We will also compare the waveforms of the Bungo Channel intra-slab earthquake Mj6.6 on April 17, 2024, and the 2016 Kumamoto earthquake Mj7.3.
2. Velocity waveforms observed in the Tokyo Bay area
The figure shows the waveforms and velocity response spectra (h=5%) obtained at Kawasaki Thermal Power Station (KWS) in Kanagawa Prefecture for the three earthquakes in the Hyuga-nada area. Earthquake observation uses a velocity sensor (VSE355-G3) and a digital recording device (CV570) manufactured by Tokyo Sokushin. Due to trigger observation, the onset of the seismic waves is missing, but a long-time record has been obtained.
First, we will compare the waveforms of the earthquake on August 8, 2024 (Mj7.1, depth (H=)31 km, epicenter distance(R=) 855 km) and the earthquake on January 13, 2025 (Mj6.6, H=36 km, R=865 km). These two earthquakes are reverse fault earthquakes at plate boundaries, and their epicenters are close. The waveforms of the two earthquakes have long periods for all three components. The amplitude of the August 8 event is larger than the January 13 event, but the shape and timing of the appearance of the long-period phases are almost the same. On the other hand, the waveform of the intra-slab earthquake on January 22, 2022 (Mj6.6, H=45 km, R=774 km) has a relatively short period, and no noticeable wave groups are found. Multi-filter analysis shows that the frequencies below 0.1 Hz were predominant in the large amplitude wave packets of the August 2024 and January 2025 earthquakes, and the dispersion is clear. No predominance of components below 0.1 Hz is observed in the January 2022 earthquake waveform.
3. Velocity response spectrum
The velocity response spectra (h= 5 %) of three earthquakes at KWS are shown on the right side of Figure. For the earthquakes in August 2024 and January 2025, a clear peak can be seen at about 15 seconds. In the 2 to 10 seconds period, the August 2024 earthquake is larger than the January 2025 earthquake, but in less than 2 seconds, both earthquakes are almost the same amplitude. On the other hand, the spectrum of the January 2022 earthquake does not show a peak for about 15 seconds. However, the spectral amplitude of 4 seconds or less is about the same for the April 2024 and August 2024 earthquakes.
4. Comparison with records from nearby earthquakes
A comparison was made with earthquake records in the surrounding area of Hyuga-nada. The earthquake in Bungo Channel on April 17, 2024 (Mj6.6, H=39 km, R=723 km) was an intra-slab earthquake. No significant long-period wave groups were observed in the waveforms, and the shape and amplitude of the velocity response spectra were the same as the January 2022 earthquake. The waveform of the 2016 Kumamoto Earthquake (Mj7.3, H=12 km, R=884 km) that occurred in inland Kyushu has a larger velocity amplitude than the August 2024 earthquake (Mj7.1). It is about 10 times the horizontal components and about 3 times the vertical component. However, the duration of later phases is short, and the peak period of the response spectrum is about 10 seconds. The characteristics of the seismic source and the propagation path of the seismic waves influence the shape of the observed seismic waves.