5:15 PM - 6:45 PM
[SSS10-P02] Long-period strong ground motion in the Tokyo Metropolitan area during earthquakes in the Noto Peninsula area
Keywords:Long-period strong ground motion, Coast area of the Tokyo Bay, The 2024 Noto Peninsula earthquake, Response Spectrum
Introduction
The 2024 Noto Peninsula earthquake was a significant event with Mj7.6 (Mw7.4). During the event, seismic intensity 2 - 3 and long-period ground motion class 2 were observed in the Tokyo Metropolitan area, over 300 km from the epicenter. Since this event triggered many strong-motion seismometers, this is a valuable opportunity to investigate long-period strong ground motion in the Tokyo Metropolitan area. Furthermore, since 2006, seismic observations have been conducted at thermal power plants on the Tokyo Bay coast using a broadband velocity strong-motion seismograph (VSE-355G3). Using those records, we compared this earthquake with past large earthquakes and earthquakes of Mj6 or higher that occurred around the Noto Peninsula.
The records on the Tokyo Bay coast
The velocity records obtained at a thermal power plant on the Tokyo Bay coast show a waveform with a maximum velocity amplitude of 3 to 5 cm/s, a long duration, and a frequency-dispersed waveform. The duration tends to be longer on the east coast than the west coast, and frequency dispersion is also progressing. The velocity response spectrum (h=5%) has a significant amplitude between 5 and 10 s at all observation points. In addition, compared with records of large earthquakes obtained in the past, the response amplitude of around 10 seconds was the third highest. The first is the 2011 off the Pacific Coast of Tohoku Earthquake (Mw9), and the second is its largest aftershock (off the coast of Ibaraki Prefecture, Mj7.6).
Spatial variation of long-period strong ground motion
The pseudo-velocity response spectrum was calculated using records of thermal power stations, records of the Japan Meteorological Agency, and records of K-NET and KiK-net of the National Research Institute for Earth Science and Disaster Prevention, and the geometric mean of the two horizontal components was plotted on a map. [See figure].
At 5 and 7 seconds, observation points with large response values extend from the Tokyo Bay coast to eastern Saitama Prefecture. In 5 seconds, it is large in the northeastern part of Saitama Prefecture, and in 7 seconds, it is large at the border between Tokyo and Chiba Prefecture. With a period of 10 seconds, it is large around Chiba City.
Comparison of earthquakes that occurred around the Noto Peninsula
Using the records obtained at the thermal power plant, we compared the records of the 2024 Noto Peninsula earthquake with those of four earthquakes around the Noto Peninsula. The target earthquakes are (1) March 25, 2007 (Mj6.9), (2) May 5, 2023, 14:42 (Mj6.5), (3) May 5, 2023, 21:58 (Mj5.9), (4) January 9, 2024, 17:59 (Mj6.1). The epicenter distance of these earthquakes is 300-350 km. As seen from the Tokyo Bay coast, it can be assumed that the seismic waves are coming from almost the same direction.
The logarithm of the maximum velocity amplitude due to these earthquakes is approximately proportional to the magnitude, indicating that the earthquake scale has a significant influence. In addition, the velocity waveforms (1), (2), and (3) are very similar to the 2024 event, and it is thought that the influence of propagation and site effects is significant. However, the near Sado event [(4)] has a slightly larger amplitude, and the waveform of the later arrivals is different.
Comparing the velocity Fourier spectra, we see differences between earthquakes at frequencies higher than 0.1 Hz. The earthquake (1) has a step around 0.4 Hz, and the amplitude on the high-frequency side is large. In the earthquake (3), the spectrum on the higher frequency side than 0.5 Hz is large. The earthquake in (4) appears to get louder from around 0.2 Hz. Furthermore, when looking at the velocity response spectrum (h=5 %), earthquake (1) has a clear peak with a period of 2 seconds, and earthquake (3) has a peak around 1.5 seconds in the period. Earthquake (4) does not have a clear peak, but the spectral amplitude is relatively large.
The 2024 Noto Peninsula earthquake was a significant event with Mj7.6 (Mw7.4). During the event, seismic intensity 2 - 3 and long-period ground motion class 2 were observed in the Tokyo Metropolitan area, over 300 km from the epicenter. Since this event triggered many strong-motion seismometers, this is a valuable opportunity to investigate long-period strong ground motion in the Tokyo Metropolitan area. Furthermore, since 2006, seismic observations have been conducted at thermal power plants on the Tokyo Bay coast using a broadband velocity strong-motion seismograph (VSE-355G3). Using those records, we compared this earthquake with past large earthquakes and earthquakes of Mj6 or higher that occurred around the Noto Peninsula.
The records on the Tokyo Bay coast
The velocity records obtained at a thermal power plant on the Tokyo Bay coast show a waveform with a maximum velocity amplitude of 3 to 5 cm/s, a long duration, and a frequency-dispersed waveform. The duration tends to be longer on the east coast than the west coast, and frequency dispersion is also progressing. The velocity response spectrum (h=5%) has a significant amplitude between 5 and 10 s at all observation points. In addition, compared with records of large earthquakes obtained in the past, the response amplitude of around 10 seconds was the third highest. The first is the 2011 off the Pacific Coast of Tohoku Earthquake (Mw9), and the second is its largest aftershock (off the coast of Ibaraki Prefecture, Mj7.6).
Spatial variation of long-period strong ground motion
The pseudo-velocity response spectrum was calculated using records of thermal power stations, records of the Japan Meteorological Agency, and records of K-NET and KiK-net of the National Research Institute for Earth Science and Disaster Prevention, and the geometric mean of the two horizontal components was plotted on a map. [See figure].
At 5 and 7 seconds, observation points with large response values extend from the Tokyo Bay coast to eastern Saitama Prefecture. In 5 seconds, it is large in the northeastern part of Saitama Prefecture, and in 7 seconds, it is large at the border between Tokyo and Chiba Prefecture. With a period of 10 seconds, it is large around Chiba City.
Comparison of earthquakes that occurred around the Noto Peninsula
Using the records obtained at the thermal power plant, we compared the records of the 2024 Noto Peninsula earthquake with those of four earthquakes around the Noto Peninsula. The target earthquakes are (1) March 25, 2007 (Mj6.9), (2) May 5, 2023, 14:42 (Mj6.5), (3) May 5, 2023, 21:58 (Mj5.9), (4) January 9, 2024, 17:59 (Mj6.1). The epicenter distance of these earthquakes is 300-350 km. As seen from the Tokyo Bay coast, it can be assumed that the seismic waves are coming from almost the same direction.
The logarithm of the maximum velocity amplitude due to these earthquakes is approximately proportional to the magnitude, indicating that the earthquake scale has a significant influence. In addition, the velocity waveforms (1), (2), and (3) are very similar to the 2024 event, and it is thought that the influence of propagation and site effects is significant. However, the near Sado event [(4)] has a slightly larger amplitude, and the waveform of the later arrivals is different.
Comparing the velocity Fourier spectra, we see differences between earthquakes at frequencies higher than 0.1 Hz. The earthquake (1) has a step around 0.4 Hz, and the amplitude on the high-frequency side is large. In the earthquake (3), the spectrum on the higher frequency side than 0.5 Hz is large. The earthquake in (4) appears to get louder from around 0.2 Hz. Furthermore, when looking at the velocity response spectrum (h=5 %), earthquake (1) has a clear peak with a period of 2 seconds, and earthquake (3) has a peak around 1.5 seconds in the period. Earthquake (4) does not have a clear peak, but the spectral amplitude is relatively large.