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[SSS09-04] Evaluation of three-dimensional seismic wave propagation using dense array observation and deep borehole observation
Keywords:Array analysis, Vertical array, Multi-reflection waves
1. Introduction
TEPCO has operated a dense surface array (30 stations at the ground surface) and two deep-borehole seismic observations [S-hole (1022m depth), E-hole (1500m depth)] in the Kashiwazaki-Kariwa Nuclear Power Station after the 2007 Niigata-ken Chuetu-oki earthquake. These records help understand three-dimensional seismic wave propagation characteristics in the deposit layers. This report shows case studies of two Mj4-5 class earthquakes combined with the dense surface array and the deep-borehole seismic observations.
2. Data
The earthquakes studied in this report were the 2020 Niigata Chuetsu earthquake (2020.08.06: Mj5.1, H=186 km) and the 2021 Niigata Chuetsu-oki earthquake (2021.09.24: Mj4.1, H= 16km). Since the epicentral distances are 20-23 km, the incident angles of seismic waves from these events are very different. Velocity waveforms of both earthquakes showed a pulse-like initial S-wave and pulse-like later-arrival waves that appeared 5 to 7 seconds after the initial S-wave.
3. Analysis using dense surface array data
To investigate the arrival directions and apparent velocities of seismic waves, we applied a Semblance analysis [Neidell and Taner (1971)] to the dense surface array data (Fig.1). We used the velocity waveforms at 25 points for the 2020 earthquake and 26 for the 2021 earthquake.
In the 2020 earthquake data, the initial P- and S-waves incident almost vertically. Although the epicenter is in the southeast direction, the S-wave came from the northwest side. It is because the variation of elevation and the thickness variation of the low-velocity surface layers in the site affected the arrival time of seismic waves. The later-arrival wave propagated from the south side with about 2.0 km/s.
In the 2021 earthquake data, the initial P-wave propagated from the epicenter direction with a velocity of 5 km/s, and the initial S-wave and the later-arrival waves came from the West with about 3.0 km/s.
3. Analysis using deep-borehole observation
We tried to identify the up-going wave and the reflection phase (down-going wave) from the ground surface using the deep-borehole observation records of the S-hole and E-hole. We detected the up-going and the down-going waves in the initial P- and S-waves for both earthquake data. The travel time between the borehole bottom and the ground surface was almost the same as the PS logging for the 2020 earthquake and was slightly shorter for the 2021 earthquake.
Next, we examined the later-arrival waves in opposite phases to the initial S-wave. In the 2020 earthquake data, we recognized the up-going and the down-going waves, and the interval between them was shorter than that of the initial S-wave part. The arrival time of the later-arrival wave at the E-hole was later than at the S-hole. As for the 2021 earthquake data, we recognized the up-going and the down-going waves, and the interval was the same as that of the initial S-wave part. These waves appeared 5.6 to 5.7 s after the initial S-wave and corresponded to the round-trip time above the seismic bedrock.
4. Conclusion
We investigated the three-dimensional wave propagation of the initial P-, initial S-, and later-arrival waves in the deep and shallow events from the dense surface array and deep-borehole seismic observations.
In the deep event data, the initial P- and S-wave incident almost vertically, but the later-arrival wave propagated from the south side. The irregular shape of the basement affects the propagation of the reflected waves.
In the shallow earthquake data, the initial P-wave propagated from the epicenter direction, and the initial S-wave and the later-arrival waves came from almost the same direction with the same apparent velocity. We recognized the up-going and the down-going waves in the later-arrival wave part. We considered that the later-arrival wave was a body wave (S-wave) and was the seismic bedrock reflected wave between the epicenter and the observation point.
TEPCO has operated a dense surface array (30 stations at the ground surface) and two deep-borehole seismic observations [S-hole (1022m depth), E-hole (1500m depth)] in the Kashiwazaki-Kariwa Nuclear Power Station after the 2007 Niigata-ken Chuetu-oki earthquake. These records help understand three-dimensional seismic wave propagation characteristics in the deposit layers. This report shows case studies of two Mj4-5 class earthquakes combined with the dense surface array and the deep-borehole seismic observations.
2. Data
The earthquakes studied in this report were the 2020 Niigata Chuetsu earthquake (2020.08.06: Mj5.1, H=186 km) and the 2021 Niigata Chuetsu-oki earthquake (2021.09.24: Mj4.1, H= 16km). Since the epicentral distances are 20-23 km, the incident angles of seismic waves from these events are very different. Velocity waveforms of both earthquakes showed a pulse-like initial S-wave and pulse-like later-arrival waves that appeared 5 to 7 seconds after the initial S-wave.
3. Analysis using dense surface array data
To investigate the arrival directions and apparent velocities of seismic waves, we applied a Semblance analysis [Neidell and Taner (1971)] to the dense surface array data (Fig.1). We used the velocity waveforms at 25 points for the 2020 earthquake and 26 for the 2021 earthquake.
In the 2020 earthquake data, the initial P- and S-waves incident almost vertically. Although the epicenter is in the southeast direction, the S-wave came from the northwest side. It is because the variation of elevation and the thickness variation of the low-velocity surface layers in the site affected the arrival time of seismic waves. The later-arrival wave propagated from the south side with about 2.0 km/s.
In the 2021 earthquake data, the initial P-wave propagated from the epicenter direction with a velocity of 5 km/s, and the initial S-wave and the later-arrival waves came from the West with about 3.0 km/s.
3. Analysis using deep-borehole observation
We tried to identify the up-going wave and the reflection phase (down-going wave) from the ground surface using the deep-borehole observation records of the S-hole and E-hole. We detected the up-going and the down-going waves in the initial P- and S-waves for both earthquake data. The travel time between the borehole bottom and the ground surface was almost the same as the PS logging for the 2020 earthquake and was slightly shorter for the 2021 earthquake.
Next, we examined the later-arrival waves in opposite phases to the initial S-wave. In the 2020 earthquake data, we recognized the up-going and the down-going waves, and the interval between them was shorter than that of the initial S-wave part. The arrival time of the later-arrival wave at the E-hole was later than at the S-hole. As for the 2021 earthquake data, we recognized the up-going and the down-going waves, and the interval was the same as that of the initial S-wave part. These waves appeared 5.6 to 5.7 s after the initial S-wave and corresponded to the round-trip time above the seismic bedrock.
4. Conclusion
We investigated the three-dimensional wave propagation of the initial P-, initial S-, and later-arrival waves in the deep and shallow events from the dense surface array and deep-borehole seismic observations.
In the deep event data, the initial P- and S-wave incident almost vertically, but the later-arrival wave propagated from the south side. The irregular shape of the basement affects the propagation of the reflected waves.
In the shallow earthquake data, the initial P-wave propagated from the epicenter direction, and the initial S-wave and the later-arrival waves came from almost the same direction with the same apparent velocity. We recognized the up-going and the down-going waves in the later-arrival wave part. We considered that the later-arrival wave was a body wave (S-wave) and was the seismic bedrock reflected wave between the epicenter and the observation point.