10:45 AM - 11:00 AM
[SSS11-13] Long-period incident waves to the Echigo basin during the 2011 off the Pacific coast of Tohoku Earthquake
Keywords:Rayleigh wave, Coherence analysis, Attenuation curve
During the 2011 off the Pacific coast of Tohoku Earthquake, the long-period ground motion was observed in the Echigo basin, Japan, and Rayleigh waves were predominant in periodic zones with a period of about 10 seconds or more [Uetake et al. (2016). )]. To know the characteristics of the incident wave are essential to consider the response of the basin structure. This report confirms whether the Rayleigh wave was predominant in the incident wave during the 2011 off the Pacific coast of Tohoku Earthquake.
To examine the incident wavefield to the Echigo basin, we selected 23 strong motion observation stations from the eastern part of Fukushima prefecture facing the Pacific Ocean to the Chuetsu region of Niigata prefecture. All of them are the observation stations of K-NET and KiK-net of the National Institute for Earth Science and Disaster Resilience and are lined up from east to west within the epicenter distance of the 2011 event in the range of 174 km to 347 km.
First, the Fourier spectrum characteristics of acceleration data were investigated. The spectral amplitude of the high-frequency band changes significantly due to the influence of ground characteristics, and at the observation point located west of the volcanic front, the spectral amplitude sharply decreases as the frequency increases. On the other hand, no significant change was observed at low frequencies below 0.1 Hz. Also, similarities were found in the spectral shapes of UD and EW in this frequency band.
Velocity waveforms were created by integrating the acceleration waveforms with a bandpass filter with a frequency of 0.05 to 0.2 Hz. Two wave groups propagating from east to west were observed in the EW- and the UD-component, continuous with the wave groups found in the Echigo Basin records. The apparent velocities of these two wave groups were approximately 3.4 km/s. When the particle locus was drawn with the EW- and the UD-component at observation stations other than the eastern part of Fukushima Prefecture, both wave groups have retrograde rotations characteristic of Rayleigh waves. The waveform of the NS component is different from that of the EW- and the UD-component. Next, the distance attenuation characteristics of the maximum value (PGV) of the velocity waveform concerning the epicenter distance (r) were investigated. For the UD-and EW-components, the attenuation curve of PGV shows the surface wave's attenuation characteristics. For the NS-component, the attenuation curve shows the body wave's attenuation characteristics.
Coherence analysis between the components was performed for each observation station. The coherence was calculated for 300 seconds from the head of the data. Focusing on the frequency of 0.05 to 0.1 Hz, the coherence between the UD-component and the EW- component was the highest, and the phase shift was about 90 degrees at many observation points. Also, coherence analysis was performed for 80 seconds from the start of recording and for the next 80 seconds, taking into account the two wave groups' appearance times confirmed in the velocity waveform, and similar results were obtained. Different results were obtained at the observation stations in the eastern part of Fukushima Prefecture.
It is considered that the Rayleigh wave component was predominant in the seismic motion with a period of 10 seconds or more that was incident on the Echigo Plain during the 2011 off the Pacific coast of Tohoku Earthquake. At least the seismic motions of the UD and EW components should be considered as Rayleigh waves propagating westward. The NS component is considered to be mainly body waves from the viewpoint of distance attenuation.
Acknowledgments: We used the strong motion data from K-NET and KiK-net of the National Research Institute for Earth Science and Disaster Prevention.
To examine the incident wavefield to the Echigo basin, we selected 23 strong motion observation stations from the eastern part of Fukushima prefecture facing the Pacific Ocean to the Chuetsu region of Niigata prefecture. All of them are the observation stations of K-NET and KiK-net of the National Institute for Earth Science and Disaster Resilience and are lined up from east to west within the epicenter distance of the 2011 event in the range of 174 km to 347 km.
First, the Fourier spectrum characteristics of acceleration data were investigated. The spectral amplitude of the high-frequency band changes significantly due to the influence of ground characteristics, and at the observation point located west of the volcanic front, the spectral amplitude sharply decreases as the frequency increases. On the other hand, no significant change was observed at low frequencies below 0.1 Hz. Also, similarities were found in the spectral shapes of UD and EW in this frequency band.
Velocity waveforms were created by integrating the acceleration waveforms with a bandpass filter with a frequency of 0.05 to 0.2 Hz. Two wave groups propagating from east to west were observed in the EW- and the UD-component, continuous with the wave groups found in the Echigo Basin records. The apparent velocities of these two wave groups were approximately 3.4 km/s. When the particle locus was drawn with the EW- and the UD-component at observation stations other than the eastern part of Fukushima Prefecture, both wave groups have retrograde rotations characteristic of Rayleigh waves. The waveform of the NS component is different from that of the EW- and the UD-component. Next, the distance attenuation characteristics of the maximum value (PGV) of the velocity waveform concerning the epicenter distance (r) were investigated. For the UD-and EW-components, the attenuation curve of PGV shows the surface wave's attenuation characteristics. For the NS-component, the attenuation curve shows the body wave's attenuation characteristics.
Coherence analysis between the components was performed for each observation station. The coherence was calculated for 300 seconds from the head of the data. Focusing on the frequency of 0.05 to 0.1 Hz, the coherence between the UD-component and the EW- component was the highest, and the phase shift was about 90 degrees at many observation points. Also, coherence analysis was performed for 80 seconds from the start of recording and for the next 80 seconds, taking into account the two wave groups' appearance times confirmed in the velocity waveform, and similar results were obtained. Different results were obtained at the observation stations in the eastern part of Fukushima Prefecture.
It is considered that the Rayleigh wave component was predominant in the seismic motion with a period of 10 seconds or more that was incident on the Echigo Plain during the 2011 off the Pacific coast of Tohoku Earthquake. At least the seismic motions of the UD and EW components should be considered as Rayleigh waves propagating westward. The NS component is considered to be mainly body waves from the viewpoint of distance attenuation.
Acknowledgments: We used the strong motion data from K-NET and KiK-net of the National Research Institute for Earth Science and Disaster Prevention.