3:30 PM - 3:45 PM
[SSS11-01] High-density estimation of site amplification characteristics from DAS observation along Route 4
Keywords:DAS, Site Amplification, Dense Observation
In the past few years, dense seismic observations using the distributed acoustic sensing (DAS) and fiber optic cables have been used in seismology. Compared to the conventional observation using seismometers, the cost of the DAS observation is low. Only one DAS equipment and a pre-existing telecommunication cable are required for the DAS observation. DAS observations have been used for the shallow-structure estimations and the hypocenter determination by using the record obtained in a high spatial density. Site amplification characteristics estimated from dense DAS records at a volcano was found to be in good agreement with the geological structure [Nishimura et al., 2021]. In this study, we report the site amplification characteristics calculated from the DAS observation with the fiber optic cable deployed along Route 4.
We installed the DAS equipment at Furukawa Maintenance Branch Office of Ministry of Land, Infrastructure, Transport and Tourism. The fiber optic cable is lined to the south along Route 4. The axial strain rates along the 50 km long fiber with an interval of 5m are recorded with a sampling rate of 500 Hz. The observation period is 1 month from 10 January 2020. We analyze 9 earthquakes whose magnitudes range from 2.7 to 4.8. Their hypocenters are widely distributed from Ibaraki to Hokkaido. The effect of the traffic noise can be reduced by taking the average of the results from different earthquakes.
Bandpass filters of 0.5-1, 1-2, 2-4, 4-8 and 8-16 are applied to the strain rate at each channel. The RMS amplitude of the strain rate in a 10 s time window starting from 2 s before the arrival of S-wave is calculated at each frequency range. The RMS amplitude is normalized so that its average over the channels to be one and then the average RMS amplitude over the earthquakes is calculated. We apply similar processing to P-wave as well, but the available number of earthquakes is only one.
The difference in the spatial distribution of the RMS amplitude among the earthquakes is not significant. This means that the RMS amplitude reflects the effect of the heterogeneity near the site. The large RMS amplitudes are observed at channels 10 km from the southern end of the cable. This may be affected by the road construction and the low signal to noise ratio due to the large distance from the interrogator. The local variation in the RMS amplitude ranges from 1/3 to 3. The large-scale spatial distribution of the RMS amplitude coincides with that of the amplification factor between the engineering bedrock with Vs=400 m/s and the surface provided by NIED Japan Seismic Hazard Information Station (J-SHIS). The large RMS amplitude and the large amplification areas are located in Osaki city, Taiwa town and around Sendai city. However, small-scale differences exist between the RMS amplitude and the J-SHIS amplification factors are different and the variation scale of the RMS amplitude is smaller than that of the amplification factor. The result from P-wave shows the same characteristics as that of the S-wave. The spatial power spectrum density of the RMS amplitude exhibits the power-law decay with the exponent of 3 at the high wavenumber range.
In this study, we have shown the possibility to estimate the site amplification characteristics in a very high spatial density from the DAS observation. We will investigate the small-scale variation of the site amplification in more detail by taking into account the effect of the coupling between the cable and the ground. The output of the DAS observation is the single component of the axial strain rate along the cable. Since the strain is the spatial derivative of the translational signal, the sensitivity to the small-scale heterogeneity of the structure can be strong. It is necessary to examine the effect of the small-scale heterogeneity on wave propagation in terms of the strain.
Acknowledgement:
DAS observation was supported by Ministry of Land, Infrastructure, Transport and Tourism, Tohoku Regional Development Bureau, Sendai Office of River and National Highway.
We installed the DAS equipment at Furukawa Maintenance Branch Office of Ministry of Land, Infrastructure, Transport and Tourism. The fiber optic cable is lined to the south along Route 4. The axial strain rates along the 50 km long fiber with an interval of 5m are recorded with a sampling rate of 500 Hz. The observation period is 1 month from 10 January 2020. We analyze 9 earthquakes whose magnitudes range from 2.7 to 4.8. Their hypocenters are widely distributed from Ibaraki to Hokkaido. The effect of the traffic noise can be reduced by taking the average of the results from different earthquakes.
Bandpass filters of 0.5-1, 1-2, 2-4, 4-8 and 8-16 are applied to the strain rate at each channel. The RMS amplitude of the strain rate in a 10 s time window starting from 2 s before the arrival of S-wave is calculated at each frequency range. The RMS amplitude is normalized so that its average over the channels to be one and then the average RMS amplitude over the earthquakes is calculated. We apply similar processing to P-wave as well, but the available number of earthquakes is only one.
The difference in the spatial distribution of the RMS amplitude among the earthquakes is not significant. This means that the RMS amplitude reflects the effect of the heterogeneity near the site. The large RMS amplitudes are observed at channels 10 km from the southern end of the cable. This may be affected by the road construction and the low signal to noise ratio due to the large distance from the interrogator. The local variation in the RMS amplitude ranges from 1/3 to 3. The large-scale spatial distribution of the RMS amplitude coincides with that of the amplification factor between the engineering bedrock with Vs=400 m/s and the surface provided by NIED Japan Seismic Hazard Information Station (J-SHIS). The large RMS amplitude and the large amplification areas are located in Osaki city, Taiwa town and around Sendai city. However, small-scale differences exist between the RMS amplitude and the J-SHIS amplification factors are different and the variation scale of the RMS amplitude is smaller than that of the amplification factor. The result from P-wave shows the same characteristics as that of the S-wave. The spatial power spectrum density of the RMS amplitude exhibits the power-law decay with the exponent of 3 at the high wavenumber range.
In this study, we have shown the possibility to estimate the site amplification characteristics in a very high spatial density from the DAS observation. We will investigate the small-scale variation of the site amplification in more detail by taking into account the effect of the coupling between the cable and the ground. The output of the DAS observation is the single component of the axial strain rate along the cable. Since the strain is the spatial derivative of the translational signal, the sensitivity to the small-scale heterogeneity of the structure can be strong. It is necessary to examine the effect of the small-scale heterogeneity on wave propagation in terms of the strain.
Acknowledgement:
DAS observation was supported by Ministry of Land, Infrastructure, Transport and Tourism, Tohoku Regional Development Bureau, Sendai Office of River and National Highway.