10:45 〜 12:15
[SSS12-P12] Estimating detail of sediment thickness off Muroto cape from P to S converted wave observed by Distributed Acoustic Sensing
★Invited Papers
キーワード：堆積層、光ファイバーセンシング、地震
Seafloor observation network (for example, ocean bottom seismometer; OBS) contributes to earthquake and tsunami early warning. To improve earthquake early warning (EEW) for offshore earthquakes, it is important to understand the characteristics of strong motions in the offshore area. However, the waveforms at the OBSs are likely to be affected by presence of thick and soft sediments that amplifies the seismic waves and results in large site amplifications.
DAS (Distributed Acoustic Sensing) measures the strain changes along an optical fiber cable using variations of the phase of backscattered laser pulses traveling in the optical fiber cable (e.g., Hartog 2017; Lindsey et al. 2020). DAS is expected as a low-cost and long-term monitoring geophysical tool, and have observed a variety of seismic waveforms (e.g., Biondi et al., 2017; Lindsey et al., 2017; Wang et al., 2018; Ide et al., 2021). DAS is also useful for estimating velocity structures beneath cables, which is estimated from ambient noise records (Dou et al., 2017; Lellouch et al., 2019; Tonegawa et al., 2021).
In this study, we investigated detailed thickness variations of the sediment layer under the sea in the Nankai subduction zone, Japan, using P-wave and P to S converted wave observed by a submarine cable deployed off Cape Muroto. The Muroto cable is 128 km long (Momma et al., 1997). We used a 55-km section of the cable, which extends from the coast, for the DAS observation (Ide et al., 2021; Matsumoto et al., 2021). The DAS data are recorded by AP Sensing (model N5200 A) over a sensing length of 55 km along the cable, with a gauge length of 40.4 m and a sensor spacing of 5.1 m, which included ~10790 channels. Since DAS does not have a vertical component, we calculated the difference in arrival time between P and PS (TPs–TP) using STA/LTA and waveform correlation between channels. Using the Vs structure (Tonegawa et al., 2022) and Vp structure which was converted from Vs using empirical relationships (Brocher, 2005), we estimated the thickness of the sediment that fits TPs–TP for each channel. We focused three earthquakes (M4.0, M4.2, and M4.9) that occurred in Tokushima Prefecture, and estimated the sediment thickness between 2000 and 7000 ch.
Channels from 2000 to 5000 (section 1) had a thickness of about 500-1000 m. It was possible to finely estimate the sediment thickness at approximately 50-meter intervals. Conversely, channels from 5000 to 7000 (section 2) had a thickness of about 1500-2000 m. Furthermore, we estimated the distribution of S-coda amplitude for the three earthquakes in each channel, and found that its variation was inversely correlated with the spatial variation of the sediment thickness obtained from TPs–TP. Section 2 is located at the western edge of Tosa-bae, where sediment is pushed out by a subducted seamount. In addition, it has been confirmed in section 2 that a high velocity zone is rising from deep to shallow depths (Tonegawa et al., 2022), and a high Bouguer anomaly can also be observed there (Kimura et al., 2014). Therefore, we suggest that, in section 2, the bottom boundary of the shallow sedimentary layer may not be sharp and have a velocity gradient, and that the observed Ps waves correspond to a deeper boundary.
DAS (Distributed Acoustic Sensing) measures the strain changes along an optical fiber cable using variations of the phase of backscattered laser pulses traveling in the optical fiber cable (e.g., Hartog 2017; Lindsey et al. 2020). DAS is expected as a low-cost and long-term monitoring geophysical tool, and have observed a variety of seismic waveforms (e.g., Biondi et al., 2017; Lindsey et al., 2017; Wang et al., 2018; Ide et al., 2021). DAS is also useful for estimating velocity structures beneath cables, which is estimated from ambient noise records (Dou et al., 2017; Lellouch et al., 2019; Tonegawa et al., 2021).
In this study, we investigated detailed thickness variations of the sediment layer under the sea in the Nankai subduction zone, Japan, using P-wave and P to S converted wave observed by a submarine cable deployed off Cape Muroto. The Muroto cable is 128 km long (Momma et al., 1997). We used a 55-km section of the cable, which extends from the coast, for the DAS observation (Ide et al., 2021; Matsumoto et al., 2021). The DAS data are recorded by AP Sensing (model N5200 A) over a sensing length of 55 km along the cable, with a gauge length of 40.4 m and a sensor spacing of 5.1 m, which included ~10790 channels. Since DAS does not have a vertical component, we calculated the difference in arrival time between P and PS (TPs–TP) using STA/LTA and waveform correlation between channels. Using the Vs structure (Tonegawa et al., 2022) and Vp structure which was converted from Vs using empirical relationships (Brocher, 2005), we estimated the thickness of the sediment that fits TPs–TP for each channel. We focused three earthquakes (M4.0, M4.2, and M4.9) that occurred in Tokushima Prefecture, and estimated the sediment thickness between 2000 and 7000 ch.
Channels from 2000 to 5000 (section 1) had a thickness of about 500-1000 m. It was possible to finely estimate the sediment thickness at approximately 50-meter intervals. Conversely, channels from 5000 to 7000 (section 2) had a thickness of about 1500-2000 m. Furthermore, we estimated the distribution of S-coda amplitude for the three earthquakes in each channel, and found that its variation was inversely correlated with the spatial variation of the sediment thickness obtained from TPs–TP. Section 2 is located at the western edge of Tosa-bae, where sediment is pushed out by a subducted seamount. In addition, it has been confirmed in section 2 that a high velocity zone is rising from deep to shallow depths (Tonegawa et al., 2022), and a high Bouguer anomaly can also be observed there (Kimura et al., 2014). Therefore, we suggest that, in section 2, the bottom boundary of the shallow sedimentary layer may not be sharp and have a velocity gradient, and that the observed Ps waves correspond to a deeper boundary.