9:53 AM - 10:15 AM
[STT42-04] Seismic observation around the Tsugaru Strait by distributed acoustic sensing
★Invited Papers
Keywords:Distributed Acoustic Sensing, Tsugaru Strait
In offshore areas, the number of permanent seismic stations is smaller than in inland areas; therefore, the detection capability and the resolution of locations of offshore earthquakes are low. Recently, distributed acoustic sensing (DAS) measurement, which uses an optic fiber cable as a high-density strain rate sensor, has been widely used for seismological observations. DAS measurement using offshore fiber-optic cable has the potential for seismological observation in the offshore area at a high resolution. In this study, we conducted DAS observation using a fiber-optic cable near the Tsugaru Strait.
The DAS observation is conducted by AP Sensing (model N5200A) from October 11, 2022, to February 8, 2023. In the period of October to December 2022, approximately 500 earthquakes listed in the earthquake catalog of the Japan Meteorological Agency (JMA) were observed (Example is shown in Figure 1). The DAS measurement can observe earthquakes with magnitudes smaller than 1 (the minimum magnitude is 0.6) in the Tsugaru Strait and some small earthquakes that are not listed in the JMA catalog; therefore, the DAS measurement shows high seismic detection capability near the cable.
For approximately 50 earthquakes within a radius of 130 km from the center of the cable, we manually picked the arrivals of P- and S-waves in channel intervals of 900 m and located their hypocenters with an earthquake location program, hypomh (Hirata and Matsu'ura, 1987). Travel times were calculated by using the JMA one-dimensional velocity structure model (Ueno et al., 2002) and corrected based on the differences between calculated and observed travel times. The hypocenters of events near the cable were located near the hypocenters of the JMA catalog at km resolution. For these earthquakes, there is a negative correlation between hypocentral distances and the maximum strain rate amplitudes of S-waves. We now investigate the relationship between the magnitudes and strain rate amplitudes of DAS measurements.
Figure 1. Example of space-time plot of strain rates of an earthquake in the Tsugaru Strait observed by the DAS measurement.
The DAS observation is conducted by AP Sensing (model N5200A) from October 11, 2022, to February 8, 2023. In the period of October to December 2022, approximately 500 earthquakes listed in the earthquake catalog of the Japan Meteorological Agency (JMA) were observed (Example is shown in Figure 1). The DAS measurement can observe earthquakes with magnitudes smaller than 1 (the minimum magnitude is 0.6) in the Tsugaru Strait and some small earthquakes that are not listed in the JMA catalog; therefore, the DAS measurement shows high seismic detection capability near the cable.
For approximately 50 earthquakes within a radius of 130 km from the center of the cable, we manually picked the arrivals of P- and S-waves in channel intervals of 900 m and located their hypocenters with an earthquake location program, hypomh (Hirata and Matsu'ura, 1987). Travel times were calculated by using the JMA one-dimensional velocity structure model (Ueno et al., 2002) and corrected based on the differences between calculated and observed travel times. The hypocenters of events near the cable were located near the hypocenters of the JMA catalog at km resolution. For these earthquakes, there is a negative correlation between hypocentral distances and the maximum strain rate amplitudes of S-waves. We now investigate the relationship between the magnitudes and strain rate amplitudes of DAS measurements.
Figure 1. Example of space-time plot of strain rates of an earthquake in the Tsugaru Strait observed by the DAS measurement.