17:15 〜 19:15
[SCG45-P30] Delayed activation of shallow tremors in the Nankai Trough after the M 7.6 Noto Peninsula earthquake
キーワード：スロー地震、浅部微動、能登半島地震、分布型音響センシング
Slow earthquakes are often activated after distant large earthquakes. The investigation of slow earthquakes triggered by large earthquakes is important from the viewpoint of their mechanism. Off the Cape Muroto area near the Nankai Trough, where shallow slow earthquake activity has been reported, distributed acoustic sensing (DAS) measurement, which can measure the temporal change in strain or strain rate along a fiber-optic cable, has been conducted continuously. In this area, an offshore seismic network DONET2 has also been operated. On January 1st, 2024, 13 tremor signals were observed by DAS and DONET2 data after the M7.6 Noto Peninsula earthquake. This activity started approximately three hours after the mainshock of the Noto Peninsula earthquake and continued for nine hours.
To locate these tremors, we utilized the envelope correlation and amplitude attenuation property with distance. First, we constrained the epicenters of the tremors based on the envelope correlation method. Then, we searched for the depth by combining the amplitude attenuation property and envelope correlation. We examined the relationship between the hypocentral distance and the maximum envelope amplitudes of tremors in 2022 located by Baba et al. (2023) in DONET2 stations. Then, we grid-searched the depth where the attenuation property can be explained by the relationship between the hypocentral distance and the amplitudes of tremors in 2022. These tremors were located on the western edge of the slow earthquake area suggested in previous studies. The broadband seismometers of some stations in DONET2 did not work correctly for a while after the Noto Peninsula earthquake probably due to the strong motion of the earthquake. However, DAS was not affected by the strong motion and worked continuously. Therefore, DAS can be an effective technology in monitoring seismic activity after a large earthquake.
Triggered tremors are often correlated with the phases of the surface waves of teleseismic events; therefore, triggered tremors are considered to be caused by the oscillation of surface waves. However, the tremors after the Noto Peninsula earthquake occurred long after the passage of surface waves of the mainshock. We interpret this tremor activity based on the clock-advance model (Gomberg et al., 1997). The transient stress change caused by the Noto Peninsula earthquake may have advanced the tremor activity from when it would have occurred due to static stress loading alone. This tremor activity seems to be a spatiotemporally smaller case of slow earthquake activity in the same area after the 2016 Kumamoto earthquake. Therefore, this area may be sensitive to stress changes due to distant large earthquakes.
To locate these tremors, we utilized the envelope correlation and amplitude attenuation property with distance. First, we constrained the epicenters of the tremors based on the envelope correlation method. Then, we searched for the depth by combining the amplitude attenuation property and envelope correlation. We examined the relationship between the hypocentral distance and the maximum envelope amplitudes of tremors in 2022 located by Baba et al. (2023) in DONET2 stations. Then, we grid-searched the depth where the attenuation property can be explained by the relationship between the hypocentral distance and the amplitudes of tremors in 2022. These tremors were located on the western edge of the slow earthquake area suggested in previous studies. The broadband seismometers of some stations in DONET2 did not work correctly for a while after the Noto Peninsula earthquake probably due to the strong motion of the earthquake. However, DAS was not affected by the strong motion and worked continuously. Therefore, DAS can be an effective technology in monitoring seismic activity after a large earthquake.
Triggered tremors are often correlated with the phases of the surface waves of teleseismic events; therefore, triggered tremors are considered to be caused by the oscillation of surface waves. However, the tremors after the Noto Peninsula earthquake occurred long after the passage of surface waves of the mainshock. We interpret this tremor activity based on the clock-advance model (Gomberg et al., 1997). The transient stress change caused by the Noto Peninsula earthquake may have advanced the tremor activity from when it would have occurred due to static stress loading alone. This tremor activity seems to be a spatiotemporally smaller case of slow earthquake activity in the same area after the 2016 Kumamoto earthquake. Therefore, this area may be sensitive to stress changes due to distant large earthquakes.