5:15 PM - 6:45 PM
[SCG54-P10] On prominent hydro-acoustic signals associated with earthquakes near Torishima Island on October 9th 2023 (JST)
Tsunamis of several tens of centimeters in height, thought to have been caused by earthquakes that occurred near Torishima Island between 4:00 a.m. and 7:00 a.m. (JST) on October 9, 2023, reached Izu and Ogasawara Islands, and several places along the south coast of Japan(JMA, 2023a). The amplitude of the observed body waves (P waves and S waves) themselves was small, and the magnitude of the earthquake cannot be determined. They are unique earthquakes in which T-waves are observed in Ogasawara Islands and some on-shore seismometers, and at least 14 similar events have been reported to occur during the same time period (JMA, 2023b). Based on the analysis of T-wave arrival times, it is estimated that the source of these events is around the Sofu Seamount to the west of Sofu Rock, and the existence of a caldera-shaped submarine topography has been confirmed from a seafloor topography survey (JAMSTEC, 2023 ). These reports suggest that these events are related to submarine volcanic activity. Therefore, I analyzed and discussed these hydro-acoustic signals detected by seismometers. First, data of ocean bottom seismometers (OBSs) of S-net installed along the Japan Trench by the National Research Institute for Earth Science and Disaster Prevention (NIED), as well as the vertical component of velocity-type seismometers at land observation points of the Japan Meteorological Agency were used. Both were downloaded from the website of NIED.
Most of the 14 events mentioned above are not listed on the JMA’s list of hypocenters, although this is likely due to the low amplitude of the seismic waves and low accuracy in determining the hypocenters. Therefore, I assume that the epicenter of the series of events is the same, and that the signal source of the hydro-acoustic signal is located at the epicenter of the 05:13 earthquake (M5.2) listed in the same list. The time axis of the observed waveform was shifted by the propagation time (subtracted from the observation time) for each observation point, and a diagram was created in which the waveforms were arranged in order of propagation distance. The sound speed was assumed to be 1472 m/s, which was estimated in Iwase (2022) as the sound speed of hydro-acoustic signals associated with the submarine volcanic eruption at Fukutoku-Oka-No-Ba. Examples of the diagrams created are shown in Fig. 1-3. The horizontal axis corresponds to the elapsed time projected at the source. Fig. 1 is 1 hour from 06:00 JST, and Figs. 2 and 3 are enlarged view of 2 minutes from 06:00 JST and 06:08, respectively. The vertical axis shows the propagation distance (km). In the waveform, the black line is the S-net observation point, and the red line is JMA’s observation point. Three JMA’s observation points are located closest to the signal source: Aogashima (AOGASM), Chichijima (CHIJI3), and Hahajima (HAHAJ2) from the closest to the source. The signals lined up vertically are the observed hydro-acoustic signals. Of the 14 events mentioned above, 6 events are recorded in Fig. 1. In Fig. 2 and Fig. 3, the signal at JMA’s observation point, which is particularly close to the source, appears to be delayed compared to the S-net observation point, and long-period components are predominant. Since JMA’s observation points are located to the north and south of the source, it is unlikely that this delay is due to a shift in the location of the signal source and the epicenter, but it may affect the detection of the T-wave arrival time. Therefore, detailed consideration is required. Furthermore, as seen in Fig. 3, it is suggested that some events are composed of multiple sub-events. In Fig.1, a downward-sloping signal to the right is recorded as a subsequent signal for each event. This is thought to be hydro-acoustic signals reflected from the north of the S-net observation network. I will analyze them in detail.
Acknowledgment
The author would like to thank National Research Institute for Earth Science and Disaster Prevention (NIED, https://doi.org/10.17598/NIED.0007) and Japan Meteorological Agency (JMA), for providing data. The web site of Geographical Survey Institute is used to calculate propagation distances.
References
JMA (2023a), https://www.jma.go.jp/jma/press/2310/09b/kaisetsu202310091100.pdf.
JMA (2023b), https://www.jishin.go.jp/main/suihon/erc391/erc391-4.pdf.
JAMSTEC (2023), https://www.jamstec.go.jp/j/about/press_release/20231121/.
Iwase (2022), Proc. USE2022, 3J1-4.
Most of the 14 events mentioned above are not listed on the JMA’s list of hypocenters, although this is likely due to the low amplitude of the seismic waves and low accuracy in determining the hypocenters. Therefore, I assume that the epicenter of the series of events is the same, and that the signal source of the hydro-acoustic signal is located at the epicenter of the 05:13 earthquake (M5.2) listed in the same list. The time axis of the observed waveform was shifted by the propagation time (subtracted from the observation time) for each observation point, and a diagram was created in which the waveforms were arranged in order of propagation distance. The sound speed was assumed to be 1472 m/s, which was estimated in Iwase (2022) as the sound speed of hydro-acoustic signals associated with the submarine volcanic eruption at Fukutoku-Oka-No-Ba. Examples of the diagrams created are shown in Fig. 1-3. The horizontal axis corresponds to the elapsed time projected at the source. Fig. 1 is 1 hour from 06:00 JST, and Figs. 2 and 3 are enlarged view of 2 minutes from 06:00 JST and 06:08, respectively. The vertical axis shows the propagation distance (km). In the waveform, the black line is the S-net observation point, and the red line is JMA’s observation point. Three JMA’s observation points are located closest to the signal source: Aogashima (AOGASM), Chichijima (CHIJI3), and Hahajima (HAHAJ2) from the closest to the source. The signals lined up vertically are the observed hydro-acoustic signals. Of the 14 events mentioned above, 6 events are recorded in Fig. 1. In Fig. 2 and Fig. 3, the signal at JMA’s observation point, which is particularly close to the source, appears to be delayed compared to the S-net observation point, and long-period components are predominant. Since JMA’s observation points are located to the north and south of the source, it is unlikely that this delay is due to a shift in the location of the signal source and the epicenter, but it may affect the detection of the T-wave arrival time. Therefore, detailed consideration is required. Furthermore, as seen in Fig. 3, it is suggested that some events are composed of multiple sub-events. In Fig.1, a downward-sloping signal to the right is recorded as a subsequent signal for each event. This is thought to be hydro-acoustic signals reflected from the north of the S-net observation network. I will analyze them in detail.
Acknowledgment
The author would like to thank National Research Institute for Earth Science and Disaster Prevention (NIED, https://doi.org/10.17598/NIED.0007) and Japan Meteorological Agency (JMA), for providing data. The web site of Geographical Survey Institute is used to calculate propagation distances.
References
JMA (2023a), https://www.jma.go.jp/jma/press/2310/09b/kaisetsu202310091100.pdf.
JMA (2023b), https://www.jishin.go.jp/main/suihon/erc391/erc391-4.pdf.
JAMSTEC (2023), https://www.jamstec.go.jp/j/about/press_release/20231121/.
Iwase (2022), Proc. USE2022, 3J1-4.