Hydroacoustic Recordings During a Submarine Seismic Swarm near Torishima Island: Comparing CTBT IMS Hydrophones and JAMSTEC Fiber-Optic Strainmeters

On 08 October 2023, a series of submarine earthquakes that took place in the Izu-Ogasawara Islands, Japan, excited at least 14 hydroacoustic signals (i.e., T -waves or H -waves) over a period of three hours. These signals were detected by the Comprehensive Nuclear-Test-Ban Treaty (CTBT) International Monitoring System (IMS) hydrophone station HA11 at Wake Island. Although the amplitude of the signals varied among the packets, the frequency content was commonly broadband, suggesting that explosive phenomena occurred during this series of episodic events. Cross-correlation analysis was performed to determine the direction of arrival (DOA) of the hydroacoustic waves at HA11 and make hypotheses about their origin . The cross-correlation analysis using the hydrophone triplets shows that the two DOAs of hydroacoustic waves were crossing near Sofugan, approximately 100 km south from Torishima Island, where numerous seismic sources were determined by the USGS. The same signals were also detected by seafloor fiber-optic strainmeters in the Nankai Trough, Japan, on the opposite side from the source area compared to HA11. In the band of frequencies up to the maximum frequency achievable by the fiber-optical method, the frequency contents observed by the fiber-optic strainmeters were very similar to those at the IMS hydrophone triplets, supporting the detection capability of hydroacoustic signals by the studied fiber-optical interferometry method. Additionally, a dispersive tsunami wave is identified on the fiber-optic strainmeter recording.