TSUNAMI WAVES DETECTED BY FIBER OPTIC STRAINMETERS DURING THE OFF TORISHIMA ISLAND EARTHQUAKES

Abstract

 A series of unknown magnitude submarine earthquakes off Torishima Island, Japan, on 08 October 2023, constituted a unique episodic event which appears to have triggered a tsunami. Signals from this event were recorded on three 200-m long fibers deployed by JAMSTEC on the seafloor (2,300 m deep) in the Nankai Trough. Microscopic changes of fiber length are measured using an optical interferometry technique based on phase difference of two fibers from a reference one. Real-time data acquisition from the fiber optic strainmeters is achieved through connection to the DONET observatory. During the above mentioned episodic event, the fiber optic strainmeters observed 14 hydroacoustic signals, followed by a dispersive tsunami wave. The two microstrain amplitudes were measured at 0.07 ∼ 0.2 µε and 0.003 µε, respectively. The closer DONET pressure gauge, located approximately 3 km away, observed a 1.4 hPa tsunami wave.