Seismic observation using distributed acoustic sensing around the Tsugaru Strait at the Japan and Kuril Trenches, northeastern Japan

Abstract

Abstract As megathrust earthquakes often have source areas in offshore regions, offshore seismic observations are important. However, the detection capability and resolution of offshore earthquake locations are low owing to the small number of permanent offshore seismic stations. Recently, distributed acoustic sensing (DAS) measurements, which use a fiber-optic cable as a high-density strain rate sensor, have been used for seismic observations. To evaluate the detectability of earthquakes using DAS measurements, locate earthquakes near the cable, and derive the empirical relationship between the magnitude and DAS S-wave strain rate amplitude, we conducted DAS measurements for 4 months using an offshore fiber-optic cable in the Tsugaru Strait, where various types of earthquakes were observed. In this observation, some earthquakes with magnitudes smaller than one or not listed in the earthquake catalog by the Japan Meteorological Agency (JMA) were observed. This suggests a high seismic detection capability for DAS measurements near the cable. We located earthquakes in the Tsugaru Strait by manually picking the arrivals of P- and S-waves. The hypocenters of events near the cable were located near those of the JMA catalog at a kilometer resolution; therefore, DAS data have the potential to locate earthquakes near the cable. In this study, an equation related to the maximum S-wave strain rate amplitude, hypocentral distance, and earthquake magnitude was derived. When the hypocentral distance increased by one order, the amplitude of the S-wave strain rate decreased by approximately 1.8 orders. This attenuation was larger than that derived mainly from inland DAS data in previous studies, which may be due to the difference in scattering or intrinsic attenuation between the inland and offshore regions. Using the derived equation, the magnitude of an earthquake can be estimated using the DAS data. We compared the S-wave amplitudes of the DAS strain rate and the acceleration of the permanent inland stations. The relationship between these two amplitudes is comparable to an apparent S-wave velocity of approximately 1500 m/s in the sediment. Graphical Abstract

Keywords

• Distributed acoustic sensing
• Tsugaru Strait
• Offshore earthquakes
• Magnitude