Development of Seafloor Seismic and Tsunami Observation System

The Japanese islands is located at the intersection of several plates and is constantly threatened by earthquakes that occur around trenches. Thus, construction of a seafloor seismic observation network is imperative for early detection and warning of earthquakes in those areas and tsunami that accompany them. We are currently developing a seafloor seismic observation network using optical fiber sensing (OFS) technologies. OFS technologies are used to measure conditions such as strain and temperature by analyzing characteristics of reflected light or back-scattered light which occur in optical fiber when pulsed light is introduced. This system under development has seafloor seismometers (accelerometers) and tsunami detectors as its sensors connected by seafloor optical cable. Those sensors use an OFS technology called fiber Bragg grating (FBG). We have conducted laboratory tests to check basic performance of the seismometers in areas such as sensitivity, frequency characteristics, and durability. We also evaluated seismometers resistance against water pressure and dynamic squeeze when loaded in cases and connected to optical fiber. Satisfactory results were obtained in both cases. Furthermore, we have performed practical tests to confirm shock resistance and operability by actually setting seafloor seismometers on the seafloor using an underwater cable construction ship. In this paper, we provide an overview of the seafloor seismic and tsunami observation system using OFS technology and report the results of tests on seafloor seismometers (accelerometers) up to now.