A seafloor-cable seismic and tsunami observation system is ideal for marine geophysical observation because the data can be obtained in real-time. We have developed a new compact seafloor cable seismic and tsunami observation system using Information and Communication Technology (ICT) since 2005. Our new system secures reliability by using Transmission Control Protocol/Internet Protocol technology and provides observational flexibility via observation nodes with a software-based system using up-to-date electronics technology. These features contribute to cost reduction and production sustainability. The system was installed on the Pacific Ocean floor off Sanriku, northeast Japan, in September 2015, in the source area of the 2011 Tohoku-oki earthquake. Our purpose is to better monitor seismic activity and to observe tsunami activity through spatially dense observation. The obtained high quality seismic and pressure data are stored continuously by the new system.