The Japanese islands are located near the plate convergent zones, where the Pacific plate and Philippine Sea plate subduct below the Japan islands, and large earthquakes have repeatedly occurred around Japan islands. For studying and understanding the generation processes of these large earthquakes, it is important to observe seismic activities on the sea floor just above these seismogenic zones. A recent pop-up type ocean bottom seismometer (OBS) performs over one-year continuous recording. The long-term OBSs (LTOBSs) are mainly used for an array monitoring of seismic activities in the plate convergent region around Japan (Kanazawa et al. 2009). However pop-up type OBS has disadvantages such as limited power, data recovery reliability, off-line observation for long-term seismic observation on the seafloor. Although it is an off-line observation network, a large scale observation array using a number of LTOBSs is a strong tool for studying earthquakes.
Ocean bottom cabled seismometers (OBCSs), where the sensors are equipped in a hermetically-sealed pressure housing and these cases are connected with cables, has many advantages for seafloor seismic observation. Therefore, OBCSs had been developed based on a submarine telecommunication cable system, and have been used over the past 25 years in Japan (e.g. Kanazawa and Hasegawa 1997). However, the OBCS system in the first generation is in-line system and has a small number of seismometers and pressure gauges. For example, an OBCS system installed off Sanriku, Japan, has three seismometers and two pressure gauges.
In 2011, Dense Oceanfloor Network system for Earthquakes and Tsunamis (DONET) (Kaneda et al. 2010) has been installed and started the seafloor observations in the boundary between the source regions of Nankai and Tonanakai earthquakes. The DONET has a main optical fiber cable loop with high reliability and junction boxes connected to the main cable. Various sensors, for example seismometers, pressure gauges, can be connected via the junction box with an underwater mateable connector (UMC). Utilization of the UMCs for scientific sensors enables various type observations and exchanges of the sensors when the sensors have malfunction or upgrade. In addition, a number of the scientific sensors can be increased. The DONET has 20 observation stations. The Neptune Canada regional cabled ocean observatory also has a function of expansion of scientific observations and exchange of sensors (Barnes et al. 2008). However it is difficult to deploy the system quickly, because this type cabled system needs remotely operated vehicle (ROV) for installation or exchange of sensors. Because of complexity of the system, construction and running cost can not be reduced effectively. Since an ROV must manipulate an UMC on a junction box, it is also difficult to bury the whole system below seafloor. This is a problem for avoiding confliction with fishing activity near a coast.
Two cabled ocean-bottom tsunami gauges of the Sanriku OBCS system successfully recorded the tsunami waveform just above the source rupture area of the 2011 off the Pacific coast of Tohoku Earthquake. The tsunami data were essential for estimated the source region of the destructive tsunami by the mainshock (e.g. Fujii et al. 2011; Maeda et al. 2011). Although the existing OBCSs have realized a significant contribution to the study of seismic activity, the number of the equipped seismometers is insufficient for high resolution observations of seismic activities in marine area. After occurrence of the 2011 Tohoku earthquake, it becomes more important to monitor seismic activity and tsunami on the seafloor near source region.
A large problem of the existing OBCS system is construction and running cost. To equip an OBCS system with a sufficient number of seismometers, this problem should be resolved. This is the critical problem in the existing OBCSs. A total cost including production, deployment and maintenance per one observation node for the new system should be less than one third of that for the existing system. In addition to the problem of construction cost, the existing OBCS has become insufficient for multidisciplinary observation and flexibility of measurements after installation. A portable type system is also required, and is expected to be used for the precise monitoring of seismic activity after large earthquakes. To satisfy these requirements, we adopted the system whose observation nodes are directly connected to seafloor optical fiber cable, i.e. the in-line system.
After substantial consideration of interdisciplinary research studies with engineers of various fields, such as ocean engineering, measurement engineering, electronic engineering, mechanical engineering, and information and communication engineering in particular, it was concluded that a new OBCS system using information and communication technologies (ICT) should be developed to resolve the addressed problems, i.e., Internet Protocol (IP) goes to seafloor. According to this concept, we have developed a new OBCS system. The new OBCS system can be assembled compact since a software processes various measurements, while complex and a large amount of hardware are used in the existing OBCSs. Reliability of the system is kept by using redundant system which is easily constructed using the ICT.
There is a tectonic zone where large earthquakes recently occurred in the central coast of the Japan Sea. The new OBCS system was first installed above the source region of the 1964 Niigata earthquake in the Japan Sea in August 2010. Although the deployed OBCS system has a cable length of 25 km and four cabled seismometers (CSs) with 5 km spacing, it has been proven that seismic data can be successfully obtained. In this paper, we describe the characteristics, layout, and system parameters of the developed OBCS system, and the characteristics of the seismic data retrieved from the system, especially the ambient seismic noise levels, and performance of the first installed system.