New compact ocean bottom cabled seismometer system deployed in the Japan Sea
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The Japanese islands are positioned near the subduction zones, and large earthquakes have repeatedly occurred in marine areas around Japan. However, the number of permanent earthquake observatories in the oceans is quite limited. It is important for understanding generation of large earthquakes to observe seismic activities on the seafloor just above these seismogenic zones. An ocean bottom cabled seismometer (OBCS) is the best solution because data can be collected in real-time. We have developed a new compact OBCS system. A developed system is controlled by a microprocessor, and signals from accelerometers are 24-bit digitized. Clock is delivered from the global positioning system receiver on a landing station using a simple dedicated line. Data collected at each cabled seismometer (CS) are transmitted using standard Internet Protocol to landing stations. The network configuration of the system adopts two dual methods. We installed the first practical OBCS system in the Japan Sea, where large earthquakes occurred in past. The first OBCS system has a total length of 25 km and 4 stations with 5 km interval. Installation was carried out in August 2010. The CSs and single armored optical submarine cable were buried 1 m below the seafloor to avoid a conflict with fishing activity. The data are stored on a landing station and sent to Earthquake Research Institute, University of Tokyo by using the Internet. After the installation, data are being collected continuously. According to burial of the CSs, seismic ambient noises are smaller than those observed on seafloor.
KeywordsCabled ocean bottom seismometer Japan Sea Earthquake observation Seafloor observation
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.
Concept of new OBCS system
The number of earthquake observatories in the oceans is quite limited on the Earth. Our objective of development is to make observations with a high density array in the marine areas that will be sufficient to achieve the same level of observation as the land-based networks. Because the characteristics of concept of the new OBCS have already been described in detail (Kanazawa and Shinohara 2009; Yamazaki et al. 2012), we summarize its concept here. In original concept, CSs are equipped with optical cables and placed at 20 km spacing. Optical cables are laid for 900 km in maximum with a continuous “S” pattern, by which the 40 CSs are distributed two-dimensionally. The area covered with the conceptual system is comparable with a source region of an earthquake with a magnitude of 8. Because the operational depth is planned to 6,000 m, the system can be available for more than 90 % area of the seafloor. The accuracy of the time stamp must be less than 0.1 ms, which is equivalent to the present accuracy of the land seismic network. The system is expected to have an operational lifetime of more than 20 years. This lifetime corresponds to that of the existing system. The most important objective of developing a new OBCS is low-costs of both production and installation. The size of the CS is a key to achieve this requirement. A smaller CS leads to lower costs for installation. The installation costs by small ordinary ship can be significantly reduced. In addition, the flexibility of measurement is also important for recent multidisciplinary researches. For example, measurement parameters after installation should be changed from the land.
Characteristic of the conceptual network for the OBCS and the land is a doubled ring (Fig. 3 in Yamazaki et al. 2012). This configuration is employed to enable both high reliability and low cost. Ethernet is used as data transmission system for this doubled ring configuration. Data collected with a time stamp at each CS are transmitted using standard IP to landing stations. The landing stations at both end of a cable are equipped with a power supply, a storage system, and access to the Internet. A global positioning system (GPS) clock at each landing station is used as a time reference to synchronize timing of each CS, and is fed to each CS through a dedicated line. Methods of clock transmission have been studied, and it was found that the latest clock synchronization system over Ethernet is sufficient for OBCS. However it was decided that a dedicated optical fiber in addition to optical fibers for Ethernet should be used for clock distribution, because a simple configuration causes high reliability. This dedicated line is also used for control the system of the CS. Under normal operation, the channel for data transmission from each CS to the landing station is chosen to reduce network traffic. When one landing station is not operated for maintenance, the data from all CS can be sent to the other landing station. This operation can be set via IP/Ethernet access from the control station or a landing station. Once an optical fiber submarine cable is broken, data from the CSs can be sent to an individual landing station through an accessible path. The OBCS can be maintained by performance monitoring. The software continuously monitors the status of the system such as the temperature within a pressure vessel, electrical voltages.
Although the existing OBCS consists of hardware only, we decided that a new CS uses a microprocessor for implementation of ICT technologies. This means the new CS is controlled by software which is also capable of processing various measurements. In addition, utilization of software makes the CS compact. Because a central processing unit (CPU) and large scale integration (LSI) can decrease the number of circuits and parts, the cost of the new OBCS can be reduced. In addition, observation parameters can be changed with high reliability from the land. The system and its parts are continuously monitored by software, and we know alert before malfunction such as power down of laser transmitters. We selected Linux operating system (OS) for the OBCS. Although the Linux OS is thought to have enough reliability, the Linux OS can be restarted from landing stations through the dedicated clock line. Additionally, the Linux OS can be accessed from both landing stations using ring configuration of the network.
Development of cabled seismometer for the new OBCS
Deployment of the first system to the Japan Sea
A landing station was constructed on the west coast of the Awashima, and the submairne cable fed into the landing station. The landing station has GPS receivers, cable termination circuits, computer servers and power supplies for the OBCS system, which are mounted a 19-inch computer rack. Because of a small size of equipment for the landing station, a building of the landing station has width of 3.2 m, depth of 2.2 m and a height of 2.7 m with air-conditioner. This small size of the landing station is also contributed to low cost of the system. The landing station is supplying the power with minus voltage compared to that of sea water at a constant current. Supplying current is approximately 0.75 A corresponding to a voltage of 130 V.
The Awashima is linked to the mainland Honshu by microwave radio communication at the present. We found that capacity of the Internet connection is not enough to send all the data from the OBCS system. Therefore the system status of the OBCS and a part of the data are sent to the ERI. The data from the seismometers are decimated at a sampling frequency of 100 Hz for real-time transmission. When a remarkable event occurs, all the data of the event will be able to be retrieved via protocol of the ftp. Commands controlling the system are sent from the ERI.
Seismic records from the OBCS system
The study area corresponds to the source region of the 1964 Niigata earthquake with magnitude of 7.5. The located hypocenters seem to form a dipping plane toward the west at an angle of 34°. From the analysis of focal mechanism of the mainshock, an angle of westward dipping nodal plane is 70° (Aki 1966). In addition, Abe (1975) estimated an angle of the plane is 56° from focal mechanism and geodetic data. The present seismic activity is not thought to relate directly to the seismic activity of the mainshock in 1964. Further continuous observation is needed to reveal a relation between the present seismic activity and the fault plane of the 1964 mainshock.
The new compact OBCS with ICT technology was developed. The application of ICT technologies makes it possible to realize remarkable and new features. ICT has enabled the new OBCS to become more compact and less expensive, and enabled IP access and the upgrade of OBCS for the flexibility and expandability of measurements. The CS of the new compact OBCS can be made so compact since software processes various measurements, while complex and a large amount of hardware are used in the existing OBCS, and lowered the costs for both production and installation. Reliability of the system is kept by using redundant system which is easily constructed using the ICT. The CSs of the OBCS on the seafloor can be accessed through IP protocol from UNIX systems on land. This will provide us an ability of changing measurement parameters of the seismometers, and upgrading the installed firmware of the FPGAs and software in the CSs.
A microprocessor controls the whole system of the CS and TCP/IP is used for communication. The CS has four Ethernet switches which are implemented on FPGA to change a communication path. The system is controlled by Linux OS. Signals from three accelerometers are digitized with a resolution of 24-bit at a sampling frequency of 1 kHz. The clock signal is sent by using GPS from the landing station. A size of the developed CS is 13 cm in diameter and 50 cm long.
The central coastal area of the Japan Sea has large strain rate, and had large earthquakes in past. The first system has a total length of 25 km and 4 CSs with 5 km interval and was deployed in the coastal area of the central part of the Japan Sea, where a large earthquake occurred in 1964. The cable route was decided by using results of marine surveys. The whole system was buried 1 meter below the seafloor to avoid a conflict with fishing activity. The data are stored on a landing station and sent to the ERI, University of Tokyo by using the Internet in real-time.
After the installation, data are being collected continuously. According to burial of the seismometers, seismic ambient noises are smaller than those observed on the seafloor. The level of ambient seismic noise recorded by the deployed OBCS is comparable to a noise floor of the used accelerometer. Wind on the surface affects the noise level for frequency around 1 Hz. We relocated hypocenter of earthquakes occurring near the Awashima using the OBCS data. The depths of the events become 4-10 km shallower than those determined by a land network, because spatially high density observation gives results with high resolution. The system has collected continuous seismic data for three years since the deployment. We will continue to gather data from a view of both scientific and technological researches.
The success of the development and first deployment was made possible by the active co-operation of many scientists, engineers, technicians from various institutions. In particular, the authors express thanks to Drs. H. Utada, Y. Morita, H. Shiobara, K. Mochizuki, Messrs. T. Yagi, Y. Hirata, S. Hashimoto from Earthquake Research Institute, University of Tokyo, Dr. K. Yamazaki from Nagaoka University of Technology, Mr. Y. Shirasaki from Marine Eco Tech Ltd., Mr. J. Kojima from KDDI R&D Labs., Inc., Ms. Y. Jyono, Messrs. K. Yamamoto, H. Kainuma, and S. Chiba from LINK Lab. Inc., Mr. K. Furukawa from Intertechno Co., Ltd., Messrs. R. Morikawa and H. Shirani from OCC Corp., Messrs. N. Fukushima and T. Etoh from KCS Co., Ltd., Dr. K. Asakawa from JAMSTEC, who have all made substantial contributions to the development and deployment of the new OBCS system. We are also grateful to two anonymous reviewers for their critical reviews for improvement of this manuscript. This study is partly supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan, and by the cooperative research program of the Earthquake Research Institute, University of Tokyo. Most of the figures were created using GMT (Wessel and Smith 1991).
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