Geophysical observations at the ocean bottom

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Geophysical observations at the ocean bottom Junzo Kasahara, Earthquake Research Institute, University of Tokyo. 1,1,1, Yayoi, Bunkyo, Tokyo, 113-0032, Japan [email protected] Abstract-Trench systems surrounding the circum Pacific Ocean are the subudction zones of the oceanic plates according to the Plate Tectonics. These subduction zones are source regions generating large earthquakes. The seismic waves generated by these large earthquakes caused huge damages in Japan and Taiwan. Seismological studies during three decades suggest that these earthquakes are caused by frictional slip between the subducting oceanic plate and the continental plate. If we can monitor any precursor slips at the plate boundary in prior to the large earthquakes, it may be possible to reduce the earthquake damages. The precursory-seismic activities and/or slip can be observed by seismometers, and/or geodetic instruments at the ocean bottom. In order to monitor seismic activities at deep ocean, we developed freefall-popup digital Ocean Bottom Seismometers (OBS). In addition to non-real time seismic observation using freefall-popup OBSs, submarine cable OBSs enabling real-time observation have been used. There are many scientific submarine cables systems in Japan. Each submarine cable system is nearly 100km long. Some use optical fibers and others do coaxial cables. We also examined the possibility of using decommissioned telecommunication submarine cables. The GeO-TOC cable OBS located at the 2800m depth of ocean bottom along the Izu-Boin Trench was operated for 5 years and brought important seismic and hydrophone data to the Japanese seismic network continuously. The VENUS cable system using the GOGC submarine cables was the test system deployed off Okinawa Island and operated for two months. It was an interdisciplinary ocean bottom observatory. In USA and Canada, the NEPTUNE (North East Pacific Time-series Undersea Networked Experiment) project is intending to install scientific submarine cables systems near Vanquvor Island, Montley Bay and off Cascadia-Juan de Fuca. In Japan, the ARENA project has been proposed. To make such project economically and successfully, the international cooperation is requested because the branching from the main submarine cable needs technological breakthrough. The development of power supply systems, undersea mate-able connectors and fail safe system are key points. This development of new technologies may greatly contribute to the ocean engineering field.

I. INTRODUCTION

0-7803-8541-1/04/$20.00 @2004 IEEE

According to the Plate Tectonics theory, the Earth is covered by several rigid plates with thickness of 100-150km. The ocean part is covered by oceanic plates, and the continental part is covered by the continental plates. In the northern part of Taiwan and the southwestern part of Japan, the Philippine Sea plate subducts beneath the Eurasian Plate with rate of several cm/year. At the north-eastern part of Japan, the Pacific Plate subducts beneath the Eurasian Plate with slightly faster rate than the subduction rate of the Philippine Sea plate. The plate subduction initiates at the trench axes and penetrates into the Earth down to the depth of 640km. The seduction movements generate earthquakes. The most intense earthquakes occur along the interface of two plates. For example, the Sanriku Earthquakes in 1896 (M8.5) and 1933 (M8.1), the Tonakai Earthquake in 1944 (M7.9) , the Nankai Earthquake in 1946 (M8.0), and the Great Kanto Earthquakes in 1923 (M7.9) are those examples. Among those the Great Kanto Earthquake in the Tokyo area was one of the most destructive earthquakes during the 20 centuries in Japan. In this catastrophe, 142,000 people were killed by this earthquake. In Taiwan, the Chi-chi earthquake (M7.7) in 1999 occurred on land part, but this earthquake is interpreted as plate boundary earthquake. There are many large earthquakes also occurred in the ocean region surrounding Taiwan. Fig. 1 shows epicenters of destructive earthquakes near Japan [1]. Seismological studies during three decades suggest that these earthquakes were caused by frictional slip between two plates. Some portion of the plate boundary, however, is loosely coupled. The stiffly coupled place is called “asperity” and loosely couple one is called “non-asperity”[e.g., 2]. The slip along such loosely coupled boundary may aseismically occur, but it may trigger large earthquakes. If we can monitor the pre-slips at the plat boundary in prior to the large earthquakes, it may be possible to reduce the damages due to earthquakes and tsunamis. The precursors can be observed by seismometers, tilt meters, and/or geodetic instruments at the ocean bottom. To observe the seismic geodetic movement, Ocean Bottom Seismometer (OBS), Ocean Bottom Tiltmeter (OBT) and GPS linked acoustic transponder system have been developed in Japan and USA. Hydrophone arrays have been used for defense purpose to watch submarines. However, the end of cold war allows geophysicists to use the US-navy hydrophone arrays. The SOSUS (SOund SUrbillance System) array was built in early 1960. Use of the hydrophone arrays was extremely useful to detect earthquakes in the north-eastern Pacific Ocean [3] and to track marine mammals. The SOSU is also used by acoustic tomography [4]. Ocean bottom magnetometer has been developed to observe electro-magnetic change at deep sea floor.

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-25Hz band. Although this analog cassette-base OBS could make one-month seismic recording, the waveform fidelity and the dynamic range of cassette tapes were poor.

Fig. 2. Digital autonomous OBS in a 17” glass hemisphere. Fig.1a. Destructive earthquakes in Japan before 1006 (JMA[1])

Fig. 1b. Destructive earthquakes between 1996 and 2003 in Japan (JMA[1]).

II. OBSERVATION USING AUTONOMOUS OCEAN BOTTOM SEISMOMETERS (OBS) Since 1970, the author developed various kinds of OBS's. Using a cassette-tape recorder, a glass-sphere housing and an acoustic release, we developed a pop-up OBS system [5]. The waveform data were saved on a cassette tape in analog form. This OBS could record four channel seismic data with 0.5

Fig.3. P-wave velocity structures along EW (top) and NS (bottom) lines in the forearc slope of the Japan Trench region using OBSs, controlled seismic sources and travel time tomography method (after [11, 12]). In order to improve the waveform fidelity, digital OBSs were developed since 1984. The first generation of digital OBS's used four 12-bit A/D converters and a ADPCM data-compression technique. The compressed data were stored on a quarter-inch cartridge tape [6]. Even using the data compression technique and a 20 Mb tape, the amount of storage was insufficient for one-month continuous recording. The second generation of the digital OBS (the MOOBS/H-1) [7] used four 16-bit A/D converters and a 5-inch 320 Mb MO-disk

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unit. The latest version of the digital OBS, the MOOBS/H-24 [8 ](Fig. 2), used six 24-bit A/D converters, and a 4.2 Gb MO-disk unit. At present, the MO disk was replaced by two 40 Gb hard disks. The latest two systems have been extensively used for seismic surveys during the past several years. Using the above digital OBS’s, significant scientific results were obtained. Along the Nankai Trough, the crustal structure revealed a discontinuity at the Shiono-misaki Canyon, south of Kii Peninsula, Japan [9-10], which is estimated to be the boundary of the Tonankai Earthquake of 1944 (M 7.9) and of the Nankai Earthquake of 1946 (M 8.0). In the Japan Trench region, seismically bright spots were found at 38°45' N - 39° N along the 139ºE line by the OBS-artificial source experiments in 1996[11] and 2001 [12] (Fig. 3). To interpret these high reflections of seismic waves, it is necessary to introduce low compressional-wave velocity materials at the top of subducting oceanic lithosphere[12].

A. Use of decommissioned submarine cables Real-time geophysical monitor on the deep-seafloor is eagerly sought in viewpoint of knowledge of environmental changes at deep-sea and to determine earthquake hazards. One of the best technologies to achieve real time monitor is the use of submarine cables, which have a long history of the technological development and proven field records in telecommunication. Although technologies of the fiber-optic submarine cables are very advanced and reliable, the use of new fiber-optic submarine cables is extremely expensive. Another kind of submarine cable is the coaxial cable, which can provide electrical power and real-time telemetry similar to fiber-optic cables. Due to the extremely rapid growth of fiber-optic technology and huge demands from international communication, a number of fiber-optic submarine cables have been deployed for the world telecommunication. Although many scientific submarine cable OBS’s have been deployed around the Japanese coast during the past two decades (Fig. 5) [e.g., 19], each system required a huge investment and therefore, the construction of similar system in the mid-ocean would not be easy. For example, a submarine cable OBS through the Pacific Ocean would require much longer cables and may not be practical.

Fig.4. Ocean bottom tilt-meter in the titanium hemisphere. In 1998, an OBS experiment was carried out perpendicular to the Izu-Bonin Trench and the survey line transects a serpentine diapir seamount with 30 km radius [13]. The seismic records show strong attenuation around the serpentine diapir [14]. This can be interpreted as a serpentine upwelling, in which peridotites were altered by water penetrating along the slab subduction. We have also carried out seismicity studies at the Okinawa Trough [15], at the Yap Trench [16], and the Japan Sea [17]. The ocean bottom tilt-meter (OBT) is shown in Fig. 4. This has the resolution of 50n radian and one year life. Scripps group and Japanese group have developed the ocean bottom geodetic method to use the combination of ocean bottom acoustic system and GPS on board of a ship enabling to measure geodetic changes by approximately 5-10 cm/year in accuracy [18]. The accuracy may be improved by the precise measurement of water sound speed.

III. REAL-TIME GEOPHYSICAL MONITORING ON THE DEEP-SEAFLOOR USING SUBMARINE CABLES

Fig. 5. Submarine cable geophysical observatories around Japan and in the Philippine Sea (orange and red lines)(by courtesy of Asakawa). TPC-1 was the first Japan-US submarine cable constructed in 1964 and TPC-2 was the second one constructed in 1976. With the installation of fiber-optic systems, TPC-1 and TPC-2 coaxial submarine cables terminated their long commercial service in 1990 and 1994, respectively. By reusing such decommissioned submarine cables , real-time scientific observatories on the deep-sea floor can be realized with high

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reliability and with reasonable cost [20-21]. Sections of TPC-1, between Ninomiya, Japan and Guam Island, and TPC-2 between Okinawa Island, Japan and Guam Island were obtained by ERI and the Incorporated Research Institutions for Seismology (IRIS) in 1990 and by the ERI in 1996, respectively. Both cables passed geophysically important regions. TPC-1 was routed from Guam, the Marina-Izu-Bonin Trenches to the Sagami-Bay. TPC-2 was routed from the Mariana Trough to the mid-Philippine Sea plate and to the Ryukyu Trench. These areas include seismically very active subduction zones and a rifting backarc basin. In 2004, the first generation of fiber optic submarine telecommunication systems such as TPC-3 and TPC-4 are ready to retire. These route are between Japan and USA. The TPC-4 runs along the Aleutian Trench, which is one of important regions for seismologists, and marine mammal researchers. The retirement of APC and HJK cables among Japan, Taiwan, Korea and the southeast Asia are in plan in 2004 or 2005. There were two major projects for the scientific reuse of decommissioned submarine cables in Japan. The Geophysical and Oceanographical Trans Ocean Cable (GeO-TOC) project used TPC-1 cable (hereafter GeO-TOC). The other is the Versatile Eco-Monitoring Network by Undersea-Cable System (VENUS) project. The VENUS project used TPC-2 cable (Guam Okinawa Geophysical Cable; hereafter GOGC). In the VENUS project, a multi-disciplinary observatory was installed at the Ryukyu Trench. B. IZU OBS using GeO-TOC CABLE (former TPC-1) system [21-22,24] The length of the GeO-TOC submarine cable is 2,659 km [21-23](Fig. 5). The GeO-TOC system had 74 repeaters [23], which contained dual vacuum-tube amplifiers to compensate for gain-loss caused by submarine cables. The GeO-TOC uses a 1" diameter coaxial cable. The power supply allowance for the GeO-TOC system was 1,940 V and 370 mA for instruments. If each station uses 30 W, the cable has enough power allowance. The former TPC-1 had 138 voice channels during its commercial use. If several voice channels were used for data-telemetry, there was also enough capacity for scientific use. The remaining service life of the cable system would be considered to be enough although the official life of the system was 25 years. Because the design service life of submarine electronics was roughly more than 50 years and the estimated lives of submarine cables were greater than the life of the electronics, the remaining service life of the GeO-TOC system should be sufficient for scientific observations [21-22]. The IZU OBS sensors comprised tri-axial accelerometers with gimbals, a hydrophone, a quartz thermometer for external temperature and a quartz pressure gauge for the external pressure. The frequency characteristics were flat over DC–62.5 Hz. Three 24-bit A/D converters with 125/62.5 Hz sampling rate were used for digitization of seismometer output. Data from seismometers were transmitted to Ninomiya shore station at 9,600 bps. The IZU OBS used only five 4-kHz channels for data and two 4-kHz channels for control commands. On the other hand, the hydrophone signals were transmitted in analog

form and were digitized at shore with 16bit and 1kHz sampling rate. System control commands were sent to the IZU OBS. Instruments of IZU were enclosed in a stainless-steel pressure case having a repeater-shaped fitting for deployment by a cable ship (Fig. 6). The IZU OBS using the GeO-TOC cable was deployed on January 13, 1997 at a depth of 2,708 m on the forearc slope of the Izu-Bonin Trench [21-22,24]. The location was 31°24.042'E and 140°55.038’E. The +4,170 V DC was supplied to the GeO-TOC cable from Guam shore station. One of the submarine cable ends was landed at Ninomiya shore station. Modulated signals were demodulated at Ninomiya and transferred to Tokyo using a commercial line at 64 kbps.

Fig 6. .IZU OBS using the GeO-TOC cable in the cable ship. During five years and 8 months since January 1977, continuous data were sent to the Japanese University Microearthquake Network and also were stored on the 1Gb MO-disks. The typical size of one hour of data was approximately 4.5 Mb. The IZU data were regularly processed since March 1998 by the University Micro-Earthquake Network to determine hypocenters in Japan. The data were also been sent to IRIS. However, just after an extremely powerful typhoon passed the landing station on October 1, 2002, the transmission was abruptly terminated. The study of the cause of the cable fault showed that the flooding of the Hayakawa River near the shore station flew into the Sagami Bay, and the heavy turbidity current broke the submarine cable at 11 km from the shore station. Although we tried to fix the cable fault, we gave up that due to heavy cable crossing at the fault point. Since the accident, the GeO-TOC cable has been used only for electrical potential measurement. C. VENUS-GOGC (former TPC-2) SYSTEM [24-27] The objective of the VENUS project (1995-1999) [24-27] was to construct a multi-disciplinary observatory to study deep-sea environmental changes due to the subduction of the Philippine Sea plate at the Ryukyu Trench using GOGC (Fig. 5) (former TPC-2 cable system). The observatory [24-25] was installed at a depth of 2,200 m on the forearc slope of the Ryukyu Trench 50 km from the mainland of Okinawa in the fall of 1999. Nine Japanese institutions worked together on this project. The cable length of the GOGC was 2,400 km. The

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system uses 1.5"-diameter coaxial cables. The former TPC-2 system had 845 voice channels. During TPC-2 use, the +1,080 V DC from Okinawa and -1,080 V DC from Guam were supplied to the cables at constant current during commercial use. In the VENUS project, the electric power supply was modified to a single supply from Okinawa. The system comprised bottom sensors, a data-telemetry system, main coaxial cables, and land equipment. The total power dissipation caused by the VENUS Okinawa observatory was approximately 53.5 W. All pressure housings for bottom units were made of titanium to resist corrosion during the long observation period. The telemetry system comprised a data-coupling unit, a data-telemetry unit and a junction box. The telemetry system generated 24 DC using 3,000 V and separated the high-voltage DC component from high-frequency carriers, and again mixed the high-frequency carrier with the DC component. The data-telemetry unit multiplexed the data and sent these data to the shore using a 240-kHz-carrier bandwidth. The transmission rate for the multiplexed data was 96 kbps. Each instrument, however, used particular transmission rate, e.g., 19.2 kbps for the BBS (BroadBand Seismometer), etc. Hydrophone data used another 240-kHz bandwidth. The junction box had nine so-called "ROV (Remotely Operated Vehicle) undersea mate-able connectors". The ROV connectors allowed units to be plugged in and unplugged on the ocean floor with the assistance of a manned submersible or an ROV.

hydrophones with 700 m spacing. Sixteen-bit data were transmitted to shore. The geodetic changes were acoustically determined by precise baseline measurements between two transponders. Three units were placed in triangular formation and the distance between two units was approximately 1 km. The estimated accuracy of geodetic measurement was a few cm, which might be smaller than the expected crustal deformation near the trench. The geoelectric-geomagnetic unit comprised a proton magnetometer, flux-gate magnetometers, and orthogonal geo-potentiometers. The length of the geo-potential measurement was 10 m. The mobile unit consisted of an acoustic communication unit and a remote instrument.

Fig. 8. VENUS BBS (BroadBand Seismometer) at the ocean bottom. The 300m cable was connected to the junction box.

Fig. 7. Instruments and telemetry system configuration of the VENUS Okinawa multi-disciplinary observatory. The observatory comprised seven instrument units: broadband seismometers, a tsunami pressure sensor, a hydrophone array, a multi-sensor unit, geodetic instruments, geoelectric-geomagnetic instruments, and a mobile unit (Fig. 7). The seismic measurement used "Guralp CMG-1T" triaxial BBS with gimbals (Fig. 8) [26-27]. The seismometers responded to periods between 300 and 0.05 seconds. The BBS outputs were digitized at 24-bit and 100 Hz. The tsunami gauge used a quartz pressure sensor and the resolution for sea-level change was 0.5 mm. The multi-sensor unit comprised short-period seismometers, a hydrophone, a digital still-life camera, a CTD, a current meter, a transmission meter, and sub-bottom temperature probes. The hydrophone array was composed of 5

Fig. 9. Deployment of a VENUS instrument using a deep-tow unit. A shore station was located in Okinawa Island. Some shore equipment was obtained from the previous TPC-2 station. The Okinawa shore station supplied 3,100 V to the cable. The shore receiving unit demodulated signals from the instruments and sent them to Yokosuka data center, Kanagawa, Japan, using two 64-kbps lines. Observed data were stored in the mass storage at the Japan Marine Science and Technology Center (JAMSTEC)

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in Yokosuka. Scientists obtained their own data and also communicated to a particular instrument using a data terminal at JAMSTEC.

Five instrument units were installed at the ocean bottom in September-October 1999 by use of a deep-tow unit (Fig. 9). Using the ROV-Kaik, we connected nine ROV connectors of the cable end to connectors on the junction box at the ocean bottom in October 1999 (Figs. 10 and 11).

Fig. 10. The VENUS junction box and telemetry pressure case at the ocean floor. The front part of this photo is a part of ROV.

Fig. 12. Ms=6.1 earthquake of November 11, 1999, which occurred off Taiwan. NS (34.4 x10-5 m/s full scale), EW (the same scale as NS) and Z (14.1x10-5 m/s).Horizontal axis: 20 minutes record (after [27]). This is one of aftershocks of 1999 Chi-chi Earthquake (Ms=7.3). Two month data such as broadband seismometer, multi-sensors, geomagnetic-geoelectric sensors and others were obtained [26,27]. Fig. 12 shows one of examples of an aftershock of the Chi-chi earthquake in 1999 obtained by Broadband seismometer of VENUS instruments. Fig. 13 shows an example of the VENUS still-life camera of the multi-sensor unit.

Fig. 11. A snap shot of penetration of undersea mate-able connecter to the female part of the junction box using the manipulator of ROV. The GOGC cable route was identified on the ocean floor, with a thin sediment cover, by the deep-tow camera in February 1998. In March 1998, the submersible Shinkai 6500 cut the GOGC cable at 25°44'N and 128°02.5'E at a 2,200-m water depth using a newly developed cable cutter. Three major legs were carried out by use of one cable ship and one research boat with ROV for deployment of the telemetry system, deployment of instruments and extension-connection of cables, respectively. Instruments were confined within an approximately 1-km radius area around the junction box. The data-coupling unit was spliced into the main cable on the deck of a cable ship in August 1999. The telemetry system with the tsunami sensor and the hydrophone array was installed at the ocean bottom at 25°44.5255'N and 128°03.5201'E at a 2,157-m water depth.

Fig. 13: Photo taken by still-life camera of the Multi-sensor system. A fish is passing near the seismometer.

IV. NEW ERA FOR MARINE SCIENCE IN THE WORLD US scientists installed the H2O geophysical station using the HAW2 (Hawaii-2) submarine cable in 1998 [29]. In USA and Canada, the NEPTUNE (North East Pacific Time-series Undersea Networked Experiment) project as a part of ORION

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(Ocean Research Interactive Observatory Network) [30-31] project is planning to install scientific submarine cables systems near Vanquvor Island, Montley Bay and off Cascadia-Juan de Fuca. The intension of ORION project is to build infrastructure of marine network to obtain 4D continuous data in the ocean to avoid aliasing due to missing of time history. In Japan, the ARENA project has been proposed [32]. To make such project economically and successfully, the international cooperation is requested because the branching from the main submarine cable needs technological breakthrough. The development of power supply systems, undersea mate-able connectors and fail safe system are key points. This development of new technologies may greatly contribute to the ocean engineering field. As the first generation of fiber-optic cable systems are going to retire soon, scientists can construct real-time ocean-bottom networks by reasonable cost and reliable technologies using such submarine cables. ACKNOWLEDGEMENT The Ministry of Education, Science, Culture and Sports, Japan supports the GeO-TOC project. The Science and Technology Agency, Japan supports the VENUS project. The author thanks JAMSTEC for its great assistance in operating research boats and submersible vehicles. The author also thanks ERI for permission to use the GeO-TOC and GOGC cables for seismological research.

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[30] NRC, Enabling Ocean, Research in the 21st Century, Implementation of a network of ocean observatories, National Academy, 220pp., 2003. [31] H. L. Clark and A. Isern, Cabled observatories for ocean research: a component of the ocean observatories initiative, in “Proceedings of the 3rd international workshop on Scientific Use of Submarine Cables and Related Technologies”, Tokyo, Japan, IEEE 03EX660, pp.209-214, 2003. [32] Y. Shirasaki et al., ARENA: A versatile and multidisciplinary scientific submarine cable network of next generation, in “Proceedings of the 3rd international workshop on Scientific Use of Submarine Cables and Related Technologies”, Tokyo, Japan, IEEE 03EX660, pp.226-231, 2003. Related home page ARENA: http:/homepage.mac.com/ieee_oes_japan/ARENA/ARENA-E .html Canadian VENUS (Victoria Experiment NEtwork Under the Sea): http://www.venus/uvic.ca Costal Ocean Processes and Observatories: Advanced Coastal Research: http://www.geo-prose.com/oiti/report.html DEOS: http://www.coreocean.org/deos/ ESONET: http://www.abdn.ac.uk/ecosystem/esonet HUGO (Hawaii Undersea Geo-Observatory): http://www.soest.hawaii.edu/HUGO/hugo.html NEPTUNE: http://www.neptune.washigton.edu/. Ocean Institute: http://www/ocean-institute.org ORION:http://www.coreocean.org/dev2go.web?id=249051 SCOTS: Scientific Cabled Observatories for Time Series, http://www.geo-prose.com/projects/scots_rpt.html