Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15 Natsushima-cho, Yokosuka, Kanagawa, , JAPAN

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1 LARGE EARTHQUAKE AND ASSOCIATED PHENOMENA OBSERVED WITH SEAFLOOR CABLED OBSERVATORY NEAR EPICENTER - AN IMPLICATION FOR POSSIBLE ADDITIONAL MEASUREMENT WITH TELECOMMUNICATION NETWORKS FOR IDENTIFICATION OF NATURAL HAZARD Ryochi Iwase (JAMSTEC) <iwaser@jamstec.go.jp> Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15 Natsushima-cho, Yokosuka, Kanagawa, , JAPAN Abstract: Some significant phenomena on the deep seafloor associated with large earthquakes, such as strong benthic water current or mudflow, have been observed with JAMSTEC s multidisciplinary cabled observatories. Although real-time and long-term observation on seafloor is most effective and important for both geophysical research and natural hazard mitigation like early warning of disastrous subsea earthquakes, the number of cabled observatory is far less than that of submarine telecommunication network because of its huge construction cost. An idea of adding seafloor observation function on some feasible observation items to submarine telecommunication networks in future is proposed with introduction of multidisciplinary observed phenomena. 1. INTRODUCTION In order to mitigate large subsea earthquake and associated tsunami hazard, direct observation on seafloor around those earthquake generating regions off the coast is far more effective than such observation on land as teleseismic wave detection which is caused by subsea earthquake. It is not only because seafloor observation is advantageous to early detection and warning of earthquakes and tsunamis, but because only through seafloor observation one can obtain observational data on precise seismic activity, crustal deformation and other subsea phenomena associated with earthquakes or tsunamis which are indispensable in seismological or geophysical research. In addition, for those purposes, real-time and continuous long-term observation is highly necessary, which can be most surely realized by cabled observatories capable of continuous power supply and data transmission. A number of cabled observatories have been constructed so far and are planning to be constructed in near future (for example, [1]-[4]). However, they have been far less constructed than global subsea telecommunication networks have, because they need as large cost as the telecommunication networks do and yet it is almost impossible to manage them commercially at present. Although several projects have been conducted to utilize retired submarine telecommunication cable systems for observatories with less construction cost [5], [6] than custom-built observatories, continuous operation has been less successful so far than the latter, partly because those cable systems had basically passed their durable period and were less reliable, and yet they need as much cost for repair as telecommunication cable systems do. On the other hand, telecommunication cable systems have sometimes faced large earthquakes or associated landslides and Copyright 2010 SubOptic Page 1 of 5

2 have been destroyed in some worst case. Since it is rare to deploy some observation equipments in advance to catch those events in situ, it could be of some use for both geophysical research and cable system maintenance if some kind of observational data were obtained from cable systems. Meanwhile, on September 26 th 2003, Off Tokachi Earthquake, magnitude of which was as large as M 8.0, occurred in the vicinity of JAMSTEC s cabled observatory at Kurile Trench off Hokkaido, northeast Japan. The earthquake was accompanied by strong benthic water current (or mudflow) which was observed with multidisciplinary sensors mounted on the observatory [7], [8]. In Sagami Bay, central Japan, mudflow which accompanied M 5 class earthquake and was not as strong as the Off Tokachi Earthquake case has been observed several times with multidisciplinary sensors including a video camera mounted on another cabled observatory [9]. Motivation of this presentation is to propose to add some kind of seafloor observation function to submarine telecommunication cable systems constructing in future, not retired cables, and to probe into its feasibility, through the introduction of multidisciplinary observation result on phenomena associated with earthquakes. It is of course important that seafloor observation function must not disturb reliability of telecommunication cable system. There is some discrepancy between them that most of electronics except optical components have been removed from submarine repeaters to promote reliability as telecommunication system, although electronics are necessary to observe seafloor for sensing and acquiring data. In spite of this discrepancy, the author would like to find out the feasible observation items and methods that can share telecommunication cable systems in future. 2. TEMPERATURE FLUCUTUATION OF OCEAN BOTTOM SEISMOMETER Before describing observational result on phenomena on seafloor associated with earthquakes, the following is the introduction of temperature fluctuation observed inside chassis of ocean bottom seismometer (OBS) that was not associated with earthquakes but was caused by seafloor experiment conducted with an remotely operated vehicle (ROV). Figure 1: Location of off Kushiro- Tokachi cabled observatory. One of JAMSTEC s cabled seafloor observatories is located at Kurile Trench off Kushiro-Tokachi in Hokkaido, northeast Japan (Figure 1). The observatory consists of three in-line ocean bottom seismometers (OBSs), two in-line type tsunami pressure gauges and a cable end station composed of multidisciplinary sensors. Pressure chassis of OBS has the same cylindrical shape as those of repeaters of submarine telecommunication systems. Since the OBSs were deployed in the same manner as submarine telecommunication system construction, they were just laid on the unconsolidated Copyright 2010 SubOptic Page 2 of 5

3 sediment, mainly consisting of sandy mud. An experiment was carried out to lay OBSs under the ocean bottom by using suction system mounted on ROV in June and July 2002, in order to reduce background noise caused by bottom water current [10]. Figure 3: Spectra of temperature corresponding to X-component at OBS1. Figure 2: Waveforms and temperature profiles observed with OBS1. OBS consists of three-component accelerometer. X-component is parallel to, Y- and Z- components are perpendicular to cylindrical axis of the chassis, respectively. Figure 2 shows 2 month waveforms of ground motion and profiles of temperature fluctuation inside chassis of OBS1 from June to July in Waveforms from top in Figure 2 correspond to X-, Y- and Z- component ground motion respectively, and bottom is temperature profiles of three thermometers attached to the circuit corresponding to each component of seismometer, respectively. The experiment at OBS1 was conducted on July 9 th 2002, as is recognized with ground motion waveforms in Figure 2. Not only ground motion, but also temperatures were largely fluctuated at the same time. Temperatures increased about 3 degrees after the experiment. Those temperatures mainly reflect heat balances between heat emission from electronics inside chassis and circumstance around. Since the power consumption of OBS seemed to be constant during this period, the temperature increase was caused by the change of diffusivity of material around OBS, i.e. OBS was surrounded by sediment. In addition, the amplitudes of harmonic temperature fluctuations decreased after the experiment. Figure 3 shows the spectra of temperature fluctuation corresponding to X-component of OBS1. Red curve is the spectrum for 16 day profile before the experiment and black curve is the one after the experiment. Before the experiment, semi-diurnal fluctuation was dominant, which mainly reflected water temperature fluctuation. After the experiment, amplitude of semi-diurnal fluctuation decreased to less than 20 %. Since the amplitude of harmonic temperature fluctuation decreases exponentially according to depth from the sea bottom, considering heat conduction in solids, this phenomena also reflected increased depth of OBS1 from sea bottom. In short, average temperature increase and decrease in harmonic temperature fluctuation amplitude indicate depth increase of OBS1 from sea bottom. 3. EARTHQUAKE-PHENOMENA OBSERVED WITH CABLED OBSERVATORY In the vicinity of off Kushiro-Tokachi Observatory, M8 earthquake, whose epicentre was located about 25 km WNW of cable end station, was occurred on September 26 th Besides seismograph, some significant phenomena on the deep seafloor associated with the earthquake were observed. Copyright 2010 SubOptic Page 3 of 5

4 conference & convention Figure 4: Water temperature, current velocity and current direction observed at the cable end station. Figure 4 is 2day profiles of water temperature, current velocity and current direction observed at the cable end station composed of multidisciplinary sensors from September 26th to 27th Main shock occurred at 04:50 JST and about 2 hours later strong benthic water current up to about 1.5 m/s which indicates the occurrence of mudflow was observed with the cable end station. The mudflow is usually accompanied by water temperature increase as was shown in Figure 4 that reflects the scale of the mudflow and the temperature profile of water column. In case of this event, the water temperature increased up to about 0.5 degree C. profiles observed with OBS1 in the same period as in Figure 4. As was described in the former chapter, theses temperature increases indicate depth increase of OBS from sea bottom, possibly because of its own weight and strong vibration. Another cabled observatory off Hatsushima Island in Sagami Bay, central Japan, mudflow which accompanied M5.8 earthquake occurred on April 21st 2006 was observed with multidisciplinary sensors including a video camera. Figure 6 shows the location of the observatory and the epicentre of the earthquake. Figure 6: Location of off Hatsushima Is. cabled observatory and epicenter. Figure 7: Profiles of current velocity (top, black), current direction (top, red), light transmission (middle, black), water temperature (middle, red), salinity (middle, green) and electric potential difference between both cable ends (bottom). Figure 5: Waveforms and temperature profiles observed with OBS1 at Off Tokachi Earthquake in Temperature increases were also observed with all OBSs of the observatory apart from the cable end station. Figure 5 shows 2day waveforms and temperature Copyright 2010 SubOptic Figure 7 shows the profiles from 02:40 to 04:40 JST on April 21st 2006 of bottom water current, light transmission, water temperature and an electric potential difference between the underwater cable ends i.e. between the observatory and the shore station which are about 8 km apart. Page 4 of 5

5 The earthquake occurred at 02:50 JST and about 5 minutes later arrival of mudflow was recognized with a video camera. Maximum current velocity was 27 cm/s. The significant increase of about 20 mv in electric potential difference between submarine cable ends was detected, which probably indicates the motional induction caused by the mudflow that went down on the slope west of the observatory. 4. DISCUSSION Development and management of cabled observatory in commercial base is not practical so far. However, there are various kinds of observation items as are partly shown above, some of which are not necessarily require high bit rate to transmit data and one chip circuit may be enough for some sensors. It may not be impractical to add some observation function into repeaters of telecommunication networks, though some idea of data transmission is necessary. However, it should be of great use to identify risks not only of nautral hazard but also for cable system maintenance, and moreover to understand unknown global subsea environment. The author hopes strongly to find some solution in near future. 5. CONCLUSION An idea of adding some seafloor observation function to submarine telecommunication networks is proposed with introduction of observed phenomena. 6. REFERENCES [1] H. Momma, et al., Preliminary results of a three-year continuous observation by a deep seafloor observatory in Sagami Bay, central Japan, Phys. Earth Planet. Inter., 108, 1998, pp [2] K. Hirata, et al., Real-Time Geophysical Measurements on the Deep Seafloor using Submarine Cable in the Southern Kurile Subduction Zone, IEEE Jour. Ocean. Eng., 27, 2002, pp [3] K. Kawaguchi, et al., The DONET: A real-time seafloor research infrastructure for the precise earthquake and tsunami monitoring, Proc. OCEANS MTS/IEEE Kobe Techno-Ocean, DOI: /OCEANSKOBE [4] P. Fairley, NEPTUNE rising, IEEE Spectrum, 42, 2005, pp. 38. [5] J. Kasahara, et al. Submarine cable OBS using a retired submarine telecommunication cable: GeO-TOC program, Phys. Earth Planet. Inter., 108, 1998, pp [6] J. Kasahara, et al. An Experimental Multi-disciplinary observatory (VENUS) at the Ryukyu Trench using the Guam- Okinawa Geophysical Submarine Cable, Annals of Geophys., 49(2/3), 2006, pp [7] R. Iwase and K.Mitsuzawa, A study on the data quality of the observed phenomena on deep seafloor environment associated with The Tokachi oki Eathquake in 2003, JAMSTEC J. Deep Sea Res., 24, 2004, pp (in Japanese with English abstract). [8] H. Mikada et al., New discoveries in dynamics of an M8 earthquake-phenomena and their implications from the 2003 Tokachi-oki Earthquake using a long term monitoring cabled observatory, Tectonophys., 426, 2006, pp [9] R. Iwase et al., Earthquake Accompanied by Mudflow Observed by a Cabled Observatory off Hatsushima Island in Sagami Bay in April 2006, Proc Symposium on Underwater Technology and Workshop on Scientific Use of Submarine Cables and Related Technologies, 2007, pp [10] H. Matsumoto et al., Effects of background noise due to deep sea environments on ocean bottom seismometers attached on the real-time cabled- observatory off Kushiro-Tokachi, JAMSTEC J. Deep Sea Res., 24, 2004, pp (in Japanese with English abstract). Copyright 2010 SubOptic Page 5 of 5