Great earthquake East Japan observation by superconducting gravimeter in Antarctica

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<ul><li><p>by superconducting gravimeter</p><p>ShTsukpan</p><p>y thatir byonmat0 (m</p><p>g gravimeter at Syowa StationvationStationce 200nductind obs, effoeasurthroug</p><p>earthquake on December 26, 2004 (magnitude: M 9.1) and Chileearthquake on February 27, 2010 (magnitude: M 8.8).</p><p>2. Superconducting gravimeter</p><p>There are twomainmethods to measure gravity. One is absolutegravity measurement which measures the absolute gravity and the</p><p>ence and change in gravity over time. An absolute gravimeter usessprings as typied by FG5 [4]. On the other hand, a superconducting</p><p>3. Setup in Syowa Station in Antarctica</p><p>On December 18, 2009 the superconducting gravimeter and li-quid helium were air lifted from the ice breaker Shirase to SyowaStation. At Syowa Station, the gravimeter was carefully transportedfrom the heliport, unpacked, and launched for the actualobservation.</p><p>Launching consisted of evacuating the insulated vacuum cham-ber for 2 days until vacuum of 2.5 105 Torr was reached. Then itwas pre-cooled over night with liquid nitrogen and the liquidnitrogen was ushed out after verifying that the sensor reached</p><p> Corresponding author. Tel./fax: +81 29 853 2484.</p><p>Cryogenics 52 (2012) 704707</p><p>Contents lists available at</p><p>ge</p><p>seE-mail address: ikeda@bk.tsukuba.ac.jp (H. Ikeda).ment of Earths gravity using a superconducting gravimeter [3].Since this time we introduced a new third generation superconduc-ting gravimeter (CT # 058), the following is a report of its launching.</p><p>Furthermore, the seismic wave caused by the Great East JapanEarthquake that occurred on March 11, 2011 at 14:46 JST (magni-tude: M 9.0, location: 38 6.2 min N, 142 51.6 min E, depth:32 km) was clearly observed approximately 20 min later by thesuperconducting gravimeter at Syowa Station about 14,000 kmaway from Japan. The observation of the free oscillations of theEarth is reported compared to the observed records of the Sumatra</p><p>1 nano-Gal (1011 m/s2) (1 lGal = 10 mm resolution). Therefore, asuperconducting gravimeter is used by the international observa-tion project GGP, which was organized to observe the deep Earthdynamics, and measurements are being continued all over theworld (about 30 locations). The superconducting gravimeter atJapans Syowa Station is an important observation point in thisproject because it is the only one in Antarctica, where centrifugalforce is small and gravity is 0.5% greater than that at the equator.The history of the superconductinwill be explained. Continuous obserconducting gravimeter at Syowastarted in 1993 (TT-70 # 16) [1]. Sinto a second generation small supercowith 4 K GM refrigerator (CT # 043) auntil December 2009 [2]. Up to nowunderstand the Earths dynamics by mment and background free oscillation0011-2275/$ - see front matter 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.cryogenics.2012.08.002of gravity using super-, Antarctica was rst3, it has been upgradedng gravimeter equippedervation was continuedrts have been made toing Earths inner move-h continuousmeasure-</p><p>gravimeter is a relative gravimeter that measures the change ingravity by detecting the displacement of an 1-in. niobium sphereoating in an extremely stable magnetic eld created by supercon-ducting coils. Fig. 1 shows the niobium sphere superconductingsensor that detects gravity [5]. Since the niobium sphere sensor isused in the temperature region of liquid helium, all thermal noisesare cut to provide sensitivity greater than that of an absolute gravi-meter by at least three digits and enables measurements of up to1. Introduction other is relative gravity measurement which measures the differ-Great earthquake East Japan observationin Antarctica</p><p>H. Ikeda a,, Y. Aoyama b, H. Hayakawa b, K. Doi b, K.aResearch Facility Center for Science and Technology, Cryogenics Division, University ofbNational Institute of Polar Research, Midori-Machi, Tachikawa-shi, Tokyo 190-8518, Ja</p><p>a r t i c l e i n f o</p><p>Article history:Available online 30 August 2012</p><p>Keywords:Superconducting gravimeterRefrigeratorGreat East Japan EarthquakeAntarctica</p><p>a b s t r a c t</p><p>The seismic wave caused bJST (magnitude: M 9.0, locapproximately 20 min latefrom Japan. The observatiobserved records of the Suquake on February 27, 201Cryo</p><p>journal homepage: www.elll rights reserved.ibuya b</p><p>uba, Tsukuba, Ibaraki 305-8577, Japan</p><p>e Great East Japan Earthquake that occurred on March 11, 2011 at 14:46on: 38 6.2 min N, 142 51.6 min E, depth: 32 km) was clearly observedthe superconducting gravimeter at Syowa Station, about 14,000 km awayof the free oscillations of the Earth will be reported compared to thera earthquake on December 26, 2004 (magnitude: M 9.1) and Chile earth-agnitude: M 8.8).</p><p> 2012 Elsevier Ltd. All rights reserved.SciVerse ScienceDirect</p><p>nics</p><p>vier .com/locate /cryogenics</p></li><li><p>enics 52 (2012) 704707 705Fig. 1. A superconducting niobium sphere sensor to detect gravity.</p><p>H. Ikeda et al. / Cryogliquid nitrogen temperature of 77 K. After ushing the liquid nitro-gen, liquid helium was transferred from a 60 l helium container webrought from Japan until the helium level of the cryostat reached100%.</p><p>After tilt adjustment, levitation of Nb superconducting sphere,and sensitivity adjustment, observation of tidal signal with thenew superconducting gravimeter was conrmed on December24. Then after nal adjustment, stationary continuous observationwith the superconducting gravimeter was started on January 7,2010. Observation data and observation condition captured witha web camera were sent from Syowa Station to Japan over theIntelsat network enabling us to observe them in real time. Fig. 2shows the nal layout of the new superconducting gravimeter in-stalled in the gravimeter room at Syowa Station, Antarctica. Thegure shows the UPS, barometer, control box, liquid helium con-tainer, superconducting sensor, 4 K refrigerator, and compressorfrom right to left.</p><p>As an example of observation data, Fig. 3 shows the change intide during a month from mid April to mid May. A single sine waverepresents the tidal signal for 1 day and the larger period sine waverepresents the period of the full moon and newmoon. Other signif-icant amplitudes are signals from earthquakes. The graph proceeds</p><p>Fig. 3. Tidal changes in signal obtained approximaFig. 2. The overall layout of the new superconducting gravimeter in gravimeterroom.from right to left. The blue1 curve shows the change in air pressure.Pressure uctuates wildly at Syowa Station, making it necessary tocorrect for pressure in order to accurately observe gravity.</p><p>4. Earths free oscillations</p><p>Free oscillations of the Earth are vibrations of the Earth that oc-cur mainly when there is a massive earthquake with period rangingfrom several minutes to an hour. For example, just as a small bellrings at high pitch and a large bell rings at low pitch, frequency isa function of the size andmaterial of a substance, and amplitude de-pends on the size of the fault activity. In other words, if the Earth isassumed to be a large bell, the internal structure of the Earth (dis-tribution of the type and density of substances forming the Earthsinterior) can be examined by continuously observing with a super-conducting gravimeter, the low frequency vibrations caused byEarths oscillations as change in gravity.</p><p>tely 1 month by superconducting gravimeter.</p><p>1 For interpretation of color in Fig. 3, the reader is referred to the web version ofthis article.</p></li><li><p>nics706 H. Ikeda et al. / CryogeUp to now, the Sumatra earthquake on December 26, 2004 (M9.1) and Chile earthquake on February 27, 2010 (M 8.8) were ob-</p><p>Fig. 4. Position and distance from Syowa Station in Antarctica great</p><p>Fig. 5. State of the superconducting gravimeter signal changes caused by g52 (2012) 704707served with the superconducting gravimeter at Syowa Station asexamples of large earthquake. Fig. 4 shows the waveform of the</p><p>earthquake Sumatra event, Chilean event and East Japan events.</p><p>reat earthquake Sumatra event, Chilean event and East Japan events.</p></li><li><p>favorable conditions. This superconducting gravimeter is the prop-</p><p>We would like to thank the 50th and 51th Japanese Antarctic</p><p>by g</p><p>H. Ikeda et al. / Cryogenics 52 (2012) 704707 707gravity change observedwith the superconducting gravimeter com-pared to the observation data of the Great East Japan Earthquake.Each gure shows the residual gravity with tidal signal subtracted.These observation results also show that the Great East Japan Earth-quake is similar in magnitude to the Sumatra earthquake.</p><p>Fig. 5 shows the magnitude of each earthquake and the distancefrom its epicenter to the Syowa Station where the superconductinggravimeter is installed. The Great East Japan Earthquake was14,431 km away from Syowa Station.</p><p>Fig. 6 shows the decay curve of the Earths free oscillation mode0S0 excited by the two large earthquakes (a mode that stretchesuniformly in the radial direction of the Earth). The vertical axis isthe gravity and the horizontal axis is the number of days sincethe earthquake. At the moment of the earthquake, gravity in-creases in the order of East Japan, and Chile, proportional to themagnitude. As for the decay curve of the Sumatra earthquake, de-cay cannot be determined because the noise level is too high whenobserved with the second generation (CT # 043) superconductinggravimeter. As for the Chile earthquake and East Japan Earthquake,resolution up to the noise level (0.2 lGal/rHz) can be assumed. The0S0 oscillation mode was observable up to 98 days after the quakefor the Chile earthquake and up to 113 days after the East JapanEarthquake.</p><p>Fig. 6. Earths free oscillation signals changes caused5. Conclusion</p><p>A third-generation superconducting gravimeter was setup atSyowa Station. Observation is continuing smoothly 22 monthsafter installation without any cryogenics-related problem. So far,research expedition members, for the installation support.</p><p>References</p><p>[1] Sato T, Shibuya K, Tamura Y, Kanao M, Okano K, Fukuda Y, et al. J Geol Soc Jpn1995;42:145.</p><p>[2] Ikeda H, Doi K, Fukuda Y, Tamura Y, Shibuya K. Polar Geosci 2005;18:49.[3] Ikeda H, Aoyama Y, Doi K, Shibuya K. In: 29th Symposium polar geosciences,</p><p>vol. 89; 2009. p. 45.[4] http://www.microglacoste.com/.[5] Warburton R, Brinton E. In: Proceedings of 2nd Workshop. Non-tidal gravityerty of the National Institute of Polar Research.</p><p>AcknowledgementsEarths free oscillations excited by large earthquake have been ob-served for Chile and East Japan Earthquakes. Antarctica is ideal forgravity observation because the value of gravity is large and thenoise level is low. Better understanding of the Earths internalshape and structure can be expected by studying the spatial distri-bution of the amplitude of the observed Earths free oscillationmode 0S0. Therefore, it is important to continue observation under</p><p>reat earthquake Chilean event and East Japan events.changes. Les Cahiers du Centre European de Geodynamique et Seismologie;1995. p. 23.</p><p>Further reading</p><p>[6] Ikeda H, Aoyama Y, Hayakawa H, Doi K, Shibuya K. Japan Geoscience UnionMeeting; 2010 [SGD002-P02].</p><p>Great earthquake East Japan observation by superconducting gravimeter in Antarctica1 Introduction2 Superconducting gravimeter3 Setup in Syowa Station in Antarctica4 Earths free oscillations5 ConclusionAcknowledgementsReferencesFurther reading</p></li></ul>