Cryogenics 52 (2012) 704–707
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Cryogenics
journal homepage: www.elsevier .com/locate /cryogenics
Great earthquake East Japan observation by superconducting gravimeterin Antarctica
H. Ikeda a,⇑, Y. Aoyama b, H. Hayakawa b, K. Doi b, K. Shibuya b
a Research Facility Center for Science and Technology, Cryogenics Division, University of Tsukuba, Tsukuba, Ibaraki 305-8577, Japanb National Institute of Polar Research, Midori-Machi, Tachikawa-shi, Tokyo 190-8518, Japan
a r t i c l e i n f o
Article history:Available online 30 August 2012
Keywords:Superconducting gravimeterRefrigeratorGreat East Japan EarthquakeAntarctica
0011-2275/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.cryogenics.2012.08.002
⇑ Corresponding author. Tel./fax: +81 29 853 2484.E-mail address: [email protected] (H. Ikeda).
a b s t r a c t
The seismic wave caused by the Great East Japan Earthquake that occurred on March 11, 2011 at 14:46JST (magnitude: M 9.0, location: 38� 6.2 min N, 142� 51.6 min E, depth: 32 km) was clearly observedapproximately 20 min later by the superconducting gravimeter at Syowa Station, about 14,000 km awayfrom Japan. The observation of the free oscillations of the Earth will be reported compared to theobserved records of the Sumatra earthquake on December 26, 2004 (magnitude: M 9.1) and Chile earth-quake on February 27, 2010 (magnitude: M 8.8).
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1. Introduction
The history of the superconducting gravimeter at Syowa Stationwill be explained. Continuous observation of gravity using super-conducting gravimeter at Syowa Station, Antarctica was firststarted in 1993 (TT-70 # 16) [1]. Since 2003, it has been upgradedto a second generation small superconducting gravimeter equippedwith 4 K GM refrigerator (CT # 043) and observation was continueduntil December 2009 [2]. Up to now, efforts have been made tounderstand the Earth’s dynamics by measuring Earth’s inner move-ment and background free oscillation through continuous measure-ment of Earth’s 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.
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 Sumatraearthquake on December 26, 2004 (magnitude: M 9.1) and Chileearthquake on February 27, 2010 (magnitude: M 8.8).
2. Superconducting gravimeter
There are two main methods to measure gravity. One is absolutegravity measurement which measures the absolute gravity and the
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other is relative gravity measurement which measures the differ-ence and change in gravity over time. An absolute gravimeter usessprings as typified by FG5 [4]. On the other hand, a superconductinggravimeter is a relative gravimeter that measures the change ingravity by detecting the displacement of an 1-in. niobium spherefloating in an extremely stable magnetic field 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 nano-Gal (10�11 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 atJapan’s 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.
3. Setup in Syowa Station in Antarctica
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.
Launching consisted of evacuating the insulated vacuum cham-ber for 2 days until vacuum of 2.5 � 10�5 Torr was reached. Then itwas pre-cooled over night with liquid nitrogen and the liquidnitrogen was flushed out after verifying that the sensor reached
Fig. 1. A superconducting niobium sphere sensor to detect gravity.
Fig. 2. The overall layout of the new superconducting gravimeter in gravimeterroom.
Fig. 3. Tidal changes in signal obtained approximately 1 month by superconducting gravimeter.
1 For interpretation of color in Fig. 3, the reader is referred to the web version ofthis article.
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liquid nitrogen temperature of 77 K. After flushing 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%.
After tilt adjustment, levitation of Nb superconducting sphere,and sensitivity adjustment, observation of tidal signal with thenew superconducting gravimeter was confirmed on December24. Then after final 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 final layout of the new superconducting gravimeter in-stalled in the gravimeter room at Syowa Station, Antarctica. Thefigure shows the UPS, barometer, control box, liquid helium con-tainer, superconducting sensor, 4 K refrigerator, and compressorfrom right to left.
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 new moon. Other signif-icant amplitudes are signals from earthquakes. The graph proceeds
from right to left. The blue1 curve shows the change in air pressure.Pressure fluctuates wildly at Syowa Station, making it necessary tocorrect for pressure in order to accurately observe gravity.
4. Earth’s free oscillations
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 and material 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 Earth’sinterior) can be examined by continuously observing with a super-conducting gravimeter, the low frequency vibrations caused byEarth’s oscillations as change in gravity.
Fig. 4. Position and distance from Syowa Station in Antarctica great earthquake Sumatra event, Chilean event and East Japan events.
Fig. 5. State of the superconducting gravimeter signal changes caused by great earthquake Sumatra event, Chilean event and East Japan events.
706 H. Ikeda et al. / Cryogenics 52 (2012) 704–707
Up to now, the Sumatra earthquake on December 26, 2004 (M9.1) and Chile earthquake on February 27, 2010 (M 8.8) were ob-
served with the superconducting gravimeter at Syowa Station asexamples of large earthquake. Fig. 4 shows the waveform of the
Fig. 6. Earth’s free oscillation signals changes caused by great earthquake Chilean event and East Japan events.
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gravity change observed with the superconducting gravimeter com-pared to the observation data of the Great East Japan Earthquake.Each figure 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.
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.
Fig. 6 shows the decay curve of the Earth’s 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.
5. Conclusion
A third-generation superconducting gravimeter was setup atSyowa Station. Observation is continuing smoothly 22 monthsafter installation without any cryogenics-related problem. So far,
Earth’s 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 Earth’s internalshape and structure can be expected by studying the spatial distri-bution of the amplitude of the observed Earth’s free oscillationmode 0S0. Therefore, it is important to continue observation underfavorable conditions. This superconducting gravimeter is the prop-erty of the National Institute of Polar Research.
Acknowledgements
We would like to thank the 50th and 51th Japanese Antarcticresearch expedition members, for the installation support.
References
[1] Sato T, Shibuya K, Tamura Y, Kanao M, Okano K, Fukuda Y, et al. J Geol Soc Jpn1995;42:145.
[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,
vol. 8–9; 2009. p. 45.[4] http://www.microglacoste.com/.[5] Warburton R, Brinton E. In: Proceedings of 2nd Workshop. Non-tidal gravity
changes. Les Cahiers du Centre European de Geodynamique et Seismologie;1995. p. 23.
Further reading
[6] Ikeda H, Aoyama Y, Hayakawa H, Doi K, Shibuya K. Japan Geoscience UnionMeeting; 2010 [SGD002-P02].
