超 伝 導 重 力 計 - 神 岡 観 測 の 意 義 Superconduction

Transcription

1 平 成 17 年 度 共 同 利 用 研 究 成 果 発 表 研 究 会 ( 東 京 大 学 宇 宙 線 研 究 所 ) 超 伝 導 重 力 計 - 神 岡 観 測 の 意 義 Superconduction gravimeter observation at Kamioka - its significance in geophysics- 佐 藤 忠 弘 ( 国 立 天 文 台 ) Tadahiro Sato National Astronomical Observatory of Japan Content: 1. Observation of the Earth s gravity 2. Superconducting gravimeter and the GGP network 3. Contribution to the study of Earth s structure (Results of the M9.3 Sumatra-Andaman Earthquake)

2 Kamioka SG observation stated on October 22 1n Inuyama

3 Gravity on the Earth s surface Its temporal changes are mainly driven in three ways; (1) Vertical curstal motion, (2) Redistribution of the mass above and/or within the Earth, (3) Changes in the acceleration at the observation site due to the seismic motion and the external force such as the tidal force.

4 Unit of Gravity / 1 m/s/s = 100 cm/s/s 1 cm/s/s = 1Gal / Mean value of the Gravity on the Earth s surface: 980 cm/s/s = about 1,000 Gal / 1μGal = 1 over 1,000,000,000 of the surface gravity (1 over 1 billion) Sensitivity of the superconducting gravimeter: more than 1 ngal = 1 over 1,000,000,000,000 of the surface gravity (1 over 1 trillion).

5 Superconducting Gravimeter (SG) / A gravimeter uses the magnetic force as a spring force, which is produced by the super-conducting current (SC) being realized at the LHe temperature of / Due to the proper stability of the SC with zero resistance, a very stable spring force is realized in the SG system. / The cryogenic temperature gives a very stable environment for the measurement. This helps to reduce a thermal noise of the sensor.

6 SG network of GGP (Global Geodynamics Project) Note: Officially, GGP started in 1997 as an international observation project. Quite recently, South Korea and Taiwan participated in GGP.

7 GGP observation site at Ny-Alesund Alesund, Svalbard in Arctic

8 Gravity signals on the Earth s s surface SNM:Seismic Normal mode, BFO:Back-groud Free Oscillation, ST:Slichiter triplet, C.M:core under tone, FCN:Free core nutation, TDT:Terdiurnal tide, SDT:Semidiurnal tide, DT: Diurnal tide, WT:Weekly tide, FNT: Fortnightly tide, MT:Monthly tide, SAT:Semiannual tide, AT:Annual tide and Seasonal gravity change, CW: Chandler tide.

9 Noise level at several GGP sites at the frequency band of Earth s s normal mode NLNM (New Low Noise Model by Peterson, 1993) Cross over point

10 Calibration of the SG#016 Result for the calibrations : At Syowa in Jan. 2001: (0.061) μgal/volt At Kamioka in July 2005: (0.077) μgal/volt Difference: 2% Calibration of the Kamioka SG in July 2005 by the Kyoto FG5.

11 Observation of the gravest Earth free oscillations excited by the 2004 Sumatra-Andaman earthquake

12 Noise levels at Kamioka and Matsushiro

13 Characteristics of the 2004 Sumatra-Andaman Earthquake / It was the largest event after the Chilean event in / The low-frequency normal modes were strongly excited. / This earthquake also excited very huge tsunami, which can be observed with enough amplitude at Syowa Station about 9000 km away from the source area.

14 Importance of the precise observation of the low-frequency Earth s s normal modes including those related to the core motion 1. Contribution to the study for source mechanism 2. Improvement of the low-degree density structure inside the Earth. This information is important for the physical interpretation of the 3-D mantle structure obtained by the tomography. 3. Observed splitting and coupling of the normal modes give us very useful information on the Q structure and the heterogeneity inside the Earth, especially useful to constrain the long-wave length structure.

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16 1. The 2004 Sumatra-Andaman earthquake, it was a huge event. Low-frequency normal modes observed from the Canberra SG F r e e Rosat et al., GRL, 2005

17 2. It radiated much seismic energy into the low-frequency band. Fig.5 of Park et al. (Science, 2005) Fig.6 of Park et al. (Science, 2005) Harvard CMT Finite-rupture Models II & III (Ammon et al.,2005) Waves mainly used surface wave body and surface waves Rupture duration (s) Fault size (km) Fault geometry steeper than CMT Moment (N m) 4.0 x x MW Note: Model III prescribes a peak rate of rupture propagation somewhat earlier than Model II.

18 M W 9.0 M W 9.3

19 Example of the observed splitting of the low-frequency seismic normal modes

20 Splitting of the normal mode: ω m = ω k (1+a+bm+cm 2 ) by Dahlen & Sailor (1979) ω k : angular frequency in the non-rotating spherically symmetric Earth a: coefficient for the Earth s ellipticity dominate in the higher frequency a = αε h + 2 nd order terms for Ω/ω k, b: Coriolis splitting coefficient for the 1 st order effect dominate in the lower frequency b = βω/ ω k = Ω/ ω k ρ(r) {kinetic energy of the oscillation} dr c: coefficient for the 2 nd order effect of the Coriolis force.

21 Observation of the 0 S 0 mode A notable characteristic of the 2004 Sumatra- Andaman earthquake is it also strongly excited 0 S 0.

22 The time decay of 0S0 amplitude was followed till the second Sumatra event of M8.3 on March

23 0 S 0, is it spherically symmetric oscillation? The answer is no!

24 Observed Latitude Dependency of 0 S 0 Note 1: Observed magnitude far exceeds the nominal calibration accuracy of the SGs (better than 0.2%). Note 2: KA, MA, CB SU locate at the similar latitude in absolute value. Model computation consists with the observation at KA and MA, but it shows a large discrepancy at the sites of southern semishpere, especially at CB.

25 Mode coupling due to the Earth s s rotation: Coriolis coupling Observation of Coriolis coupled modes below 1 mhz W. Zürn, G. Laske, R. Widmer Schnidrig, F. Gilbert (G. J. Int., Volume 143, 2000) We present observations of spectral energy at toroidal mode frequencies in vertical seismic recordings of the 1998 Balleny Islands earthquake. Since toroidal modes on a spherically symmetric, non rotating Earth have horizontally polarized particle motion these observations call for an explanation. We first rule out local and instrumental effects as being responsible for the vertical component signal of the toroidal modes 0T3 (0.59 mhz) and 0T4 (0.77 mhz). The global effects that we consider are general heterogeneous mantle structure, ellipticity of figure and rotation. We find that rotation through Coriolis coupling of low order spheroidal and toroidal oscillations is the dominant mechanism. Strasburg SG BFO L& R Gravimeter Boulder SG BFO Tilt meter BFO Strain meter

26 An experiment on the improvement in the estimation of the pressure effect

27 Summary: 1. The SG observation at Kamioka successfully started on October just before the huge Sumatra- Andaman earthquake. 2. Low-frequency normal modes were well observed at Kamioka as well as other many sites of the GGP network. 3. 0S0 Our analysis results clearly reveal the latitude dependency of the 0S0 amplitude. It may be the first finding from the observation. The observation shows a clear discrepancy in amplitude to the model computation especially in the sites in the southern semisphere.

28 4. This finding is important to study the inhomogeneity inside the Earth, because of the simple excitation mechanism of 0S0 mode. 5. Collocation observation Collocation observation of the SG and strain meter is important to study the coupling between the spheroidal and toroidal modes of the Earth s free oscillation. On this purpose, the comparison at Kamioka, Matsushiro and Inuyama may give us a useful data to study the mode coupling. 6. Calibration Through the study introduced here, we have recognized again an importance of the calibration of the observation instrument, although it is an inconspicuous work.

29 Thanks for your attention and Thanks for the cooperation of ICRR on the SG observation at Kamioka...