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New Observations in Earth’s Normal Modes of Free Oscillation

New Observations in Earth’s Normal Modes of Free Oscillation. 趙丰 Benjamin F. Chao, 雷湘鄂 Xiang’E Lei 1 朱澄音 Amelie C. Chu, Institute of Earth Sciences, Academia Sinica , Taiwan 1 also at Inst. Geodesy and Geophysics, Chinese Academy of Sciences, Wuhan. Normal modes ||.

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New Observations in Earth’s Normal Modes of Free Oscillation

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  1. New Observations in Earth’s Normal Modes of Free Oscillation 趙丰 Benjamin F.Chao, 雷湘鄂Xiang’E Lei1 朱澄音 Amelie C. Chu, Institute of Earth Sciences, Academia Sinica, Taiwan 1also at Inst. Geodesy and Geophysics, Chinese Academy of Sciences, Wuhan

  2. Normal modes || Traveling-wave – Standing-wave Equivalence sin(wt + kx) + sin (wt – kx) = 2 sinwt * coskx

  3. A typical seismogram

  4. (3-D) (discrete) Eigenfunction , Eigenfrequency nulm,nωlm degree overtone number order n=0: Fundamental modes ~ Surface waves n>0: Overtone modes ~ Body waves

  5. Earthquake Displacement Field • Equation of motion • Solve by expanding displacement field Normal mode eigenfunctions for SNREI Earth Expansion coefficients (note the static limit) (Gilbert, 1970 )

  6. Deep Earthquakes: 1970 Colombia, M 8.0, depth = 650 km 1994 Bolivia, M 8.2, depth = 630 km 2013 Okhotsk Sea, M 8.3, depth = 609 km

  7. Product power spectrum (logarithm) V ~60 stations H ~80 stations

  8. Product power spectrum (logarithm) V ~60 stations H ~80 stations

  9. Product power spectrum (logarithm) V ~60 stations H ~80 stations

  10. Product power spectrum (logarithm) V ~60 stations H ~80 stations

  11. Product power spectrum (logarithm) V ~60 stations H ~80 stations

  12. Radial “breathing” modes nS0 (n = 0 ~ 11) Observation vs. model

  13. GSI’s GNSS stations • GSI (Geospatial Information Authority of Japan) • GEONET(GNSS Earth Observation Network System) • Total 1230 stations at an average interval of about 20km for crustal deformation monitoring and GNSS surveys. • # Station used in this study: 1019 (http://dbx.cr.chiba-u.jp)

  14. (Mitsui et al.,2012)

  15. GPS Data Processing: • GIPSY/OASIS-II (Ver. 6.1) • Precise Point Positioning (PPP) technique • Sampling Rates: 30 sec. • Data Length: 21hrs (starts from 06:00 (UTC) 11th, March) 30hrs (starts from 18:00 (UTC) 11th, March) • We carry out simple spectral stacking, reducing the variance of the noise level

  16. Spectral Stack – GSI, Japan 0S2 0S3 0S4 0T2 |Y(f)|2 0S9 2T1 0T15 2T2 2T7 0T20 1T11 1T12 0T17 1T9 0S10 0T3 0T11 0T19 0T13 0T22 0T4 0T10 0T12 0T14 0T21 0T18 - - - fundamental toroidal modes

  17. Spectral Stack – GSI, Japan 0S2 X 0S3 0S4 0S0 0T2 0S8 0S7 0S12 0S9 0S6 0S21 0S18 0S20 1S4 －: Fundamental spheroidal modes

  18. PBONetwork • The Plate Boundary Observatory (PBO) ofEarthScopemeasures Earth deformation through its arrays of GPS receivers, strainmeters, seismometers. • Ref. Frame: IGS08 • # GPS Records: 1548

  19. Stack – PBO, Western U.S. 0S6 1S4 0S3 0S2 x 0S0 0S9 0S5 0T8 0S10 1S10 0S8 0T18 1S8 0T23 2T1 0T13 0T12 x 2T2 0T6 0S4 0T10 0T11

  20. International GNSS Service IGS Network# GPS Station used : 337

  21. DART of NOAADeep-ocean Assessment and Reporting of Tsunamis network

  22. 24 BPR records used (15 for 2010 Chile, 5 for 2011 Japan, 3 for 2005 Sumatra/Nias, 1 for 2004 Sumatra): After arrival of seismic waves but before tsunami, typically several hours long, at “event mode” 15-sec sampling.

  23. A typical BPR earthquake record(4 days long, after de-tide)

  24. Spectrum of a single DART (BPR21413) record for 2010 Chile earthquake Blue: pre-earthquake Red: during earthquake Pink line: spheroidal 0Sn modesGreen line: toroidal 0Tn modes

  25. Spectral Stack of 24 DART records for 4 earthquakes Blue: pre-earthquake Red: during earthquake Pink line: spheroidal 0Sn modesGreen line: toroidal 0Tn modes

  26. What does ocean BPR record? • Assuming hydrostatic (OK for <60 mHz) • Pressure changes because the “g” in P=ρgHchanges, from g to g±a(equivalence principle), where a is the vertical acceleration associated with the given mode, or dω2 => spheroidal modes only? (but we see toroidal modes too…) • Additionally the dynamic drag produces an opposing pressure ½ CDρv2 which is tiny.

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