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Capturing the “hum” of the Earth on low frequency seismic arrays

Capturing the “hum” of the Earth on low frequency seismic arrays. Barbara Romanowicz Univ of California, Berkeley in collaboration with: Junkee Rhie. 0S8. 0S4. 0S5. 3S2. 0S9. 0S10. 0S11. 0S13. 2.2. 0.6. mHz. Quiet Days Station SUR. 0S22. 0S65. Kobayashi and Nishida, 1998.

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Capturing the “hum” of the Earth on low frequency seismic arrays

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  1. Capturing the “hum” of the Earth on low frequency seismic arrays Barbara Romanowicz Univ of California, Berkeley in collaboration with: Junkee Rhie

  2. 0S8 0S4 0S5 3S2 0S9 0S10 0S11 0S13 2.2 0.6 mHz

  3. Quiet Days Station SUR 0S22 0S65 Kobayashi and Nishida, 1998

  4. 437 quiet days over 10 years Tanimoto, 2000

  5. Observed Synthetic with real distrib. of M<5.8 Tanimoto, 2000

  6. Ekström, 2000 Phase shifted 15 days with respect to winter solstice

  7. Location of “hum” using an arraymethod • Two main broadband seismic arrays equipped with STS-1 seismometers: • BDSN, California • F-NET, Japan • 3rd array: GRSN (STS-2) • Data filtered in the band 150-400 sec, or use of Gaussian filter centered at 240 sec.

  8. BDSNarray

  9. Stack Amplitude as a function of time and azimuth, FNET Jan, 2, 2000 Mw 5.7 D = 74.17o Verify size of eq

  10. Quiet Day: 2000.031 BDSN FNET

  11. January 31, 2000 FNET+BDSN+Europe

  12. FNET BDSN Red = winter average(oct-march) Blue = summer average (april-sept) Black = yearly average Year 2000 2 x 10-10 m/s 0.5 ngal

  13. Winter 2000 Summer 2000

  14. Seismic background noise Significant wave height (Topex-Poseidon) Rhie and Romanowicz, 2004

  15. Excitation mechanism • Involves coupling between atmosphere/ocean and solid earth • energy dissipation in the oceans • Further understanding: • need for STS-1 or equivalent large aperture arrays, especially in the southern hemisphere (e.g. Australia) • arrays in the ocean spanning the entire water column

  16. Degree 1 term in azimuth

  17. BDSN FNET Red: winter Blue: summer black: array response Spectra

  18. Year 2000 Correl. Coef. =0.78 Summer Winter Winter Red = BDSN; back-azimuths = 295o (winter); 105o (summer) Blue = FNET; back-azimuths = 65o (winter); 235o (summer)

  19. “Quiet day”: 01/31/2000

  20. Day 200l.031 54500-55500 sec

  21. BDSN BDSN Jan 31, 2000 FNET

  22. Jan 2, 2000 Mw 5.8 D = 36.31o

  23. Year 2001

  24. Excitation mechanism • Must involve coupling between atmosphere, ocean and solid earth: • Wind generated waves (~10-20sec) • Conversion to long period infragravity waves (100-300 sec) by non-linear interactions near the shore • bounded and free infragravity waves • Coupling of infragravity waves to solid earth in the deep ocean • depends on ocean basin width, depth and shape of coasts

  25. Very few “point source” like events found: • “Slow earthquakes” (e.g. Beroza and Jordan, 1990) detectable in the period band 150-400 sec are very rare, in contrast to the shorter period band 30-150 sec (e.g. Ekström et al., 2003). • Further analysis: • Distributed sources • random around the globe?

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