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

3. 0S8 0S4 0S100S90S130S113S20S5 0.6 2.2 mHz

4. Kobayashi and Nishida, 1998 0S220S65 Quiet Days Station SUR

6. Tanimoto, 2000 437 quiet days over 10 years

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

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

9. Location of “hum” using an array method 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.

10. BDSN array

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

12. Quiet Day: 2000.031 FNET BDSN

13. January 31, 2000 FNET+BDSN+Europe

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

15. Winter 2000 Summer 2000

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

17. 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

19. Degree 1 term in azimuth

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

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

22. “Quiet day”: 01/31/2000

23. Day 200l.031 54500-55500 sec

24. FNET BDSN Jan 31, 2000 BDSN

25. Jan 2, 2000 Mw 5.8  = 36.31 o

26. Year 2001

31. 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

32. 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?