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

0S8 0S4 0S100S90S130S113S20S mHz

Kobayashi and Nishida, S220S65 Quiet Days Station SUR

Tanimoto, quiet days over 10 years

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

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

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 sec, or use of Gaussian filter centered at 240 sec.

BDSN array

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

Quiet Day: FNET BDSN

January 31, 2000 FNET+BDSN+Europe

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

Winter 2000 Summer 2000

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

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

Degree 1 term in azimuth

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

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

“Quiet day”: 01/31/2000

Day 200l sec

FNET BDSN Jan 31, 2000 BDSN

Jan 2, 2000 Mw 5.8  = o

Year 2001

Excitation mechanism Must involve coupling between atmosphere, ocean and solid earth: –Wind generated waves (~10-20sec) –Conversion to long period infragravity waves ( 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

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