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Capturing the “hum” of the Earth on low frequency seismic arrays
Barbara Romanowicz
Univ of California, Berkeley
in collaboration with: Junkee Rhie
0S80S4 0S100S9 0S130S113S20S5
0.6 2.2mHz
Kobayashi and Nishida, 1998
0S22 0S65Quiet Days Station SUR
Tanimoto, 2000437 quiet days over 10 years
Tanimoto, 2000
Observed
Syntheticwith realdistrib. ofM<5.8
Ekström, 2000
Phase shifted 15 days with respect to winter solstice
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.
BDSN array
Stack Amplitude as a function of time and azimuth, FNET
Jan, 2, 2000Mw 5.7 = 74.17o
Verify sizeof eq
Quiet Day: 2000.031
FNET BDSN
January 31, 2000
FNET+BDSN+Europe
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
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: winterBlue: summer
black: array response
Spectra
SummerWinter Winter
Year 2000
Red = BDSN; back-azimuths = 295o (winter); 105o (summer)
Blue = FNET; back-azimuths = 65o (winter); 235o (summer)
Correl. Coef. =0.78
“Quiet day”:01/31/2000
Day 200l.031 54500-55500 sec
FNET
BDSN
Jan 31,2000
BDSN
Jan 2, 2000Mw 5.8 = 36.31o
Year 2001
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
• 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?