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?