Infrasounds and Background Free Oscillations

1. Infrasounds and Background Free Oscillations Naoki Kobayashi [1] T. Kusumi and N. Suda [2] [1] Tokyo Tech [2] Hiroshima Univ.

2. O Free oscillations spherical harmonics • Normal modes of the solid earth • Earthquakes with Magnitude > 6 • Characteristic time where radial eigenfunction

3. What are the background free oscillations? • ~6×10-19 m2/s3 in the mHz band even on seismically quiet days • Annual and/or semiannual variations in amplitudes • Larger amplitudes at thebranch crossings with theinfrasound modes Nawa et al. 1998 ~ PSD of ground accelerations

4. PSD on seismically quiet days PSD of ground accelerations • 0Sl are observed on seismically quiet days • Peaks < 10-18 m2s-3 • Higher intensities in the summer season of the northern hemisphere • Larger amplitudes of 0S29 and 0S37 1990~2006, IRIS 25 quiet stations, 90 days-average

5. Larger intensities at the branch crossings with the infrasound modes • Peak intensities are larger at the branch crossings with the infrasound modes! all year mHz July Angular degree

6. What is the excitation? • Atmospheric turbulences • Kobayashi & Nishida (1998) • Nishida & Kobayashi (1999) • Modes are excited independently one another. • Fukao et al. (2002) • Oceanic process • Rhie & Romanowicz (2004) • Stronger wave radiations from northern and southern pacific ocean in winter season • Tanimoto (2005), Webb (2007) • Wave-wave interaction of ocean gravity waves global source region small source region

7. Atmospheric excitation turbulent cells cycles in life N Force degeneracy Mass (response) = 2×10-12 m/s2 the earth

8. Observation and synthetic acceleration pressure Fukao et al. (2002)

9. Well but … • Fukao et al. (2002) well explain the background free oscillations using observed pressure PSD. • But it fails to explain the excesses of amplitudes of 0S29 and 0S37. • We need the atmosphere! Branch crossings

10. New method of normal mode calculation • Anelasticity • Open boundary condition • Quick search for a complex eigenfrequency • Numerically stable Vertical displacement eigenfunction Kobayashi (GJI 2007) Both modes are calculated from the center of the Earth to an altitude of 1000km.

11. Excitation by atmospheric turbulence Power spectral densities of the ground accelerations N Force Response where From volumetric pressure forces

12. Comparison with observation response Obs./synthetic residual force

13. Seasonal variation due to thermal structure in the atmosphere Obs./synthetic response force residual

14. Excitation of acoustic modes by atmospheric turbulence only Too small to observe them!

15. Another estimate Excess in amplitude = a contribution of acoustic mode pressure. For a singlet of (multiplet)

16. Schematic view Acoustic waves Boundary turbulence Surface waves

17. conclusion • The Earth is oscillating incessantly due to other mechanism than earthquakes. Their amplitudes are about 10-18m2/s3 in the central mHz band and varies annually. • Amplitudes of modes are explained by the atmospheric turbulence in the boundary layer. • Excesses of amplitudes of modes at the branch crossings with the infrasound modes are also explained by the atmospheric turbulence. • We also predict pressure signals of infrasound modes at 3.7 and 4.4 mHz are about 10-4 Pa2/Hz which may NOT be detectable. But a broad band seismometer can be a good detector for the acoustic modes!

18. Atmospheric noises Earth’s hum microseisms Ground acceleration spectra New Low Noise Model (Peterson 1993)

19. Model atmosphere PREM + Globally averaged July atmosphere NRLMSISE-00 (Picone et al. 2002)

20. Discussion on the dynamic pressure • We use the same PSD as Fukao et al. (2002) for the dynamic pressure. • This is not the pressure of the B.L. turbulence. • However … • The values around 5 mHz are comparable with observed aero dynamic pressure. • Correlation length is also comparable with a scale of boundary layers. (~700m) mesoscale pressure B. L. winds temperature at Boso peninsula inJapan

21. Vertical displacement eigenfunctions altitude