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

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

What are the background free oscillations? ~6× m 2 /s 3 in the mHz band even on seismically quiet days ~6× m 2 /s 3 in the mHz band even on seismically quiet days Annual and/or semiannual variations in amplitudes Annual and/or semiannual variations in amplitudes Larger amplitudes at the branch crossings with the infrasound modes Larger amplitudes at the branch crossings with the infrasound modes PSD of ground accelerations Nawa et al ~

PSD on seismically quiet days 0 S l are observed on seismically quiet days Peaks < m 2 s -3 Higher intensities in the summer season of the northern hemisphere Larger amplitudes of 0 S 29 and 0 S 37 PSD of ground accelerations 1990~2006, IRIS 25 quiet stations, 90 days-average

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

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

Atmospheric excitation the earth turbulent cells Force N Mass (response) cycles in life degeneracy = 2× m/s 2

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

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 0 S 29 and 0 S 37. We need the atmosphere! Branch crossings

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

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

Comparison with observation Obs./synthetic force response residual

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

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

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

Schematic view Boundary turbulence Surface waves Acoustic waves

conclusion The Earth is oscillating incessantly due to other mechanism than earthquakes. Their amplitudes are about m 2 /s 3 in the central mHz band and varies annually. The Earth is oscillating incessantly due to other mechanism than earthquakes. Their amplitudes are about m 2 /s 3 in the central mHz band and varies annually. Amplitudes of modes are explained by the atmospheric turbulence in the boundary layer. 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. 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 Pa 2 /Hz which may NOT be detectable. We also predict pressure signals of infrasound modes at 3.7 and 4.4 mHz are about Pa 2 /Hz which may NOT be detectable. But a broad band seismometer can be a good detector for the acoustic modes!

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

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

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) at Boso peninsula in Japan mesoscale B. L. pressure winds temperature

Vertical displacement eigenfunctions altitude