Infrasounds and Infrasounds and Background Free Background Free

Oscillations Oscillations Naoki Kobayashi [1]Naoki Kobayashi [1]

T. Kusumi and N. Suda [2]T. Kusumi and N. Suda [2]

[1] Tokyo Tech [2] Hiroshima [1] Tokyo Tech [2] Hiroshima Univ.Univ.

Free oscillationsFree oscillations

Normal modes of the solid earthNormal modes of the solid earth Earthquakes with Magnitude > 6Earthquakes with Magnitude > 6 Characteristic timeCharacteristic time

R

GMPc

GM

R

c

Rff

sec1600222 3

dyn

where

radial eigenfunction

spherical harmonics

O

290O 290S 291S 292S

293S 294S 295S 296S

What are the background What are the background free oscillations?free oscillations?

~6×10~6×10-19-19 m m22/s/s33 in the mHz band even in the mHz band even on seismically quiet dayson seismically quiet days

Annual and/or semiannual variations Annual and/or semiannual variations in amplitudesin amplitudes

Larger amplitudes at theLarger amplitudes at thebranch crossings with thebranch crossings with theinfrasound modes infrasound modes

PSD of ground accelerations

Nawa et al. 1998 ~

PSD on seismically quiet PSD on seismically quiet daysdays

00SSll are observed on are observed on seismically quiet seismically quiet daysdays

Peaks < 10Peaks < 10-18-18 m m22ss-3-3

Higher intensities in Higher intensities in the summer season the summer season of the northern of the northern hemispherehemisphere

Larger amplitudes Larger amplitudes of of 00SS2929 and and 00SS3737

PSD of ground accelerations

1990~2006, IRIS 25 quiet stations,90 days-average

Peak intensities Peak intensities are larger at the are larger at the branch crossings branch crossings with the with the infrasound modes!infrasound modes!

Larger intensities at the Larger intensities at the branch crossings with the branch crossings with the

infrasound modesinfrasound modes

290S 370S 440S 550S

290S 370S 440S 550S

lP7

lP0

lS0

mH

z

Angular degree

July

all year

What is the excitation?What is the excitation? Atmospheric turbulencesAtmospheric 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 processOceanic process Rhie & Romanowicz (2004)Rhie & Romanowicz (2004)

Stronger wave radiations from northern and Stronger wave radiations from northern and southern pacific ocean in winter seasonsouthern pacific ocean in winter season

Tanimoto (2005), Webb (2007)Tanimoto (2005), Webb (2007) Wave-wave interaction of ocean gravity wavesWave-wave interaction of ocean gravity waves

small source region

global source region

Atmospheric excitationAtmospheric excitation

124

2

22

lQL

R

M

pLa l

ll

200

]km[6371

kg][101

29

2429

Q

R

M

km][1

Pa][10

L

pthe earth

turbulent cells

Force

NMass (response)

cycles in life degener

acy= 2×10-12 m/s2

Observation and Observation and syntheticsynthetic

Hz/PamHz1/102

kmmHz1/1223

15.0

f

fL

pressure

acceleration

Fukao et al. (2002)

Well but …Well but … Fukao et al. (2002) Fukao et al. (2002)

well explain the well explain the background free background free oscillations using oscillations using observed pressure observed pressure PSD.PSD.

But it fails to explain But it fails to explain the excesses of the excesses of amplitudes of amplitudes of 00SS2929 and and

00SS3737.. We need the We need the

atmosphere!atmosphere!Branch crossings

New method of normal New method of normal mode calculationmode calculation

Kobayashi (GJI 2007)

0S29

0P29

Vertical displacement eigenfunction

Both modes are calculated from the center of the Earth to an altitude of 1000km.

AnelasticityAnelasticity Open boundary Open boundary

conditioncondition Quick search for a Quick search for a

complex complex eigenfrequencyeigenfrequency

Numerically stableNumerically stable

UR

r

R

Excitation by atmospheric Excitation by atmospheric turbulenceturbulence

22

24 4

EL

RL

Power spectral densities of the ground accelerations

RXL

RURU

C

CCI

lE

kkk

k

k k

kk

k k

k

k

2

2

1

4

12*

**

where

Force

N Response

From volumetric pressure forces

Comparison with Comparison with observationobservation

Ob

s./

syn

theti

cfo

rce

resp

on

sere

sid

ual

Seasonal variation due to Seasonal variation due to thermal structure in the thermal structure in the

atmosphereatmosphere

resp

on

sere

sid

ual

Ob

s./

syn

theti

cfo

rce

Excitation of acoustic Excitation of acoustic modes by atmospheric modes by atmospheric

turbulenceturbulence

/HzPa101 2/HzPa101 24/HzPa101 26

290P lnP

Too small to observe them!

only

Another estimateAnother estimate

290S 370S 440S 550Srealfreerealreal tGfG

Excess in amplituderealreal fG = a contribution of acoustic mode pressure.

22acous.

22turb. 441.0 RRL

Hz/Pa101

Hz/Pa10226

28acous.

For a singlet of

,290P

(multiplet)

Schematic viewSchematic view

Boundary turbulence

Surface waves

Acoustic waves

/HzPa101 24

-3218 sm101

conclusionconclusion The Earth is oscillating incessantly due to The Earth is oscillating incessantly due to

other mechanism than earthquakes. Their other mechanism than earthquakes. Their amplitudes are about 10amplitudes are about 10-18-18 mm22/s/s33 in the central in the central mHz band and varies annually.mHz band and varies annually.

Amplitudes of modes are explained by the Amplitudes of modes are explained by the atmospheric turbulence in the boundary layer.atmospheric turbulence in the boundary layer.

Excesses of amplitudes of modes at the branch Excesses of amplitudes of modes at the branch crossings with the infrasound modes are also crossings with the infrasound modes are also explained by the atmospheric turbulence.explained by the atmospheric turbulence.

We also predict pressure signals of infrasound We also predict pressure signals of infrasound modes at 3.7 and 4.4 mHz are about 10modes at 3.7 and 4.4 mHz are about 10-4-4 PaPa22/Hz which may NOT be detectable./Hz which may NOT be detectable.

But a broad band seismometer can be a good detector for the acoustic modes!

Ground acceleration Ground acceleration spectraspectra

New Low Noise Model (Peterson 1993) m

icro

seis

msEart

h’s

h

um

Atm

osp

heri

c n

ois

es

Model atmosphereModel atmosphere

NRLMSISE-00 (Picone et al. 2002)

Glo

bally

ave

rage

d Ju

ly a

tmos

pher

ePREM +

Discussion on the dynamic Discussion on the dynamic pressurepressure

We use the same PSD as We use the same PSD as Fukao et al. (2002) for Fukao et al. (2002) for the dynamic pressure.the dynamic pressure.

This is not the pressure This is not the pressure of the B.L. turbulence.of the B.L. turbulence.

However …However … The values around 5 mHz The values around 5 mHz

are comparable with are comparable with observed aero dynamic observed aero dynamic pressure.pressure.

Correlation length is also Correlation length is also comparable with a scale comparable with a scale of boundary layers. of boundary layers. (~700m)(~700m)

at Boso peninsula in Japan

mesoscale

B. L.

pressure

windstemperature

Vertical displacement Vertical displacement eigenfunctionseigenfunctions

500

km0

500

alt

itu

de

270P 280P 290P 300P 310P

270S 280S 290S 300S 310S

UR

r

R

U