Infrasounds and Background Free Oscillations Naoki Kobayashi [1] T. Kusumi and N. Suda [2] [1] Tokyo...
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Transcript of Infrasounds and Background Free Oscillations Naoki Kobayashi [1] T. Kusumi and N. Suda [2] [1] Tokyo...
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