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Infrasounds generated by the China earthquake of
Nov. 14, 2001 (Mm 7.8) Analysis of
ground-coupled air waves and distant generation
of infrasounds detected at I34MN - Mongolia A.
Le Pichon, J. Guilbert, M. Vallée, B. Alcoverro
J. Virieux, JX Dessa, M. Ulziibat
Mongolian Academy Center Research Center of
Astronomy Geophysics Ulaanbaatar-51
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11/14/2001 092610 90.7E - 36.0N Mm 7.8 D
1850 km
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Results of PMCC detection
A
B
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A true seismic / acoustic coupling ?
A
MB2000
LP-Z
a gt 5 10-5 g _at_ 20 s Dz gt 0.5 cm _at_ 20 s
0.02 Pa (sens. MB2000 10 V/g) 0.05 Pa / DP
7.5 Pa c-c
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Results of PMCC detection - 1D
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Infrasound propagation and location of distant
source regions
First approach direct simulation

• Discretization of fault plane 20 x 20 km, 0-60
  km depth
• Simulation of the 3C ground displacement 20 x
  10, Bouchon method, 1976
• Computation of the synthetic pressure field
  integral equation ETOPO30
• Reconstruction of the azimuth time variation at
  I34MN
• Comparison with measurements

Second approach inverse location procedure

• Measurements at I34MN arrival time and
  backazimuth
• Origin time and epicenter coordinates
• Seismic surface waves propagation Vs 3 km/s _at_
  20s
• Acoustic celerity to be determined

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Spatial variations of the HWM-93 wind profiles
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3D Ray-tracing results
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Isd
It
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Simulation of the acoustic propagation from the
topography
Integral formulation Helmholtz-Huygens
Fraunhofer approximation first order in r/rl
Directivity pattern
Cells 5 x 5 km with T 20 s, kL 5
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Comparison between measurements and simulations
at I34MN
With a 3 azimuth correction, celerity 0.29 km/s
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CONCLUSION

• Simulation results
• - The direct and inverse approaches yield the
  same results
• - Good agreements with the measurements using
• - line-source hypothesis far-field
  approximation of the integral equation
• - velocity model provided by empirical
  atmospheric profiles
• - synthetic seismograms along the fault
• - Directivity of the radiation

• Sources of infrasonic waves
• - Seismic ground-coupled air waves
• - Diffracted pressure waves reradiated by the
  topography when surface seismic waves carry
  energy from the rupture points
• - Accurate location of distant source regions
• - propagation range 1500-2000 km
• - horizontal scale size 10 x 10
• - Infrasonic detection generated by large
  earthquake can be used to validate celerity
  models in the atmosphere 0.285-0.295 km/s