Modeling of atmospheric and ionospheric disturbances from shallow seismic sources

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Abstract

Earthquake sources, as well as contained underground explosions and volcanic explosions, initiate atmospheric waves at the air–ground interface which propagate upward and outward. The propagating atmospheric waves produced are of two types: a high-frequency acoustic wave and a low-frequency gravity wave with horizontal wavelength much longer than its vertical wavelength. Because of the exponential decrease of atmospheric density with height, the acoustic and particularly the gravity waves can grow to significant amplitude in the upper atmosphere, where they can affect the ionosphere causing changes in the distribution of neutral and charged particles. The coherent fluctuations of electron densities and ionization layer boundaries produced by these waves can be detected by electromagnetic sounding methods and hence the occurrence and character of the disturbances can be inferred. A particular application of interest is the detection and discrimination of underground and near surface chemical explosions in a nuclear test monitoring context. Specifically, identification of the different source types is enhanced by combining seismic detection methods with detection of the ionospheric disturbances caused by explosion and earthquake sources. In this study, numerical models of non-linear gravity controlled atmospheric disturbances produced by seismic sources near the surface of the Earth are investigated in order to obtain quantitative predictions that might be used in evaluating detection methods based on gravity wave excitation. Explicit numerical integration of the non-linear finite difference equations is used to simulate the transient flows produced in a three-dimensional ARDC atmosphere. Results from the simulations agree with many results from linear theory approximations and also show non-linear characteristics similar to important gravity wave observations. Electron density changes in the ionosphere are predicted with their spatial and temporal behavior found to be particularly sensitive to the type and magnitude of the dissipative mechanisms that may occur. In the numerical examples studied, the amplitudes of the ionospheric electron density fluctuations due to the gravity waves produced by large explosions and some types of large earthquakes are predicted to be well within the range of detection using E-M ionospheric sounding methods.

Keywords

  • Atmosphere;
  • Ionosphere;
  • Earthquake
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