MODELING OF ATMOSPHERIC AND IONOSPHERIC DISTURBANCES FROM SHALLOW SEISMIC SOURCES

Earthquake sources, as well as contained underground explosions and vo lcanic explosions, initiate atmospheric waves at the air-ground interf ace which propagate upward and outward. The propagating atmospheric wa ves produced are of two types: a high-frequency acoustic wave and a lo w-frequency gravity wave with horizontal wavelength much longer than i ts vertical wavelength. Because of the exponential decrease of atmosph eric density with height, the acoustic and particularly the gravity wa ves can grow to significant amplitude in the upper atmosphere, where t hey can affect the ionosphere causing changes in the distribution of n eutral and charged particles. The coherent fluctuations of electron de nsities 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 appli cation of interest is the detection and discrimination of underground and near surface chemical explosions in a nuclear test monitoring cont ext. Specifically, identification of the different source types is enh anced by combining seismic detection methods with detection of the ion ospheric disturbances caused by explosion and earthquake sources. In t his study, numerical models of non-linear gravity controlled atmospher ic disturbances produced by seismic sources near the surface of the Ea rth are investigated in order to obtain quantitative predictions that might be used in evaluating detection methods based on gravity wave ex citation. Explicit numerical integration of the non-linear finite diff erence 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 th e type and magnitude of the dissipative mechanisms that may occur. In the numerical examples studied, the amplitudes of the ionospheric elec tron density fluctuations due to the gravity waves produced by large e xplosions and some types of large earthquakes are predicted to be well within the range of detection using E-M ionospheric sounding methods. (C) 1998 Published by Elsevier Science B.V.