Fundamental Study on the Effective Observation Method and the Propagation Mechansm of Anomalous Radio Wave Noise Occurring Just Prior to Earthquakes

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1. Survey of references : The experimental observations of the anomalous electromagnetic emissions possibly related to earthquakes have been carried out from the years of 1970. The observation points are almost near the ground surface, and the receiving frequencies are less than several MHz. As the observed data are few as yet, these data are not entirely acceptable in general. The references of the theoretical investigations or numerical examples assuming the wave source at the deep ground were not found.2. Derivation of the approximately theoretical equations and the results of the numerical calculations : We treated this problem as the simplest model consisted of two-layred (ground and air) homogeneous media, assuming the source was located at the focus of an earthquake. Equations of the electric field intensity near the ground surface and of the strength of the source were obtained, using the geometrical optics theory. Applying the observed data by other research institution to the equations, numerical estimations were carried out. These results lead to the conclusion described later section 4.3. Experiments with the small-sized ground-air model in a room : The experiments were attempted to verify the validity of the calculations. The measurements were done in the limited space including dry sand enclosed by some radio-wave absorbers. But we observed the unsatisfied data because the reflection occured from absorber and the instability of a receiving antenna.4. Consideration for the effective observation : The observation point should be located in the deep ground to remove another noise coming from above the ground, and in the low conductivity ground at lower observation frequency for lower propagation loss.