Statistical Characteristics Of Seismic Influence On Ionosphereand Study Of Earth-atmosphere Electromagnetic Coupling Besed On The Wenchuan M_s8.0 Earthquake

Ground-based electromagnetic observation is one of traditional ways to attain earthquake precursors. As the development of earthquake spatial observation, a large massive of high quality data has been available from GPS satellite, especially DEMETER satellite lunched by France. On one hand, many studies of earthquake related ionospheric parameter abnormity have been done. On the other hand, combining ground-based observation data with spatial observation data, the investigation of lithosphere-atmosphere-ionosphere electromagnetic coupling is a new study direction in earthquake science.Under these considerations, in the first part of this paper, the statistical characteristics of earthquakes on the ionosphere are studied using all IAP ion density data about 6.5 years and all earthquakes with MW≥4.8 took place globally. The results show, the earthquake detection rate increases as the magnitude, perturbation appearing time and detection distance increase under the same dectection condition. In average, the amplitude of the perturbations is related to the magnitude of the earthquakes; the number of perturbations is higher the day of the earthquake occurring and then gradually decreases for the days before. There are seismic areas on the Earth’s surface where it will not be possible to use ionospheric parameters for earthquake prediction because the number of natural ionospheric perturbations is too large. Earthquakes occurring below the sea are better detected. Most of perturbations appear one week before the events and they move spatially toward the epicentre as the earthquake comes near.In the second part of the paper, three physical models are used to investigate the Earth-air electromagnetic coupling based on the obvious electromagnetic abnormalies recorded by ground-based observation and spatial ionospheric observation 3 days before the Wenchuan MS8.0 earhquake. First, based on Freund’s electron-hole theory and Kuo’s piezoelectric model, the result shows the surface current is A101.45′ when the main rupture of the Wenchuan earthquake occurred. Second, an infinitesimally short, horizontal dipole located at a hypocenter depth in the half-space(earth) is used to estimate the magnitude of the seismotelluric current required for the Wenchuan MW8.0 earthquake observable electromagnetic ground signals(f=1 Hz, D=1440 km, A=1.3 m V/m) and the result shows the seismotelluric current magnitude is up to A101.38I′=. While, the ionospheric effect on the electromagnetic wave transmission should not be neglected when the transmitter-receiver distance is larger than the hight of the ionospheric bottom. So, last, a long current line model is employed to estimate the current for the Wenchuan MS8.0 earthquake observable electromagnetic ground signals(f=1 Hz, D=1440 km, A=1.3 m V/m) under the ionospheric influence. Comparing with the result of A101.38I′= got by the horizontal dipole, the results show the current is about AIe5′=101.3 and this is of the same order as attained using piezoelectric model. So, the result using piezoelectric model is reasonable as the “energy source” to drive the strong spatial electromagnetic phenonema before the Wenchuan earthquake.