Seismo-ionospheric precursors and disturbances of total electron content induced by the 12 May 2008 M8.0 Wenchuan earthquake

The GPS total electron content (TEC) of the global ionospheric map (GIM) is employed to simultaneously observe seismo-ionospheric anomalies during an M8.0 earthquake near Wenchuan, China on 12 May 2008. It is found that TEC above the forthcoming epicenter anomalously decreases in the afternoon period of day 6 to 4 and in the late evening period of day 3 before the earthquake, but enhances in the afternoon of day 3 before the earthquake. The spatial distributions of the anomalous and extreme reductions and enhancements indicate that the earthquake preparation area is about 1650km and 2850km from the epicenter in the latitudinal and longitudinal directions, respectively. On the other hand, a network of 6 ground-based GPS receivers in China is employed to study traveling ionospheric disturbances triggered by the Wenchuan earthquake (TIDEs). The network detects 5 TIDEs in the Southside of the epicenter area. A study on the distances of the detected TIDEs to the epicenter vs. their associated traveling times shows that the horizontal speed is 600 m/s. Applying the circle method, we find that the 5 circles intercept a point right above the epicenter when the horizontal speed of 600 m/s is given. Global searches of the ray tracing and the beam forming techniques are employed to confirm the 5 TIDEs being induced by the Wenchuan Earthquake. Results show that TEC variations before the earthquake resulted from seismo-ionospheric signals (electromagnetic mechanisms) are much greater than those after caused by vertical motions of the Earth's surface (mechanical mechanisms).