A network of 6 ground-based GPS receivers in East Asia was employed to study seismo-traveling ionospheric disturbances (STIDs) triggered by an M 8.0 earthquake which occurred at Wenchuan on 12 May 2008. The network detected 5 STIDs on the south side of the epicenter area. A study on the distances of the detected STIDs to the epicenter versus their associated traveling times shows that the horizontal speed is about 600 m s -1. Applying the circle method, we find that the 5 circles intercept at a point right above the epicenter when the horizontal speed of 600 m s -1 is given. Global searches of the ray-tracing and the beam-forming techniques confirm that the STIDs are induced by vertical motions in the Earth's surface during the Wenchuan Earthquake.
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