Temporal and spatial characteristics of VTEC anomalies before Wenchuan Ms8.0 earthquake

GPS and the COD VTEC data were studied in search of ionospheric VTEC changes in space and time that might be associated with the Wenchuan Ms8.0 earthquake on 12 May, 2008. The result shows several significant anomalous decreases at 12: 00 UT - 16: 00 UT on April 29 and an anomalously increase at 14: 00 UT - 18: 00 UT on May 9. The anomalies had two humps, that were located on both sides of the geomagnetic equator and had a tendency of drifting towards the equator. Since the observed anomalies cannot be attributed to any other causes and since they occurred close to the time of the earthquake, we consider them to be possibly premonitory to the earthquake.