This paper presents a statistical study of the pre-earthquake ionospheric anomaly by using the total electron content (TEC) data from the global ionosphere map. A total of 736 M >= 6.0 earthquakes in the global area during 2002-2010 are selected. The anomaly day is first defined. Then the occurrence rates of abnormal days for both the days within 1-21 days prior to the earthquakes (P(E)) and the background days (P(N)) are calculated. The results show that the values of P(E) depend on the earthquake magnitude, the earthquake source depth, and the number of days prior to the earthquake. The P(E) is larger for earthquakes with greater magnitude and lower depth and for days closer to the earthquakes. The results also show that the occurrence rate of anomaly within several days before the earthquakes is overall larger than that during the background days, especially for the large-magnitude and low-depth earthquakes. These results indicate that the anomalous behavior of TEC within just a few days before the earthquakes is related with the forthcoming earthquakes with high probability.
