Total electron content variations in the ionosphere before the Colima, Mexico, earthquake of 21 January 2003

Anomalous ionospheric variations associated with the process of strong earthquake (M>5) preparation has been fairly well established. To check possible ionospheric variations connected with the recent Colima earthquake, M=7.8, of 21 January 2003, the data of five stationary GPS receivers of the National Institute of Statistics, Geography and Informatics (INEGI) of Mexico network were analyzed. It was found that the vertical total electron content showed anomalies two to three days before the seismic shock, while the daily cross correlation coefficient calculated for the different pairs of GPS receivers presented a drop within an interval of one to five days before the seismic shock. Also, the horizontal spatial (latitude-longitude) distribution of the TEC deviation had its peak of deviation in a point close to the vertical projection on the ionosphere of the impending earthquake epicenter, thus revealing the local character of the observed anomaly. We conclude, that the observed variations in the ionosphere can be regarded as a possible short-term earthquake precursors.