Ionospheric TEC disturbance study over seismically region in China

Using data of Crustal Movement Observation Network of China (CMONOC) and IGS (International GNSS Service), we attempt to give a statistical behavior of the total electron content (TEC) perturbation before 30 Mw6.0+ earthquakes from 2000 to 2010 in China. Results show that TEC anomalies are detected before 20 earthquakes, nearly 67%. The anomalies represent positive before most events and occur mostly within 2-6 days before the shocks, significantly during the afternoon period, 12-20LT. To verify the reliability of the anomalous behavior of TEC detected in the statistical study, we analyze variations of TEC over two regions during a period of low solar and geomagnetic activity in April-October 2008. In southwest China, during that time, six large earthquakes with magnitude M ≧ 6.0 occurred. And a 'check-region' with same geomagnetic latitudes as the epicentral area is selected. Known that the decisive role in the ionosphere state is performed by space weather effects, we compare the TEC variations with time series of EUV solar radiation, Bz component of the interplanetary magnetic field (IMF), index of geomagnetic activity Dst and planetary index Kp, respectively. It is found that, both in the two regions, the observed anomalies in the regional TEC are in good correlation with the variations of above index. We detect different levels of TEC anomalies before the 6 earthquakes, and most of them are amost certainly dominated by space weather expect for Wenchuan case. The work illustrates the difficulties of searching precursors of earthquake in the ionosphere, but also, it reminds the importance for considering the effect of atmospheric waves on the ionosphere as well as the applicability of statistical approach.