Statistical analyses on the ionospheric total electron content related to M ≥ 6.0 earthquakes in China during 1998 - 2015

To verify the pre-earthquake ionospheric anomaly (PEIA), statistical analyses are implemented on the relationship between the total electron content (TEC) of global ionosphere map (GIM) and 62 M ≥ 6.0 earthquakes in China during 1998 - 2015. A median-based method together with z test is employed to determine the criteria and/or characteristics of TEC anomalies related to earthquakes. It is found that the GIM TEC significantly decreases at 18:00 - 22:00 UT (universal time, post-midnight to pre-dawn) 4 - 5 days before 37 6.0 ≤ M < 6.5 earthquakes, at 01:00 - 04:00 UT (morning) 3 - 6 days before 18 6.5 ≤ M < 7.0 earthquakes, and 04:00 - 10:00 UT (pre-noon to afternoon) 3 - 5 days; but increases 08:00 - 12:00 UT (late afternoon to early evening), 18 - 20 days before 7 M ≥ 7.0 earthquakes. The receiver operating characteristic (ROC) curve is used to compare the TEC anomaly-based method with some competitive alternatives for predicting the earthquakes under study. We found, based on possible TEC anomalies, that the observed PEIAs are significantly earthquake-related. Moreover, the results of regression analyses show that the PEIA strength is associated with the magnitude of earthquakes.