Analysis of ionospheric anomaly preceding the Mw7.3 Yutian earthquake

On February 12,2014,a large Mw7. 3 earthquake occurred in Yutian of Xijiang Province,China.We processed the global ionosphere maps provided by CODE（ the Center for Orbit Determination in Europe）and the foF2（ the critical frequency of F2-layer） data of Chongqing ionosonde station to analyze the preearthquake ionospheric anomalies. Solar activities and magnetic storm were checked by the sliding inter quartile range method to remove their effects on the ionosphere. A positive ionospheric anomaly with the large amplitude of 20 TECU was observed near the epicenter on February 3（ 10th day before the earthquake）. In addition,the foF2 at Chongqing station had an unusual increase of more than 40% on the day,which was consistent with the TEC（ Total Electron Content） anomaly. The global disturbance represents that the peak of TEC anomaly didn’t coincide with the vertical projection of epicenter. The TEC anomalous area was closer to the equator,and it mainly occurred from local time 16 ∶ 00 to 20 ∶ 00. An enhancement of TEC with the small amplitude also appeared in the magnetically conjugated region.

Ionospheric disturbances associated with the 2015 M7.8 Nepal earthquake

Based on the total electron content （TEC） derived from Global Positioning System （GPS） observations of the Crustal Movement Observation Network of China （CMONOC） and the Global Ionosphere Map （GIM） from the Center for Orbit Determination in Europe （CODE）, we detected and analyzed the ionospheric variations during the 2015 M7.8 Nepal earthquake （including the pre-earthquake ionospheric anomalies and coseismic ionospheric disturbances （CIDs） following the main shock）. The analysis of vertical total electron content （VTEC） time series shows that the large-scale ionospheric anomalies appeared near the epicenter two days prior to the earthquake. Moreover, the pre-earthcluake ionospheric anomalies were also observed in the geomagnetically conjugated region. In view of solar-terrestrial environment, the pre-earthquake ionospheric anomalies could be associated with the Nepal earthquake. In addition, we also detected the CIDs through the high-frequency GPS observation stations. The CIDs had obvious oscillated waveforms with the peak-to-peak disturbance amplitudes of about I TECu and 0.4 TECu, which propagated approximately with the horizontal velocities of 877 ±75 m/s and 319 ± 30 m/s, respectively. The former is triggered directly by the acoustic waves which originated from the energy release of the earthquake near the epicenter, while the latter could be stimulated by the acoustic-gravity waves from the partial transformation of the acoustic waves.