Ground-based Ionospheric Inversion And Pre-earthquake Ionospheric Anomaly

Since in 1960 s,some American scientists discovered that there was a significant disturbance in the ionosphere near the epicenter before the earthquake,the case studies and statistical analysis of the pre-earthquake ionospheric anomalies have progressed gradually.Study shows that the pre-earthquake ionospheric anomaly has short-term feature,which opens up a new research and analysis idea for earthquake prediction,especially for short-term earthquake prediction.However,it must be acknowledged that the study of the pre-earthquake ionospheric anomaly is still in the stage of exploration and experiment.The mechanism of the pre-erathquake ionospheric anomaly is still unclear.It is necessary to continue the research,and more case studies and statistical analysis are needed for veryfing the temporal and spatial distribution characteristics of the pre-earthquake ionospheric anomalies.The study of pre-earhtquake ionospheric anomaly is inseparable from high-precision ionospheric observation and solution.Although the current techniques for ionospheric observation are relatively mature,there still exist possibilities for improvement of studies on the ionospheric solution and inversion methods,such as instrument differential code bias(DCB)estimation in GPS TEC solution,reconstruction algorithm problems of the ionospheric tomography,etc.,which need further researches.The study of pre-earthquake ionospheric anomaly also involved disturbance source analysis.As we known,there are many factors that can cause ionospheric disturbances,and the most important is space weather.In addition,lightning,thunderstorms,typhoons,volcanic activities,seismic activities and even human activities may cause significant disturbances in the ionosphere.However,the disturbance mechanisms of these factors are still not fully understood,and the temporal and spatial distribution characteristics of ionospheric disturbances caused by various factors are still inconclusive,which encourages people to explore and research.This thesis focuses on three topics: ground-based ionospheric observation and solution methods,ionospheric disturbance distribution under the influence of space weather(magnetic storm)and spatial-temporal distribution characteristics of the pre-earthquake ionospheric anomalies.The concrete research content of this thesis is as follows:1)Based on the observation data of the ionosonde in Xialatuo seismic station,the main factors affecting the observation quality are analyzed.The research shows that the appearance of the ionospheric Es and the Spread F will cause the deterioration of the observation quality of the ionosonde.The reliability of the final results of the ionosonde is closely related to the ionogram measurement.Reliable automatic ionogram measurement algorithm,supplemented by manual intervention,can obtain reliable observation results.2)Based on the single layer model and the spherical harmonic model,the regional ionospheric VTEC map and the receiver DCB estimation are realized.Using the observation data of Crustal Movement Observation Network of China(CMONOC)to calculate the distribution of the ionospheric VTEC above China,and analyzing the reliability of the solution results,it is found that the solution results of the ionospheric VTEC distribution are better than 0.7TECU,and the root-mean-squire errors of receiver DCB estimation results reach about 0.8 ns,which proves that the solution results are highly reliable.3)The reconstruction algorithm of ionospheric three-dimensional tomography is studied.A constraint iterative reconstruction algorithm based on inverse distance weighting is proposed.The algorithm is based on the recognition that the electron density corresponding to any voxel in the ionospheric reconstruction region can be interpolated by electron densities of the adjacent voxels.The algorithm uses the inverse distance weighting to establish this interpolation relationship,and then obtains the constraint equation of the reconstruction region.Tomographic reconstruction experiment of the ionosphere in a specific area is performed by using the observation data of CMONOC.The process involves the study-proposed constrained and unconstrained reconstruction algorithms respectively.Then the two reconstruction results are compared with the observation result of the ionosonde in Xialatuo seismic station.It is found that the reconstruction result of the new algorithm is in good agreement with the observation result of the ionosonde,which proves the validity and reliability of the algorithm.4)Using the observation data of CMONOC and the global ionospheric VTEC map of CODE,by virtue of the ionospheric TEC solution method and ionospheric tomography technology studied in this thesis,and combined with the observation results of the ionosonde,the changes of the ionosphere over the global region and China during the time of the two geomagnetic storms on March 7-9,2012 and March 17-18,2015 respectively are investigated.The temporal and spatial distribution characteristics of the ionospheric anomalies are analyzed.The study shows that the ionospheric disturbances caused by geomagnetic storms have the basic characteristics of wide distribution area,long duration and high disturbance intensity.At the same time,travelling ionospheric disturbance(TID)can also be found in the 2015 geomagnetic storm event.According to the theory of ionospheric physics,the TID should be produced by acoustic gravity waves excited in the polar region during the geomagnetic storm.5)Based on the seismic cases of the Yutian M7.3 earthquake on February 12,2014,the Tohoku M9.0 earthquake on March 11,2011,and the Wenchuan M8.0 earthquake on May 12,2008,using the global VTEC map of CODE and the observation data of CMONOC,and combined with geomagnetic indices,the changes of the ionosphere before the three earthquakes are examined.The result shows that the three cases have pre-earthquake ionospheric anomalies.By analysis on the spatial-temporal distribution characteristics of the anomalies,it is found that the anomalies are close to the epicenters and occur within 10 days before the earthquakes,which indicates that the anomalies are highly correlated with the earthquakes in space and time.The tomographic imaging technique is used to reconstruct the pre-earthquake electron density distributions of the three earthquakes.The three-dimensional distribution structures of the ionospheric anomalies are investigated.It is found that the ionospheric anomalies before the earthquakes are mainly concentrated at the height of 200-400 km,which is different from the vertical distribution characteristics of the storm-induced ionospheric anomalies.6)A total of 110 earthquakes with magnitudes greater than M5.0 are selected as seismic samples.The global ionospheric VTEC map of CODE is used to investigate the ionospheric changes before these samples.Nearly half of the earthquakes are detected to have pre-earthquake ionospheric anomalies.The pre-earthquake ionospheric anomalies are classified according to the magnitudes,depths and focal mechanisms of the forthcoming earthquakes.The possibility distributions of ionospheric anomalies before the earthquakes in different classification are investigated.It is found that the pre-earthquake ionospheric anomaly mainly has the following distribution characteristics: the larger the magnitude,the more likely it is to detect the pre-earthquake ionospheric anomaly,and more likely the anomaly is to occur within 10 days before the earthquake;The occurrence probability of the pre-earthquake ionospheric anomalies may have a negative correlation with the depth of the source;the pre-earthquake ionospheric anomaly is the most possible to occur before the reverse-fault earthquakes.