Refined seismic-ionospheric effects: case study of Mw 8.2 Chiapas earthquake on September 7, 2017

Abstract

Due to poor local accuracy and temporal resolution, the GIM (global ionospheric map) application continues to be of restricted use in obtaining the characteristics of seismic-ionospheric effects. Utilizing spherical harmonic (SH) function and generalized trigonometric series (GTS), the latest 15 min/map GIM with high precision released from the Chinese Academy of Sciences (CAS), is expected to record the short-term response of the fine ionospheric structure. From the interquartile range (IQR) method, we first present the application of CAS GIM in the Mw8.2 Chiapas earthquake in Mexico on September 7, 2017. To verify the reliability, the IGS GIM at low temporal resolution of 2 h/map was simultaneously analyzed and compared. Our results exhibit clear local anomalies that preceded the Chiapas event by 5 days. The detection of IGS GIM merely captured the basic features of pre-earthquake ionospheric anomalies, i.e., abnormal local effect with a duration of 15 h, maximum amplitude of 20 TECU, and westward migration in the seismogenic area with epicentral distance <4092 km. As a comparison, the periodic variations of motion state, energy, and abnormal morphology in the ionospheric anomalies were originally revealed by CAS GIM. The motion tracks exhibited oscillation periodicity characteristics, and the nonstationary and nonlinear intensity fluctuation implied a harmonic energy transmission. These results reflected the imbalance between energy transmission and ion\u2009+\u2009distribution in dynamic ionospheric processes. Then, the evolution of the ionospheric anomaly could be divided into three stages: formation of 9.2–12.4 LT, climax of 12.6–19.5 LT, and dissipation of 19.7–23.7 LT. During the peak period, the changes in energy transmission resulted in transforming the ionosphere to a single-double peaks shape and strong enhancement of the TEC intensity. The variations in horizontal phase velocity of 166–320 km/s in the abnormal region reflected the differences in electromagnetic waves from slow increases to rapid attenuation in the formation and dissipation periods. Based on the electromagnetic field disturbances recorded by the Swarm spacecraft, potential lithosphere–atmosphere–ionosphere Coupling (LAIC) effects could be identified through the electromagnetic coupling path. These features made it easy to remove earthquake precursor signals from other “source-type” TEC turbulences.