Investigation of the Relationship among Fault Types, Focal Depths, and Ionospheric TEC Anomalies before Large Earthquakes between 2000 and 2020

In this study, the relationships between possible ionospheric total electron content (TEC) anomalies and earthquake focal depths and fault types were investigated using 3,099 global earthquakes with $\mathrm{Mw}\ge 6.0$ between 2000 and 2020. The relationship between the ionospheric TEC changes and earthquakes has been previously discussed in the literature. However, very few studies have examined the relationship between the focal depths and the fault types of these earthquakes. In general, three indices, namely the disturbance storm-time index (Dst), the planetary geomagnetic activity index (Kp), and the solar flux (F10.7), are used to characterize the space weather conditions (SWC) that affect TEC anomalies and to identify the active/quiet days for future earthquakes. In this study, 13 different indices were used: indices that measured solar activity included proton flux (Pf) at six different energy levels, F10.7, and extreme ultraviolet (${\mathrm{EUV}}_{0.1-50\mathrm{nm}}$ and ${\mathrm{EUV}}_{26-34\mathrm{nm}}$). Meanwhile, geomagnetic storm indices included Dst, Kp, the $z\text{-}\text{component}$ of the magnetic field index (Bz), and proton density ($P_d$). By using the ionospheric TEC values derived from global ionosphere maps (GIM-TEC), both negative and positive ionospheric TEC anomalies were observed for 15 days before and 4 days after the earthquakes using a statistical analysis technique that involved a 15-day moving median. The earthquakes were grouped according to their fault types (normal, thrust, and strike-slip) and focal depths (shallow, intermediate, and deep). Their relationships with the negative and positive TEC anomalies in the quiet days before the earthquake were examined. Negative and positive anomalies were observed approximately 3–12 days before earthquakes that occurred on normal faults, approximately 5–8 days before earthquakes that occurred on thrust faults, and approximately 1–12 days before earthquakes that occurred on strike-slip faults. The average TEC anomaly changes were calculated to be 43.4% total electron content unit (TECU) for normal faults, 44.8% TECU for thrust faults, and 41.3% TECU for strike-slip faults. Positive anomaly values were exhibited by larger earthquakes on normal and thrust faults, whereas negative anomaly values tended to occur on strike-slip faults. Negative and positive TEC anomalies were detected around 3–12 days before shallow earthquakes, around 1–13 days before intermediate-depth earthquakes, and around 1–12 days before deep earthquakes. The average TEC anomaly changes were calculated to be 44.4% TECU for shallow earthquakes, 43.3% TECU for intermediate-depth earthquakes, and 42.6% TECU for deep earthquakes. Positive anomaly values were also found to be higher than the negative anomaly values in each group. We propose that there is a relationship between the ionospheric TEC anomalies that occur before large earthquakes and their fault types and focal depths.