Ionospheric earthquake signatures on GPS TEC

Abstract

The ionosphere total electron content (TEC) can be derived from a ground based receiver of the global Positioning System (GPS). The GPS consist of 24 satellites, evenly distributed in 6 orbital planes around the globe at an altitude ∼20200 km. Each satellite transmits signals on two frequencies (f1 = 1575.42 MHz and f2 = 1227.60 MHz) with two different codes, P1 (or C/A) and P2 and two different carrier phases, L1 and L2. Since the ionosphere is a dispersive medium, the speeds of propagation of the electro-magnetic waves transmitted by the GPS satellites depend on the frequency of radio waves. The carrier phase advance and group delay of GPS transmitted radio waves in the ionosphere is proportional to electron content integrated along the propagation path. From carrier phase and pseudo-range code data observed by GPS receivers located in the vicinity of Indonesia, we derived the ionospheric GPS TEC using phase levelling method. For the aim of finding the diurnal variation anomalies associated with the large earthquakes in Indonesia and surrounding areas, we used the harmonic analysis of TEC observed at several GPS stations. The results of harmonic analysis have revealed that the amplitude of diurnal variations of TEC decreases (negative anomaly) or increases (positive anomaly) several days before major earthquakes, and by using spatial analysis of the amplitude anomaly, the magnitudes of some major earthquakes can be estimated.