Sensing Ionospheric Precursors of Earthquakes by Inversion of GPS Two-Frequency Beacon Data

There is growing evidence of ionospheric precursors over earthquake preparation regions, manifesting as changes in the electron density distribution up to two weeks before strong earthquake events. The work reported here addresses imaging the ionospheric electron density structure by inversion two-frequency GPS beacon signals to three-dimensional electron density distributions using an array of GPS receivers. We present a system that processes phase and group delay time series data from a network of dual frequency GPS receivers and produces a dynamic ionospheric model that is consistent with all the input data. The system is intended for monitoring the ionosphere over a fixed geographical area with dimensions of the order of several thousand kilometers. The core of the inversion technique is Tikhonov's methodology for solving ill-posed problems. We extended the method to multidimensional nonlinear inverse problems and developed techniques for fast numerical solution. The resulting solution for the ionospheric distribution of electron density is guaranteed to be smooth in space and time and to agree with all input data within errors of measurement. The input data consist of time series of absolute TEC and relative TEC (directly calculated from the raw dual-frequency group delays and phase delays, respectively). The system automatically estimates the measurement noise and receiver-transmitter biases. We test the system using archived data from dual frequency GPS receivers in the Southern California SOPAC network and data from a vertical sounder. This code is used to study the recent Parkfield earthquake (M6.0) of 28 September 2004. Evidence of an ionospheric precursor is found as a distinct reduction of ionospheric critical frequency localized over Southern California up to thirteen days before the main shock.