GPS DETECTION OF IONOSPHERIC PERTURBATIONS FOLLOWING THE JANUARY 17, 1994, NORTHRIDGE EARTHQUAKE

Sources such as atmospheric or buried explosions and shallow earthquak es producing strong vertical ground displacements are known to produce pressure waves that propagate at infrasonic speeds id the atmosphere. At ionospheric altitudes low frequency acoustic waves are coupled to ionospheric gravity waves and induce variations in the ionospheric ele ctron density. Using Global Positioning System (GPS) data recorded by the permanent GPS network operating in southern California, we compute d ionospheric electron content time series for several days preceding and following the January 17, 1994, M(W)=6.7 Northridge earthquake. We observe an anomalous signal beginning several minutes after the earth quake with time delays that increase with distance from the epicenter. The signal frequency and phase velocity are consistent with results f rom numerical models of atmospheric-ionospheric acoustic-gravity waves excited by seismic sources as well as;previous electromagnetic soundi ng results. We believe that these perturbations are caused by the iono spheric response to the strong ground displacement associated with the Northridge earthquake.