  During earthquakes preparation periods significant disturbances in the
ionospheric plasma density are often observed. These anomalies are caused by
lithosphere-atmosphere-ionosphere interaction, particularly by the seismic
electric field penetrating from the ground surface into the ionosphere. The
seismic electric field produces electromagnetic EB drift changing plasma
density over the epicenter region and magnetically conjugated area. The paper
is devoted to analysis of regular Global Positioning System observations and
revelation of seismo-ionospheric precursors of earthquakes in Total Electron
Content (TEC) of the ionosphere. Global and regional relative TEC disturbances
maps (%) have been plotted for 2005-2006 M6, D<60 km seismic events and
analyzed in order to determine general features of precursors. The obtained
results agree with the recent published case-study investigations.
