The paper gives the analysis of seismogeological data and probable large earthquakes parameters for the territory of Irkutsk. The suggested methods form the signal for original seismicity of the territory with using rather large local seismic events records (K > 13). Original accelerograms obtained using this method despite of a number of restriction during forming are theoretically substantiated for the territory. They are allowed to use for calculations of seismic ground motions parameters with different ground conditions.