In this paper the authors propose to determine the parameters of initial sea surface displacement at tsunami source by a part of the measured tsunami wave time series until the first maximum. This is a clear and easy to apply stop criteriuon. Determination of the tsunami source parameters is conducted by the orthogonal decomposition of synthetic time series, calculated in advance. By a number of numerical experiments mutual position of the initial disturbance zone (as well as the earthquake epicenter) and the sensor are determined. It is expected that the proposed approach will save time (from one to several minutes) for security and evacuation measures.