In case of strong earthquake offshore Japan, tsunami wave approaches the nearest coast in approximately 20 minutes. So, time required for evaluation of tsunami danger is the key parameter for any warning system. The authors propose to determine the parameters of initial sea surface displacement at tsunami source (initial data to simulate numerically wave propagation) by only a part of the measured tsunami wave time series. The necessary part of wave profile is determined by tracking the relative changes of source parameters. 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 (up to few minutes) for security and evacuation measures.