In order to evaluate the decay process of far-field tsunamis, we have revised the definition of tsunami coda and related measures - moving root mean squared amplitude (MRMS amplitude), and non-dimensional tsunami amplitude (NDA) - in this paper. We applied these measures to the tsunami waveform data of 2006 and 2007 Kuril Island earthquake tsunamis observed by tidal stations located in Japan. Analytical results of tsunami coda of these events indicate that decay time constants obtained at tidal stations along the Pacific coast of Japan have only small differences. Further we confirmed that the NDA seldom exceed three times of MRMS amplitude. Then we show the possible application of utilizing these characteristics of tsunami decay process to realize appropriate cancelation of tsunami warnings.