The nature of tsunami energy decay with epicentral distance in the open ocean for two large trans-Pacific tsunamis

The dynamics of a tsunami wave in a zone of propagation in the open ocean includes examination of time evolution of tsunami amplitude whilst propagating over a large distance. In this study, we used NOAA Center for Tsunami Research database from field measurements of vertical displacement of sea surface elevation referred here to tsunami maximum amplitudes for two trans-Pacific tsunamis, namely the 2011 Tohoku, Japan and 2014 Iquique, Chili events. The data were all recorded by DART buoys located at various geographical positions across the Pacific and were analyzed to examine tsunami energy decay in terms of tsunami amplitude decrease with increasing epicentral distance, defined here as tsunami travel distance measured from the epicenter. The results show that the amplitude decreases dramatically in a near-field region of observation and decreases gradually in distant propagation before advancing with relatively constant energy. Extracted from the amplitude decay curve, this finding suggests that different mechanisms of energy conversion and conservation are responsible for energy release in the propagation zone. This is of significance for the development of tsunami early warning as accurate prediction of tsunami run-up in a zone of mitigation requires knowledge of tsunami wave height when the wave approaches shorelines.