New Insights on Tsunami Genesis and Energy Source

Conventional tsunami theories suggest that earthquakes with significant vertical motions are more likely to generate tsunamis. In tsunami models, the vertical seafloor elevation is directly transferred to the sea-surface as the only initial condition. However, evidence from the 2011 Tohoku earthquake indicates otherwise; the vertical seafloor uplift was only 3~5 meters, too small to account for the resultant tsunami. Surprisingly, the horizontal displacement was undeniably larger than anyone’s expectation; about 60 meters at the frontal wedge of the fault plate, the largest slip ever recorded by in-situ instruments. The question is whether the horizontal motion of seafloor slopes had enhanced the tsunami to become as destructive as observed. In this study, we provide proof: (1) Combining various measurements from the 2011 Tohoku event, we show that the earthquake transferred a total energy of 3.1e+15 joule to the ocean, in which the potential energy (PE) due to the vertical seafloor elevation (including seafloor uplift/subsidence plus the contribution from the horizontal displacement) was less than a half, while the kinetic energy (KE) due to the horizontal displacement velocity of the continental slope contributed a majority portion; (2) Using two modern state-of-the-art wave flumes and a three-dimensional tsunami model, we have reproduced the source energy and tsunamis consistent with observations, including the 2004 Sumatra event. Based on the unified source energy formulation, we offer a competing theory to explain why some earthquakes generate destructive tsunamis, while others do not.