Determine the depth of the ocean through which the tsunami is passing to determine its speed. You can find ocean depths at the National Oceanic and Atmospheric Administration website or The World's Oceans website. The depth is represented by "d" in the equation for calculating the tsunami's speed.
Multiply the depth (d) by acceleration due to gravity (g) -- 9.81 meters per second.
The square root of the d x g sum is the speed of the tsunami.