WHY WAS there no tsunami from Magnitude 8 Tonga earthquake?
Tsunamis are very rare events and require very special conditions for their generation. Most tsunamis take place in subduction zones or oceanic trenches where one tectonic plate is diving beneath another. The earthquake must exert a sudden vertical and significant displacement of the seafloor to generate a significant tsunami, which usually requires an event of magnitude 6.5 or greater.
Conditions were perfect for the Sumatra earthquake of Dec 26, 2004: the last estimate of magnitude was 9.3 up from 8.9 immediately following the quake making it the third largest earthquake we know of. The earthquake occurred as a result of slip or displacement on the interface between two tectonic plates - an interplate event. This resulted in a significant upthrow of the seafloor perpendicular to the trench axis which was in direct contact with the sea water and displaced an enormous amount of water creating the world's most destructive tsunami to date.
Not so for the recent Tonga earthquake of May 3. The magnitude was 7.9, large enough and could be revised upwards in coming months. The mechanism involved vertical displacement on the fault. However, the earthquake occurred in the interior of the diving plate - an intraplate earthquake - resulting in no significant displacement of the overriding plate, the seafloor or the water above it.
