Geologically speaking, a real tsunami is generated when converging plate boundaries move abruptly, vertically displacing the overlying water. Since tsunamis mainly occur in the Pacific Ocean following shallow-focus earthquakes over magnitude 6.5 on the Richter scale, one of the best means of prediction is the detection of such earthquakes on the ocean floor with a seismograph network. Tsunamis may be detected by wave gauges and pressure monitors, such as those emplaced as part of the U.S. Tsunami Warning System, established in 1949.
On April 1, 1946, an earthquake of 7.8 on the Richter Scale occurred near the Aleutian Islands, Alaska, generating a tsunami which inundated Hilo on the island of Hawai'i with a 14m-high surge. The area where the earthquake occurred is where the Pacific Ocean floor is being pushed downwards under Alaska. Examples of tsunami being generated at locations away from convergent boundaries include Storegga during the Neolithic era, Grand Banks in 1929, and Papua New Guinea in 1998. In the case of the Grand Banks and Papua New Guinea tsunamis an earthquake caused sediments to become unstable and subsequently fail. These slumped and as they flowed down slope a tsunami was generated. These tsunami did not travel transoceanic distances.