Just as scientists cannot predict precisely when earthquakes or volcanic events will occur, neither can they determine exactly when a tsunami will be generated. However, by studying past tsunamis, scientists can predict where they are most likely to occur in the future and what their impact on specific coastal locations is likely to be. Using physical evidence and, where available, eyewitness accounts, geologists work to piece together entire tsunami histories, beginning with the causal event and ending with the aftermath.
From numerous studies, scientists know that not all underwater or near-shore seismic events cause tsunamis. To generate a tsunami, an earthquake -- the most commonly cited cause of tsunamis -- must be very large and centered at depths no greater than 50 kilometers (30 mi). About 85 percent of all recorded tsunamis originated in the Pacific Ocean, which is replete with faults, and scientists can create prediction models based on known subduction zones that fit the profile.
In addition to earthquakes, volcanoes may also initiate tsunamis. Violent eruptions or massive slope failures can result in the displacement of the large volumes of water needed to trigger a tsunami. Although less frequently than volcanic events, rock falls and submarine landslides may also produce tsunami waves. Sudden slope failures like these are sometimes triggered by strong earthquakes. Although evidence suggests that wave energy generated by point-source events such as landslides tends to dissipate quickly, historical events in Japan, Alaska, and elsewhere demonstrate that the local effects of tsunamis caused by these sources can be devastating.
In areas where tsunamis have occurred in the last several hundred years, eyewitness reports or written accounts sometimes provide scientists with the information needed to gauge the potential impact of future tsunamis in the same area. In such cases, scientists seek to learn how many waves came ashore, how high they were, which one was the biggest, and how far the tsunami moved inland.
When no human documentation exists, geologists rely on the fact that tsunamis erode, transport, and deposit sediments. For example, the presence of sand or gravel deposits normally found on a coastline in an inland location might indicate that a tsunami transported the material. Thicker deposits suggest larger waves because larger waves are capable of carrying more sediment from the seafloor. Also, the discovery of flattened plants entombed in a layer of sand suggests that powerful waves rolled quickly in and out of an area.