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What is it about an earthquake that causes a tsunami?

Although earthquake magnitude is one factor that affects tsunami generation, there are other important factors to consider. The earthquake must be a shallow marine event that displaces the seafloor. Thrust earthquakes (as opposed to strike slip) are far more likely to generate tsunamis, but small tsunamis have occurred in a few cases from large (i.e., > M8) strike-slip earthquakes.

Note the following general guidelines based on historical observations and in accordance with procedures of the Pacific Tsunami Warning Center.

Magnitudes below 6.5

Earthquakes of this magnitude are very unlikely to trigger a tsunami.

Magnitudes between 6.5 and 7.5

Earthquakes of this size do not usually produce destructive tsunamis.  However, small sea level changes may be observed in the vicinity of the epicenter. Tsunamis capable of producing damage or casualties are rare in this magnitude range but have occurred due to secondary effects such as landslides or submarine slumps.  

Magnitudes between 7.6 and 7.8

Earthquakes of this size may produce destructive tsunamis especially near the epicenter; at greater distances small sea level changes may be observed. Tsunamis capable of producing damage at great distances are rare in the magnitude range.

Magnitude 7.9 and greater

Destructive local tsunamis are possible near the epicenter, and significant sea level changes and damage may occur in a broader region. Note that with a magnitude 9.0 earthquake, the probability of an aftershock with a magnitude exceeding 7.5 is not negligible.

Tags: Seismology, Tsunamis, Seismic, Prediction