An earthquake generally needs to be at least magnitude 6.5 to 7 on the Richter scale to cause a tsunami, but commonly destructive or ocean-crossing tsunamis are more likely when the earthquake reaches magnitude 7.5 or higher.

Key Factors for Tsunami Generation

• The earthquake usually must occur under or near the ocean to disrupt the sea floor and displace water vertically.

• The quake must be shallow (less than 70 km deep) so the energy efficiently moves water above.

• The earthquake’s motion should move the sea floor upward or downward (vertical movement), as horizontal slip does not typically generate tsunamis.

• Subduction zone earthquakes, where one tectonic plate moves under another, are the most frequent sources of large tsunamis because they provide significant vertical sea floor movement.

Magnitude Thresholds

Magnitude Range| Tsunami Potential
---|---
Below 6.5| Unlikely to generate a tsunami
6.5–7.0| May cause tsunamis; usually small and local
7.0–7.5| Likely to cause noticeable/localized tsunamis
Above 7.5| Frequently causes regionally or globally significant tsunamis1356

Conclusion

A shallow, vertical-motion earthquake under the sea, typically magnitude 7 or greater, poses the greatest risk for producing a tsunami. Magnitude and the type of movement are both critical in determining whether a tsunami will result.