How large of an earthquake is needed to create a tsunami?

• A. Magnitude 6
• B. Magnitude 8
• C. Magnitude 9
• D. Magnitude 7

Answer: (D)

Tsunamis form when the seafloor moves vertically and displaces a large amount of water rapidly. The energy released by a quake grows with magnitude, and once you reach around seven, the quake is typically large enough to cause substantial vertical movement along an ocean-floor fault (usually a shallow, undersea thrust fault in a subduction zone). That rapid displacement launches waves that can propagate across oceans and rise near shores. Smaller quakes (around magnitude six) very rarely produce enough vertical displacement to generate a noticeable tsunami, while much larger quakes (magnitude eight or nine) certainly can, but they’re not required to create one. So roughly magnitude seven is the threshold where tsunamigenic potential becomes significant.