In some regions—including the Caribbean (pictured: St. John's Waterlemon Cay)—tsunamis can be triggered by smaller earthquakes, especially if they set off underwater landslides.
Geophysicist Matthew Hornbach has discovered that even the 2010 magnitude 7.0 earthquake in Haiti was large enough to produce a ten-foot (three-meter) wave.
Reached via email, Hornbach, of Texas's Southern Methodist University, is currently on a ship off Montserrat, drilling into ancient volcanic "flank collapses" to determine the region's overall tsunami risk.
While he's not yet at liberty to discuss his latest findings, Hornbach wrote that, when it comes to tsunamis, "the Caribbean is an important place that is often overlooked, and I will say that some of these waves can be massive."
Northwestern's Okal agrees, pointing specifically to the U.S. Virgin Islands, which had a magnitude 7.5 earthquake and associated tsunami as recently as 1867.
The big risk there, he said, is to cruise ships, of which five or six can be in port at once, each with several thousand passengers and crew.
Cruise-ship captains have said they could be out of port within five minutes of an earthquake, Okal said, but he doesn't believe it. "You're going to have a traffic jam trying to move these enormous things," he said.
And, he added, "If you do get your ship out in five minutes, you're going to leave people on the shoreline to be washed away by the waves.
"It's a tsunami trap," he said, noting that a disaster wouldn't require a magnitude 9 earthquake. "A high 7 would do it."
(Japan Tsunami Pictures: Nuclear Reactor and Cities Burn.)