That's because tsunamis generated by underwater landslides might be more common than thought, based on a new analysis of monster waves created by the magnitude 7 earthquake that struck Haiti on January 12. (See pictures of the Haiti earthquake aftermath.)
Tsunamis are generally thought to be produced by earthquakes in which one part of the crust is thrust above or beneath another. This motion causes the sea to slosh as the seabed rises or falls, setting off a mighty wave, said team leader Matthew Hornbach, a geophysicist at the University of Texas in Austin.
But in the Haiti earthquake, the plates slid past each other with a largely sideways motion, Hornbach said. Instead of seabed lifting, it was underwater landslides—on slopes as steep as 30 degrees—that likely triggered the tsunamis, which in some places reached heights of ten feet (three meters), he said.
In fact, of the nine tsunamis known to have hit Haiti and neighboring islands in the past 318 years, three can be "directly linked" to underwater landslides, Hornbach said.
"When you look through the records, you realize that submarine landslides occur frequently," he said.
Similar sideways-moving faults, known to geologists as strike-slip faults, exist in other coastal regions, including along California's notorious San Andreas Fault. (Related: "Deadly San Andreas Fault Longer Than Thought.")
That fault was responsible for Northern California's 1989 Loma Prieta earthquake, which was a magnitude 7.1. Records show that the quake was accompanied by unusual water motions, Hornbach noted, which now appear to have been small tsunami-like sloshings.
"If I had been in Los Angeles before this study, and I was on the beach during a large earthquake, I would think about tsunamis but not be all that concerned," Hornbach said. "Now I would be."
"Abnormal" Tsunamis a Global Risk?
The new study also suggests that earthquakes don't have to be huge to trigger underwater landslides and thus potential tsunamis.
"A 1907 earthquake in Jamaica, where there were multiple slide-generated tsunamis, was only magnitude 6.5," he said. "Usually a tsunami warning isn't issued for under a magnitude 7."
Danger zones include areas where there is a lot of loose sediment perched on the brinks of steep submarine slopes, he said. On Haiti and other Caribbean islands, such conditions are exacerbated by deforestation, which increases the rate of soil erosion and contributes to the precarious coastal mounds, according to the study, published last week in the journal Nature Geoscience.
"You see places in Jamaica where huge developments have been built on these deltas," Hornbach said. But "you don't want to build in those places or be around them in an earthquake."
Similar landslide-driven tsunamis appear to have occurred in western Taiwan (map), a region also known for its strike-slip faults, said Kuo-Fong Ma of National Central University in Jhongli City.
"Several findings have discovered abnormal tsunamis accompanied by moderate-sized (e.g., magnitude 6.5) earthquakes," she said by email.
Such tsunamis might not always be the most severe, but they could still pose a significant risk to highly populated areas, added Chris Goldfinger, a marine geologist at Oregon State University.
"Southern California, Seattle, and San Francisco are good examples of localities where this could occur," he said via email. "While these waves likely won't be large by tsunami standards, they could be damaging to the infrastructure onshore, such as Los Angeles Harbor, which was not constructed with this type of hazard in mind."
California Ready for Tsunamis
Although the new study might surprise some experts, the risk of tsunamis—even from strike-slip earthquakes—has already been included in California's preparedness plans, said Costas Synolakis, director of the Tsunami Research Center at the University of Southern California.
"We have known since the Papua New Guinea tsunami that killed over 2,200 in 1998 that even moderate onshore earthquakes near the coast can generate large tsunamis by triggering submarine landslides," Synolakis said by email.
And Synolakis is not convinced that the Haiti tsunamis were caused by landslides: "They could have been generated by complex tectonic motions that have yet to be convincingly excluded as the culprits."
In any event, he said, what's important is to be aware of the tsunami risk if you're on a coast anywhere in the world during any type of earthquake. (Get tsunami facts.)
"If [you] feel a ground shake that lasts over 30 seconds, or if [you] observe any unusual water motions, move inland or to high ground immediately."