Photos: Where Will Next Mega-Tsunami Hit? (Japan Quake Anniversary)

Northwestern North America

Photograph by Phil Schermeister, National Geographic

When it comes to tsunamis, North America's most threatening fault is the Cascadia subduction zone, which stretches offshore from Northern California to Canada and includes Oregon's Devil's Elbow State Park (pictured). A subduction zone is where one tectonic plate is diving under another.

Every few hundred years, geologists believe, this fault unleashes a giant earthquake, comparable to, or larger than, the one that devastated Japan on March 11, 2011.

The most recent megaquake in the area was in 1700, according to analysis of  growth rings in trees killed by the temblor.

It was big enough to send a tsunami all the way to Japan and produce Native American legends of the day the ocean rose up and threw canoes into trees, USGS geologist Brian Atwater reported a few years ago, when scientists were first waking up to the subduction zone's threat.

Unfortunately, U.S. coastal residents aren't likely to react as well as the Japanese did, Oregon State's Corcoran said. "The Japanese are culturally attuned to be aware of earthquakes and tsunamis. We are not."

How many people are in this danger zone is difficult to estimate. Corcoran put the figure at roughly a hundred thousand. But some coastal towns, he noted, can see their populations swell by a factor of ten during peak tourist weekends.

"It's musical chairs," he said. "When the earthquake happens, whoever is going to be there is going to be there."

(Read "Japan's Nuclear Refugees" in National Geographic magazine.)

Published March 9, 2012

Eastern Mediterranean

Photograph by Scott S. Warren, National Geographic

The least studied and least understood subduction zone in the world is the Hellenic arc, a trench running through the eastern Mediterranean south of Greece and Turkey, said civil engineer Costas Synolakis of the University of Southern California (USC) and the Hellenic Center for Marine Research.

Some the hazards of the area, which includes Lemesos, Cyprus (pictured), are known. In the past 2,000 years the eastern Mediterranean has produced two giant earthquakes—in A.D. 365 and 1303—with magnitudes estimated to have exceeded 8.5 and "with tsunamis to match," Synolakis said.

The earlier tsunami devastated ancient Alexandria, Egypt, added Emile Okal, a geophysicist at Northwestern University. That raises serious questions about the tsunami threat to modern Egypt, he said.

On average, USC's Synolakis added, the Mediterranean appears to have seen about one or two tsunamis per century—and is very vulnerable to the next one.

"The exposure is huge. The Mediterranean is heavily populated along its shores, with millions of tourists in the summer," he said. Furthermore, "the awareness is abysmal, and there is no tsunami warning center."

(Also see "One Year After Fukushima, Japan Faces Shortages of Energy, Trust.")

Published March 9, 2012

Peru

Photograph by Abraham Nowitz, National Geographic

Offshore from Peru is another subduction zone, also with a history of big earthquakes.

For example, since "the Spanish conquest in 1543, Lima [pictured] has been destroyed three times by a major earthquake with tsunamis," Northwestern's Okal said. "How large, we are not sure, but enough to completely wreck the city as it was built [at the time].

"Now construction would be a bit better, but the population has increased, so I would put Lima as a city very much at risk from a major earthquake and local tsunamis."

(Japan Tsunami: 20 Unforgettable Pictures.)

Published March 9, 2012

Caribbean

Photograph by David Doubilet, National Geographic

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.)

Published March 9, 2012

Turkey

Photograph by Richard Nowitz, National Geographic

The most dangerous fault for Turkey (pictured: Istanbul) is the North Anatolian Fault. Since 1939 it's been rupturing, piecemeal, starting at the eastern end.

The resulting earthquakes have been irregularly spaced in time and magnitude but each has been one step west of its predecessor, Okal said.

The most recent major Turkish quake was in 1999, a magnitude 7.4 in Izmit, which left 17,000 dead. It also produced a small tsunami in the Sea of Marmara, between the Black Sea and the Aegean.

That tsunami wasn't a major source of destruction. But the next fault segment apparently in line for a rupture lies directly beneath the Sea of Marmara, Okal said. If that section snaps, it's likely to produce a more substantial tsunami, "right in front of Istanbul."

Not only does that city have millions of people, but it doesn't effectively enforce building standards, he added.

Even though a Sea of Marmara earthquake wouldn't likely to be much stronger than magnitude 7.5, he said, the quake and its tsunami would be a humanitarian disaster. "We can bet on this."

(Tsunami Pictures: Epic Waves, Earthquake Shock Japan.)

Published March 9, 2012

Indonesia

Photograph by Thomas Cockrem, Alamy

Indonesia has seen more than its share of giant earthquakes, including the Indian Ocean quake of December 2004—whose tsunami killed more than 200,000—followed months later by a magnitude 8.7 temblor on an adjacent segment of the same fault line, known as the Sunda subduction zone.

But not all of that fault line released its tension in 2004 and 2005. One segment, along the middle of the island of Sumatra may still be carrying stresses accumulated since its last major earthquake, in 1833, Okal said.

The stressed segment lies off the densely populated city of Padang, which is several times larger than Banda Aceh, the largest Indonesian city hit by the 2004 tsunami.

"There are places where we know of very large historical earthquakes, which are more or less ripe for [another]," Okal said. "I would put Padang as [such] a bad place."

(Video: Tsunamis 101.)

Published March 9, 2012

More Pictures: Japan Tsunami, Before and After

Photograph from Miyako City Office via Reuters

Published March 9, 2012