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Ask the Expert Responses
Set 2: Posted March 31, 2005

Wave That Shook the World homepage

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Q: What were the differences in the quakes of December 26, 2004 and March 28, 2005 that one caused such a terrific tsunami and the second caused none?

Linda Allen
Jacksonville, Texas

A: You ask a very good question and the jury is still out. But to begin with, the March 28 earthquake did produce a tsunami. I just heard reports from a field team that is heading into the region that wave heights of three meters are reported on the back side of Simeleu Island, and the water heights are expected to be larger elsewhere on the island. So it is very clear that a tsunami was generated and may locally have caused damage. But it is also clear that the tsunami was much smaller than the December 26 tsunami. So the big question is why. An 8.7 earthquake is quite capable of producing a very large tsunami.

There are several possibilities why this one wasn't so very large. First, the fault deformation may have mainly been beneath the relatively shallow water between the small islands and Sumatra, and didn't displace a very large volume of water. Second, the rupture characteristics —rupture speed and direction—may have affected the efficiency of tsunami generation. Third, the islands may have helped to break up the wave energy. It is very important first to get as detailed a picture as possible of what the actual wave heights were before the correct answer can be found.

Q: Has anyone gone down to look at and film the sea bottom where the December quake and tsunami originated? Would changes be noticed, such as an uplifted ledge, ridges, or the release of gases? Did anyone survey/explore the area for any reason before the quake so a comparison can be made? Thank you.


A: Yes. The British ship HMS Scott visited the epicentral area last month with a team of scientists to image the seafloor bottom. They found evidence of pressure ridges, fresh scarps, and submarine slides. You can view many of the images online. Just type "hms scott tsunami" into any search engine and you will come up with several links to these images. I don't know of any pre-earthquake surveys of the exact same area, although there may have been some conducted as part of exploration reconnaissance surveys. I'm sure the scientists involved with the Scott expedition are combing all possible sources for such information.

Q: How realistic is the threat of a "mega-tsunami" to the eastern seaboard of North America?

Dan Maynard
Gainesville, Florida

A: Any place on the coast is at risk of tsunamis, and the East Coast is no exception. In 1929, a magnitude 7.2 earthquake in Newfoundland triggered a large underwater slump that ruptured the new transatlantic cable in 12 places. The slump also produced a tsunami that was recorded along the eastern seaboard as far south as South Carolina and across the Atlantic Ocean in Portugal. In Newfoundland, the tsunami reached heights of over 20 feet and claimed 29 lives. Earthquakes do occur along the east coast. The 1886 Charleston, South Carolina earthquake was felt as far away as New York and Wisconsin, and it had a probable magnitude of about 6.8. It didn't produce a tsunami but demonstrates that large earthquakes can happen on the east coast.

Large earthquakes do occasionally occur elsewhere in the Atlantic that may pose a tsunami risk to the east coast. The 1755 Lisbon, Portugal earthquake produced a very large tsunami that was observed in the Caribbean and probably produced waves along the east coast.

There has been quite a bit of media attention on east coast tsunamis caused by large landslides. In 2000, potential landslide sites off the coast of Virginia and North Carolina hit the headlines. Failures of unstable sections of the continental slope are thought capable of sending 18-foot-high tsunamis towards the coast. There has been even more attention paid to a mega-tsunami caused by the partial collapse of one of the Canary Islands in the Eastern Atlantic. Of particular concern is Cumbre Vieja volcano, which shows signs of fracture. A large failure could impact most of the Atlantic Ocean. There is no agreement on the probability of such a failure, but there is no question that such an event is far rarer than large earthquake-driven tsunamis typical of the Pacific basin.

Because of the potential for an east coast tsunami, a bill to strengthen the U.S. tsunami warning system currently winding its way through Congress proposes to install several deep pressure sensors in the Atlantic and Caribbean as well as add more in the Pacific basin.

Q: If a tsunami were to take place close to North America, at which location would it most be expected to take place? The least?

Berwyn, Illinois

A: The most likely? Alaska by a long shot. Faults associated with the Alaska-Aleutian subduction zone are capable of producing more earthquakes and larger-magnitude earthquakes than any other region of North America. In the past century this zone produced great tsunamis in 1946, 1957, and 1964, with numerous more localized tsunami events, including the highest tsunami wave heights ever recorded—over 1,700 feet in Lituya Bay in 1958. The least likely coast is also a no-brainer—the arctic coastline of Canada. There are no plate boundaries and few faults in the Arctic Ocean.

If I were to rank the North American coastlines from most likely to least likely, I would probably list them like this:

  1. Alaska
  2. Middle America trench along Mexico and Central America
  3. West Coast of North America
  4. Gulf Coast
  5. Atlantic Coast
  6. Arctic Coast

If you want to know which city in the U.S. continental mainland (excluding Alaska and Hawaii) is most vulnerable, it's Crescent City, California, which is located about 80 miles away from where I live. Eleven people in Crescent City died in the 1964 tsunami, there was significant damage in 1960, and observable tsunamis in 1946, 1952, 1957, and 1965.

Q: What are the chances of a tsunami occuring in the Gulf of Mexico? Is that possible? What would be the total damage?


A: Tsunamis have occurred in the Gulf of Mexico and they will happen again. In 1991, a magnitude 7.6 earthquake in Costa Rica produced a two-meter-high tsunami that flooded nearly 1,000 feet inland on the Caribbean side of the country. There are a number of older events listed in tsunami catalogs. The Caribbean contains a number of fault systems that are capable of producing earthquakes large enough to cause tsunamis. There are also active submarine volcanoes. The Gulf is also at risk from very large tsunamis generated elsewhere in the Atlantic, like the one produced by the 1755 Lisbon earthquake or a possible future one produced by the very rare but potentially catastrophic collapse of Canary Island volcanoes. While possible, we have no quantitative estimate of probability.

I know of no tsunami hazard mapping in the Gulf area outside of Puerto Rico, where a few very dedicated scientists have conducted numerical models of tsunami inudation with almost no resources. What I am sure of, however, is that the Gulf will experience many more flooding events due to hurricane-triggered storm surge before it get hits by a large tsunami.

Q: I've heard that if a certain fault on one of the Hawaiian islands ruptures, it could conceivably launch a 1,000-foot-high wall of water crashing into the West Coast of the U.S. Is this likely, and how far could such a wave penetrate inland?

Craig Marshall
Redondo Beach, California

A: I think you are probably referring to the East Rift Zone that extends across Kilauea's south flank. There have been many earthquakes along this zone and noticeable deformation; you can even observe cracks in the highway caused by this slip. Several scientists have postulated that a collapse of all or part of the south flank could happen and will cause a megatsunami. Others have argued that the current geometry of the slope shows buttressing of the toe in the offshore area, making such a collapse relatively unlikely in the near future.

As to how high such a tsunami might be, we really don't know for sure. Current tsunami modeling is based on historic events that allow modelers to check their results and modify their code. There have been no observable megatsunamis to use to calibrate numerical modeling. There's no question such an event would cause very large tsunami waves in Hawaii—at least hundreds of feet in height. It's less clear how high the waves would be along the west coast of North America, but they would likely exceed historic inundation.

Q: Is there or will there be a rating system for tsunamis?


A: Do you mean something similar to the magnitude scale for earthquakes? Actually, there are several different tsunami scales: there is a tsunami magnitude scale and several tsunami intensity scales. They are based on the amplitude of the tsunami. They are not used as widely as earthquake magnitudes in part because it is so difficult to get a complete picture of tsunami wave energy. But perhaps as more ocean bottom instruments are deployed, the idea of tsunami magnitude will be revisited.

Q: How did you get into this field? What were your greatest challenges in learning to do what you do?

Steve Bamford
Tallahassee, Florida

A: I certainly never planned to study tsunamis as a child! First I became interested in geology as an undergraduate at Berkeley in the 1960's. I was completely turned off by the enormous lecture classes I had to take as a Humanities major. I chose Geology for my required science course, and it was taught by Howell Williams, the volcanologist who first deciphered Crater Lake and Mt. Mazama. It was a wonderful course at the dawn of plate tectonics, and I was hooked.

Years later, after finishing a Ph.D. thesis on rock mechanics, I was fortunate to get a teaching job at Humboldt State University. Humboldt County is in the single most seismically active part of the continental United States, so I quickly became involved with the tectonics of the area. In 1992, we had a magnitude 7.1 earthquake that was on a fault closely related to the Cascadia subduction zone. It produced a small tsunami that was a wakeup call for the Pacific Northwest. That earthquake gave birth to the U.S. National Tsunami Hazard Program, and I was fortunate to be pulled into the early development of the program. I've been working on tsunamis ever since.

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