Ask the Expert Responses
Set 4: Posted April 4, 2005

Wave That Shook the World homepage

Previous set of responses

Q: Is there anything my family can do to practice for a tsunami?

Shannon
Jacksonville, North Carolina

A: Tsunamis are similar to all other hazards in that the best thing you can do ahead of time is assess, plan, and practice.

1. Assessing your hazard. Where do you live relative to the coast? Are there any tsunami hazard maps for your area? Since you live in North Carolina, the answer is no. If you live farther than two miles away from the open coast or more than 100 feet above sea level, you have no tsunami hazard and the only thing you need to plan for is what to do when you visit the coast.

2. Planning. If you do live in a possibly vulnerable area, look at the lay of the land. How would you move to a safer area? You may be able to get some planning assistance from your local office of emergency services (it may be called civil defense or homeland security) or the American Red Cross. I'm sure hurricane evacuation plans are in place for your area, and they may also offer some guidance.

3. Practice. Plans that aren't practiced or exercised are useless. Emergency managers consider exercises the bedrock of good emergency management. There are two types of exercises you can conduct with your family or at your workplace or other group. The first is called a table top. It means you sit down with your family and talk about what might happen and discuss what everyone should do. You ask questions and discuss possible alternatives. The second type is an actual practice. You either set a predetermined time/day and everyone does what your plan says they should do. You might time how long it takes you to evacuate and think about what you can do make it go more smoothly the next time.

You can use this strategy, of course, for any hazard—floods, storms, fire, etc.

Q: Can the government use a tsunami as a war tool?

Wes Smith
Houston, Texas

A: Interesting you should ask. Several of the early numerical models for estimating tsunami inundation were developed by the Department of Defense. They were interested in what would happen if a nuclear bomb were detonated in the ocean. Their calculations showed it could produce a very large tsunami. It's interesting to note there are a number of conspiracy sites on the Web claiming that the Indonesian earthquake was caused by such a bomb and not an earthquake—a claim that has absolutely zero basis in fact. But there is no question that a very large explosive devise will cause a tsunami. But it's a pretty poor war tool, as it is the opposite of a smart bomb. You can't control where the waves go and the likelihood of damaging your own interests or your allies or other parties is very high.

Q: I understand that Las Palmas island has split, the western half slipping six feet toward the ocean. Assuming it does create a 300-meter-high tsunami if it slides into the ocean, would the wave be as high all along the east coast of the U.S. after it crossed the Atlantic Ocean? Or does the wave height decrease as the area of coverage increases?

Anonymous

A: All tsunamis spread out from the source. The tsunami caused by the collapse of a flank of Las Palmas would spread out in great circles, decaying at a rate roughly proportional to the inverse of the square root of the distance from the source. This is called geometric spreading and will significantly reduce the wave heights as they travel across the Atlantic. Modeling of a tsunami from the Canary Islands is highly speculative at the moment because we have never observed a tsunami of similar scale and don't know how well our tsunami models will perform.

Q: We live approximately 500 yards from the beach but on a bluff that is about 40-50 ft tall. The area here is quite rocky, but the soil is basically sand. Will the bluffs protect our home? We live six miles north of Gold Beach at Nesika Beach. There are many huge old trees all around us, and we do feel secure, but this is why I am asking you, are we safe?

Bruce Grieve
Gold Beach, Oregon

A: Because you live in Oregon, I don't need to give you a guesswork answer. Tsunami hazard maps are available for most communities on the Oregon Coast, and I'm sure the Gold Beach is one of them. Contact your local office of emergency and ask to see the maps. They should be able to identify whether or not you are in a hazardous zone. Please don't assume you are safe!

Q: I just spent a week in a hotel overlooking Chicago's North Shore and wonder to what extent the volume and area of a body of water determines the height of the waves in a tsunami. Would a 7.0 quake in Lake Superior generate the same size waves as a 7.0 quake in the Pacific?

Anonymous

A: You ask a good question. Both the size and shape of a body of water will affect tsunami characteristics. Certainly volume has an impact. A large earthquake beneath a very shallow body of water won't create as large a tsunami as the same event under a deep one. But a 7.0 earthquake in Lake Superior just might create an even bigger tsunami than the 7.0 in the Pacific. Why? Because it could trigger a submarine landslide. The landslide would produce a tsunami, and in a closed body of water you can end up with waves exciting rhythmic oscillations (seiche).

I don't think anyone has done tsunami modeling in the Great Lakes and I have no idea what the hazards are likely to be, but I can envision a scenario where your 7 could make bigger waves there than in the Pacific. Magnitude 7 earthquakes won't even elicit tsunami warning messages from tsunami warning centers, because they aren't big enough to produce a tsunami by themselves, and if they trigger landslides the effect is very localized.

Q: How often do tsunamis happen each year?

Amanda White
Centreville, Virginia

A: Roughly 10 per year or 100 per decade are detected. Most of these can only be seen on tide gauge recordings and do no damage.

Q: Based on the simulations, it appears that a tsunami's birth is dependent on the topography of the ocean floor nearest the epicenter. This is why we didn't see another in March. Is this a true statement?

The tsunami in December traveled all the way to the coast of Africa. Would ships at sea have noticed anything strange as the tsunami waves passed under them?

Herb Bryant
Dayton, Ohio

A: No, a tsunami's birth depends on vertical displacement of the seafloor, not the pre-existing seafloor shape. The speed of the tsunami depends on how deep the water is. Seafloor shape (bathymetry) can influence a tsunami, however. Regions with many sea stacks and seafloor roughness will tend to reduce the tsunami amplitude. Near-shore bathymetry is very important and may focus or defocus waves along the coast.

As to the March 28 earthquake, it did produce a tsunami. The tsunami was measured on several tide gauges in the Indian Ocean; it was about seven inches high in Sri Lanka. It was much higher in the source region—at least 12 feet high on the island of Simuelue off the northern Sumatra coast. But there is no question that this tsunami was much smaller than the killer waves of December 26.

The reason? There is not enough information yet for a definitive answer. First, the tsunami inundation survey of the epicentral region needs to be completed. There is an International Tsunami Survey Team in the region right now, and they are working very hard to gather the data. You can follow some of their field updates online at http://walrus.wr.usgs.gov/news/field.html. I will be joining this team for the second leg of fieldwork.

There are several possible explanations as to why the March 28 tsunami was so much smaller. To begin with, the earthquake was smaller: the fault rupture zone was on the order of 100 miles in extent compared to the 700-mile-long rupture on December 26. But a magnitude 8.7 earthquake is still quite capable of producing a very big tsunami, and the March 28 tsunami does not fall into the very big category.

There has been some mention in the media of a different type of faulting. Both December 26 and March 28 were thrust earthquakes, where one side of the fault was pushed up and over the other, so this is not the reason for the difference. The location of the main earthquake slip may contribute to the difference. The March 28 fault displacement appears to be centered beneath the very shallow forearc basin between Sumatera and the band of offshore islands and not beneath the deeper waters of the Indian Ocean. The character and direction of slip may also play a role. We should have a much better answer in a few weeks.

As to what would it be like to be on a ship in deep water when the tsunami passed by—you wouldn't notice a thing. In deep water the tsunami has a very long wavelength (hundreds of miles) and a very small amplitude (a few feet). It would be no more noticeable than the tidal bulge.

Q: Tsunamis are natural occurrences. Can human beings stop them?

Shashi Bansal
West Hartford, Connecticut

A: No. Tsunamis are a natural occurrence related to the processes that shape the Earth's surface—earthquakes, volcanoes, and landslides. These processes are driven by the heat escaping from the Earth and gravity, and people cannot control these underlying forces. What people can do is learn to live with natural disasters: use technology to assess hazards, use technology and education to develop warning systems, use planning to develop evacuation zones and appropriate land use, use engineering to develop design codes so that buildings can resist both strong ground shaking and tsunamis and, where appropriate, design barrier systems, understand how vegetation and topography may reduce tsunami impact, and most of all EDUCATE, EDUCATE, EDUCATE.

People need to be aware of tsunami hazards and what to do if they feel an earthquake, see the water withdraw, or hear loud noises coming from the ocean. Right now with so much media attention, many people who three months ago had never heard the term "tsunami" are now aware of them. It may be 40 or 50 years before another great tsunami occurs. How many people will still know these lessons then?

Q: Does the depth of the ocean at the epicenter or the depth of the ocean near the shore where a tsunami hits determine how destructive it will be? When hearing about the size of an earthquake, what do the numbers mean? Example: 7.1 to 8.1 How much more powerful is one to another?

Scott Darby
Frederick, Maryland

A: The depth definitely contributes. The energy of a tsunami depends on the volume of water displaced—the deeper the water, the greater the volume. But water depth is only one factor; the type of faulting, the amount of fault slip, the direction of slip, and the rate of slip also affect tsunami size. The near shore bathymetry affects how rapidly the tsunami wave builds in amplitude and can focus tsunami energy in certain areas.

Earthquake magnitude is also an important factor. Magnitude is a measure of the total energy released at the earthquake source. It depends on the size of the fault (area), the amount of slip between the two sides of the fault, and how tightly the two sides of the fault are pushed together. The magnitude scale is logarithmic: each jump of 1 unit in magnitude (the difference between your 7.1 and 8.1) corresponds to an increase in energy release of about 32 times. The March 28 earthquake had a magnitude of 8.7, and the December 26 was rated by the USGS at 9. A difference of 0.3 in magnitude corresponds to a factor of 2 in energy release.

But this does not mean the ground will shake 32 times harder as you move up one unit in magnitude. Earthquake accelerations (how strongly you feel the ground shake) generally increase as you move up the magnitude scale until you get to the 5's. If you are very close to the epicenter of a shallow magnitude 5 earthquake, it may shake the ground just as hard as a magnitude 6, 7, 8, or 9. The difference is that the magnitude 5 will shake for only a few seconds while the 9 will shake for minutes. The epicentral area of the 5 is only a few miles—once you are farther away, shaking will get weaker very quickly. The epicentral area of the 9 may be over 500 miles long and 100 miles wide, and strong shaking will be felt at far greater distances.

Q: I understand the difference between tidal and tsunami waves, but I wonder if tidal waves act in similar ways, i.e., rapid advance, trapping people in the mudflats of areas where there are mudflats. Thank you.

Donna Day
United States

A: True tidal waves (waves caused by the daily changes in tide) appear very similar to small tsunamis. The USC tsunami program has loaded a great video clip of a true tidal wave at its Web site (select the China clip):

http://www.usc.edu/dept/tsunamis/2005/video/video.html

These tidal waves certainly have the capacity to knock people down and cause death and injury to those in the water's path. But they are much smaller than moderate or large tsunamis.

Another much more common type of wave that also behaves a lot like a small tsunami is the sneaker or rogue wave. These waves, which result from the constructive interference of swells, may be two or three times larger than typical waves hitting a coast. Where I live there are two or three deaths every year when people get caught unaware of these waves. They are most hazardous during high tides and may be preceded by a significant drawdown and a louder than average roar from the water.

Q: Does it seem ironic to you that North Coast disaster groups and private citizens have raised many tens of thousands of dollars for relief in Indonesia and Southern Asia while a project to emplace tsunami hazard signage in their own relatively tsunami-vulnerable backyards is stalled in part because of a lack of funding?

Nathan Rushton
Fieldbrook, California

A: I think it's great that so many people have opened their wallets to help tsunami victims. It's amazing to see school children to senior citizens—many who probably barely knew what a tsunami was four months ago—puttingso much effort into helping. But you ask a good question: why so much support for this disaster and not so much for something that might affect them at home, or for other disasters?

First, a clarification. The reason there are no tsunami signs posted in Humboldt County or elsewhere in California (except Crescent City) is that it's a complex process. First, tsunami inundation modeling must be completed so the tsunami hazard is understood. You can't post signs if you don't know what areas are at risk. Then you need to develop an evacuation zone and designate evacuation routes and areas of safe haven.

In our area and in most coastal communities, state or federal highways are involved. Signs can't be posted on these roads without the permission of Caltrans, the California Department of Transportation. Caltrans had a number of concerns about tsunami signs: Would people understand what they meant? Were they consistent with federal guidelines? I believe Caltrans has now approved a procedure for the posting of tsunami signs in California.

Federal funds for tsunami mitigation have been provided to California to develop inundation maps. The state started with the most populous counties and has beenworking north. This year, coarse grid inundation modeling should be completed for all of California's coastal counties. This means that it should soon be much easier to get signs posted.

The best way to make sure signs get posted is to contact your local decision makers. Call or write your County Supervisor and State Legislators. Without a groundswell of public opinion, the issue may slide to the back burner as the events of December 26 fade.

Q: No area around this part of Sarasota, Florida is more than a few feet above sea level. We are on the Gulf Coast. What would the probable effect of an earthquake in the Gulf of Mexico have on southwest Florida? Also, would it be possible to be affected in Sarasota by a tsunami somewhere in the Atlantic? Thanks.

Bob Barber
Sarasota, Florida

A: I can see why you are concerned. Low-elevation coastal areas are at the greatest risk of tsunami. Right now we don't have adequate information to answer your question. Tsunami inundation modeling takes a possible source (like an earthquake on a known fault) and, using the seafloor bathymetry (ocean floor depth) and coastline shape, mathematically computes the inundation. This is not a simple process and requires very high quality bathymetry and a painstaking process of grid development to put the bathymetry in a form that the numerical models can read. With the heightened concern about tsunamis on all U.S. coastlines and a bill winding its way through Congress to greatly enhance U.S. tsunami mitigation efforts, it is likely that tsunami hazard mapping will be expanded to the Gulf and Atlantic coasts.

It is possible for Florida to be affected by a tsunami generated farther away. The 1755 tsunami that came from Portugal likely caused some flooding in Florida. But tsunamis on the East and Gulf coast are very rare, and you will undoubtedly be affected by numerous hurricanes before the next tsunami comes your way.

Q: Regarding the comment you made [in the first set of responses] that "In Southern California, the design tsunami is a submarine landslide," how is this different from the tsunami in Sumatra? And do you think something like it has the possibility of happening in the next 50 years? I know these are all just scenarios, but I would like to know your opinion. Thanks!

Brenda Hann
Venice Beach, California

A: There is a big difference between a landslide-induced tsunami and a tectonic or earthquake tsunami source. A submarine or subaerial landslide is a relatively small source in area but potentially very large in water displacement. Landslide tsunamis tend to be very focused in effects.

The 1998 tsunami in Papua New Guinea is a good example. Most scientists believe a magnitude 7.1 earthquake triggered a large submarine slide just off the coast of Sissano Lagoon in northwestern Papua New Guinea. Wave heights exceeded 30 feet along the lagoon, and more than 2,200 people died. But the area affected by large waves was only about 25 miles in length; away from the Sissano Lagoon area the wave heights quickly died off.

An earthquake source like the December 26 event is much larger—700 miles long and nearly 100 miles wide. It produced a much less focused tsunami that affected thousands of miles of coastline. This type of source is not likely in Southern California. The offshore fault systems aren't long enough and don't have the right type of motion to produce an Indonesian-style tsunami. The Indonesian tsunami was probably a combination of the tectonic or earthquake deformation of the seafloor and local landslides triggered by the strong ground shaking.

So how likely is a tsunami in California in the next 50 years? In northern California where I live there have been several studies that have attempted to address recurrence of great earthquakes on the Cascadia subduction, a fault that most of us feel is quite capable of producing an Indonesian type of earthquake and tsunami. The odds range from about 10 to 20 percent likelihood of rupturing in the next 50 years. In Southern California there are no studies of tsunami probability from local sources. This is because the offshore tsunami source potential is not well defined. But I'm sure the potential is considerably lower than the Cascadia potential along the Pacific Northwest.

We don't know what the effects of a great earthquake on the Cascadia subduction zone will be in Southern California. But I expect we will have some fairly good estimates within the next year. The Indonesian tsunami will greatly improve tsunami inundation modeling, and the situation along the Sumatran coast south of Aceh Province is somewhat analagous to the rest of California south of Cape Mendocino. The Indonesian tsunami died off very quickly to the south. This is partly because a tsunami directs most of its energy perpendicular to the fault zone. Central and southern Sumatra were parallel to the fault. This doesn't mean there will be no effect—I'm very interested in seeing what the modeling results show—but I'm sure the wave heights will be much smaller than along the Pacific Northwest.

Q: According to tidal gauges here in Halifax, Nova Scotia, and a Web site, the tsunami made it here in Halifax harbor 26 hours after the earthquake in Asia, and it measured 40 cm in height. How did people know this, and how did it reach here around Africa?

Anonymous

A: People know the travel time of the December 26 tsunami from a global network of tide gauges. You can view many of these recordings at: http://www.pmel.noaa.gov/tsunami/sumatra20041226.html

Click on "Sea level Data."

Q: If the San Andreas fault slips, will there be a tsunami in Hawaii? If so, how big will it be?

A.J.
Honolulu, Hawaii

A: No. In fact, there probably won't be much of a tsunami in California either. The 1906 San Francisco earthquake did produce a tsunami, but it was very small and only detected on instruments and was probably caused by a step in the fault rupture offshore of the Golden Gate. The San Andreas fault produces strike-slip earthquakes where the ground moves horizontally. These types of earthquakes may trigger landslides or a local tsunami due to a stepover like what happened in 1906. In either case, the effects will be very localized and won't be a problem for Hawaii. The Cascadia subduction zone is another story: when it ruptures, Hawaii will be affected, as will Alaska and much of the Pacific basin.

Previous set of responses