

Ask the Expert Responses
Set 3: Posted April 2, 2005

Wave That Shook the World homepage

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Q: Can an earthquake on land trigger a tsunami?


A: Yes, it can, and it has happened in the past. The earthquake deformation won't trigger the tsunami, but the shaking can trigger landslides. The 1989 Loma Prieta earthquake caused a slump in Monterey Bay and a small tsunami. Any time a strong earthquake occurs near the coast there is always a small possibility of a tsunami, and I recommend that anyone on the beach get off the beach whenever you feel an earthquake. Don't return until there has been some official declaration that no tsunami exists.

Q: What rate or percentage of change in our ecosystem affects the ability for this kind of natural disaster to happen sooner? Does El Niño play into these changes?

Ellen Gonzales
Grapevine, Texas

A: Ecosystem changes don't affect the occurrence of great earthquakes and their tsunamis but can have a significant impact on the effects once the tsunami has been generated. Modification of the shoreline can make it much easier for a tsunami to penetrate inland. Such things as removing vegetation, smoothing foredunes, and paving large expanses of coastal areas will make it much easier for a tsunami to flood the region. Of course, the biggest effect of human activities on the magnitude of a tsunami catastrophe is population growth—the increased numbers of people living in vulnerable areas.

Q: I saw the tsunami show on PBS yesterday and towards the end, if I remember correctly, there was mention of the Cascadia subduction zone and that we would need to develop buildings strong enough to resist the incredible force of a tsunami. I noticed, however, that the fault line on the computerized display showed that it was quite close to land. If this is so, how can any building be strong enough to withstand both the incredible power of a nearby earthquake and THEN the awesome force and momentum behind a tsunami?

Patrick Higgins
Alexandria, Virginia

A: You put your finger square on the center of the problem. We know quite a bit about how buildings respond to strong ground shaking, and this information is incorporated into building codes worldwide. There has been less work done on tsunami impact, but the Japanese have accumulated quite a lot of information on how structures interact with high-velocity, debris-strewn waves. Hawaii incorporates tsunami design issues into construction requirements for buildings near the coast. But there is very little information on how structures deal with the nearly simultaneous onslaught of strong ground shaking and a tsunami—the situation we expect where I live the next time the Cascadia subduction zone ruptures.

There are a number of projects underway at the moment to address this knowledge gap. The Japanese have built a large tsunami wave tank with a shake table in the middle to observe this "double-whammy" impact. The U.S. National Tsunami Hazard Mitigation Program has designated this question a high priority task for the mitigation leg of the program. They have convened a team of engineers and scientists to work towards code development in the next few years. And a number of International Tsunami Survey Teams are collecting field data from Indonesia right now to look at how the buildings there performed.

What we do know from past tsunamis is that many buildings can survive both strong ground shaking and tsunami inundation. They are generally strong reinforced-concrete structures. Wood buildings generally do well during earthquake shaking but not so well when a tsunami strikes.

Q: Why do tsunami waves travel so fast?

J.H. Abbott
South Holland, Illinois

A: Tsunamis travel fast because they have a very long wavelength compared to wind-driven water waves. Tsunamis originate when the entire column of water above the seafloor is uplifted or dropped down. Unlike wind waves, they are driven by gravity. As a result, their speed is only a function of how deep the water is—the deeper the water, the greater the gravitational potential energy and the gravitational acceleration.

Q: From reading your answers to other questions, I gather that the Pacific Northwest is a hot spot for possible tsunami action. I am from Canada and live on the west coast of Vancouver Island, specifically in an area called the Barkley Sound. The Sound is like a large bite out of the side of the island.

The information on this Web site only mentions that Washington, Oregon, and California are in danger of tsunamis. Is this correct, or is it just a case of "America-centricity" (no offense). Isn't the subduction zone right off the coast of British Columbia?

Ashley Goldberg

A: I think I have mentioned Canada in some of my other answers, and I always include British Columbia when I talk about the hazards of the Cascadia subduction zone (CSZ). The CSZ extends from northern California near Cape Mendocino (about 200 miles north of San Francisco) to Vancouver Island, Canada. Most scientists agree that the last rupture in the year 1700 involved the entire zone, and all the coastal regions nearby experienced both strong ground shaking and tsunami inundation. But this isn't the only region that will be affected by the next Cascadia tsunami. Alaska, Hawaii, and the northern British Columbia coast will also be affected—and all of the other countries in the Pacific basin!

Q: The fact that 12/26/04 was three microseconds longer than it should have been because of the tsunami makes me suddenly feel a lot less secure. Was there any other event in the last 500 years that caused the Earth to wobble like this? Surely hurricanes wobble the Earth just as much. The three-microsecond delay must have screwed up a lot of scientific experiments. How does that affect all those atomic clocks around the world? Must they all be recalibrated now? The launch sequence for NASA's next Mars mission must also be recalibrated, I assume. Thanks.


A: Relax! The Earth is constantly wobbling. There are two types of natural wobble of the Earth relative to its axis of rotation. The annual "forced wobble" is caused by the seasonal fluctuations in the Earth's atmosphere. The natural or free wobble (called the Chandler wobble) is caused by sudden inputs of energy like a very large earthquake or a meteor strike. Quite a few earthquakes in the past have shown a small effect on the Earth's rotation. The Earth's rotation is also changing in rate and direction in space.

I have never read anything about individual hurricanes affecting the Earth's rotation. While hurricanes seem very energetic to those in the wind's path, they don't input very much energy into the solid Earth and are nowhere near as energetic as what happened on December 26. But as I mentioned before, the annual shift in atmospheric mass due to the change of seasons does affect rotation. Scientists have not used the length of the day as a standard of time for a long time, because it is so unrealiable. I don't know of any scientific experiments that were affected, and it certainly won't change atomic clocks, which are based on vibrational rates of certain atoms and are completely independent of rotation.

Q: What organized fundraising efforts are under way for the establishment of a worldwide tsunami warning system? Friends and I vowed that it was not enough to donate to the relief effort, which we all did, but that it was also critical to donate to some process that is trying to prevent so much death from happening again.

Claudio Cambon
Los Angeles, California

A: There are many efforts under way at both the national and international level. There is an international tsunami group called ITSU that includes representatives from a number of tsunami-prone nations. ITSU works through UNESCO and the United Nations. You may visit its Web site at There are also individual efforts in Thailand, Sri Lanka, and India to develop local or regional tsunami warning systems.

But one point I want to emphasize is that there is no simple technological fix like an early-warning system that will save everyone in a tsunami. Tsunami warning systems require time—time to detect the earthquake, determine the magnitude, and disseminate the information to the people who need to know the information. Once the information is transmitted to local officials, it takes several hours to conduct an orderly evacuation. Disorderly evacuations can be hazardous—witness what happened on March 28 in Sri Lanka when two people died because of chaos during the evacuation for what turned out to be no significant tsunami.

The only effective way to reduce the loss of lives from tsunamis is a coordinated program that includes education, hazard assessment, and rapid warning dissemination. For people close to the source area of an earthquake, the ground shaking is the warning, and they need to know what to do, how to do it, and do it very fast with little or no official guidance.

Q: I read somewhere that the destruction of coastal mangrove swamps to create recreation venues in areas such as Thailand contributed greatly to the level of destruction. This article claimed that the naturally occurring dense stands of mangrove had previously served to absorb and dissipate the force of the wave. Anything to this argument?


A: I read this, too, and am interested in seeing the results of some survey work that is now being analyzed. I became very interested in how vegetation interacts with tsunamis when I studied the 1998 tsunami in Papua New Guinea. We learned that vegetation did affect inundation. The tsunami penetrated far less inland in areas covered by dense thickets of mangrove and trees compared to the very shallow adjacent lagoon. However, the mangroves proved to be very hazardous to many of the tsunami survivors, who were swept into them and in some cases impaled upon the broken branches and had a very difficult time extricating themselves. Trees were also problematic.

There are numerous examples of how vegetation and coastal topography affect inundation. Forested areas act as natural filters for debris, and hilly foredunes create turbulence that significantly reduces wave energy. We certainly saw this effect in 1964 when Crescent City was attacked by the tsunami from Alaska. In this case, the trees were willows, and they probably had a similar effect as mangroves. The worst aspect of coastal development from the perspective of tsunami safety is to remove vegetation and smooth over the landscape. I've heard stories of coastal landowners removing dunes to improve their views. Unfortunately, they are now much more likely to get a view of a tsunami coming straight at them.

Q: In about 1968, I visited Pismo Beach on the central coast of California, and an oldster told me a tsunami had once occurred there. He warned me that if we were ever on a beach and the water started to recede way out, run for our lives. That was almost 40 years ago, but I never forgot that conversation. Too bad so many people had never heard this fact before December 26. Do you know of any tsunami that ever hit Pismo Beach?


A: I know of two historic tsunamis in the Pismo Beach area. On November 22, 1878, a tsunami occurred along the central California coast. It was observed at six communities, including Pismo Beach. This tsunami doesn't appear to have been associated with a felt earthquake and is usually attributed to a submarine landslide. On November 4, 1927, a magnitude 7.3 earthquake produced a tsunami that was about six feet high in the Pismo Beach area. The oldster was absolutely correct, and I'm glad he passed his information on to you.

Q: Regarding the development or improvement of a tsunami warning system, have the major hotel chains been asked to participate? It seems as though international hotel chains would make ideal candidates for local warning systems, because:

  1. they represent consistent waterfront presence worldwide, at least in resort and highly populous areas;

  2. they tend to have e-mail and telephone systems that are near state of the art, at least for reservation systems, even if the local infrastructure offers less capability;

  3. they would be likely to be able to offer a single point of contact, i.e., headquarters e-mail/phone that would have the capability to notify all sites or targeted sites simulataneously (e.g., automatically forwarded to all sites or all sites within five miles of ocean);

  4. they tend to be staffed 24/7, at least for receipt of e-mail and telephone messages;

  5. if they do not already have a public address system in place, they would tend to have the resources to put one in and keep it tested for "clear the beach and move inland immediately"-type emergency announcements;

  6. they have a vested interest in keeping their customers and staff safe;

  7. they would presumably benefit in goodwill from very positive public relations, as it would be an asset to proudly display a decal or sign proclaiming them a part of the "Tsunami Early Warning System" or some such title—this would benefit them not only with customers but with the local community as well;

  8. they would tend to be able to afford to print and distribute tsunami safety instructions, similar to airline flight safety instructions or cruise ship lifeboat drill instructions, and monthly (during aftershocks) or annual drills could be added to their fire preparedness routines;

  9. legalities being what they are, they would probably need some sort of liability reduction in exchange for agreeing to help with the warnings.

It is only one small part of what needs to be done, but given how much more likely it is to see a hotel than a government safety office on waterfront property, it seems like a natural way to augment the government contacts who could help quickly spread the word when there is a tsunami alert.

Sharon Hope
Covina, California

A: Large hotels are very involved with tsunami evacuation and dissemination of tsunami warnings in Hawaii. Many of the hotels are designed with tsunamis in mind, with lower floor areas that are open and the waves can easily travel through without affecting the building's structural integrity. Hotel staff help manage evacuations, and in areas where the hotel may be cut off from higher ground inland, the upper floors may be used for vertical evacuation.

In the Pacific Northwest, a number of hotels have also become involved with tsunami planning efforts. In Crescent City, California, you will find tsunami information often posted in hotel rooms, and many hotels in Oregon, Washington, and British Columbia are also active in promoting tsunami awareness and appropriate behavior when an earthquake hits.

It would be great to get more hotels on the safety bandwagon. Unfortunately, there are still a number who are concerned that mentioning tsunamis may scare their customers. Others are concerned about legal ramifications. I hope one outcome of the Indonesian earthquake is a greater acceptance in the tourist industry of their responsibilities in reducing the risk for their guests.

Q: When all is said and done, it appears (to this north coast estuary dweller) that the only recourse for a local tsunami is self-help, i.e., head by foot toward the hills with one's portable emergency pack and do so within 15 minutes. Please comment.

Winston Mitchell
Seaside, Oregon

A: I know Seaside well and have attended several meetings in your lovely community. There are tsunami hazard maps posted in your community and an active public education program about tsunami hazards. Awareness and personal understanding of what to do the next time a strong earthquake occurs are the main way to reduce losses when the next tsunami hits.

For you, the earthquake is the warning and you may have no more than 10 to 15 minutes between ground shaking and the arrival of the first significant waves. Don't take a car, because the roads may be damaged from the earthquake shaking and others may have created a massive log jam. Do be aware of the tsunami hazard maps that are available for your community and know where you are going to head. The last Cascadia earthquake occurred on a night in January, so be prepared to respond in the dark and in the rain. Yes, you will need to rely on yourself, but the technical information on areas at risk should be able to guide you in your response.

There is a FEMA-sponsored pilot project that is using Seaside as the case study for combining inudation studies, structural response of buildings, and human response in a tsunami situation. We are all eager to hear the results of that study, and I hope it will provide you with more information.

Q: I live on the southwest Washington coast on the Long Beach Peninsula. Until the NOVA episode, I was under the impression that a long, shallow, sloping beach (like ours) would attenuate a tsunami wave, thereby potentially causing less damage. Would such a wave not crest farther out and be lower than one slamming against a sharply sloping beach?


A: Actually, a long, uniformly sloping beach is more likely to allow coherent buildup of tsunami amplitudes. Tsunami waves don't "crest." You are accustomed to normal wind-driven waves. In deep water, wind waves don't feel the seafloor. We call them deep-water waves, and their speed depends on the driving force of the winds. When they reach the coast, they start to feel the bottom once the water is shallow enough. This causes the wave to crest and break (it sort of falls over on itself).

Tsunami waves are never deep-water waves, because even in the deepest ocean they still feel the bottom. As the sea bottom becomes shallower, the velocity of the tsunami slows down, and to conserve energy the wave amplitude builds up. Changing the offshore slope affects how quickly the tsunami slows and builds in amplitude.

Q: Is there a maximum speed a tsunami can travel? Also, what is the fastest recorded speed of a tsunami? Thank you.

Rudy Chacon
China, California

A: Tsunami speed is limited by the depth of the ocean. The deepest ocean on Earth is the Mariana Deep in the western Pacific. A tsunami traveling here might locally reach speeds of nearly 700 miles per hour. Actually, tsunami speed had never been directly recorded prior to the Indonesian tsunami. It was determined indirectly based on the time of the source and the time of wave arrival. The Indonesian tsunami will allow some direct calculations in a few situations—from the space-based measurements of Poseidon and Jason, and from the many videos of the tsunami hitting the coast.

Q: I just watched "Wave that Shook the World," and one of the scientists mentioned that it is currently not known at what frequency or how many waves will contact land during a tsunami. But isn't the movement of water fairly predictable?

Johanna Anderson
Arcata, California

A: Unfortunately, the movement of water is not predictable. Certain things can be predicted—the daily changes in tides and the amplitudes of large swells. But once waves enter shallow water, especially in bays or inlets, the wave motion becomes very difficult to predict. This is because tsunamis almost always excite a secondary oscillation or seiche within a bay. The periodicity of the seiche is a function of the bay geometry. It is not a single period, but just like a musical instrument it will have overtones. So successive waves from the tsunami will interact with the complex oscillations in the bay. Some harbors like Hilo in Hawaii and Crescent City in California are notorious for producing a complex set of oscillations that last many hours. Tsunami modelers are getting much better at modeling this complex wave train, but it is still difficult to predict what will happen after three or four cycles.

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