On March 28, 1964, a tsunami slammed into Crescent City, California. A series of waves pushed buildings off their foundations and into other buildings, and swept cars and structures out to sea. So strong were these walls of water that a 50,000-pound concrete tetrapod was knocked from its base by the force of a timber being carried along in the rushing tumult (this tetrapod is still on display at the south entrance to the city). The inundation extended 800 to 2,000 feet inland in the commercial and residential areas of the city, and a mile inland along a low intermittent stream. Water depths reached up to eight feet in city streets and 13 feet along the shoreline. Overall, Crescent City sustained more than $7.4 million in damage and 12 people lost their lives.

We’ve all heard of tsunamis. We think of them as being colossal, powerful, and unpredictable, much like earthquakes: coming as if from nowhere and capable of causing great devastation within minutes. Is it possible to anticipate these destructive events and take action? The National Oceanic and Atmospheric Administration (NOAA) thinks so and, as a result, in September NOAA certified Crescent City as California’s first official “Tsunami Ready Community.” Key elements of this designation are knowledge about tsunamis and a carefully developed plan that will be implemented when the next tsunami occurs. Tsunamis are often improperly called “tidal waves.” Tidal waves do exist. They are generated by the gravitational pull of the moon and sun, and we experience them as the daily rise and fall of the sea along our shores. A tsunami, in contrast, is a series of very long waves generated by a sudden dramatic event, such as an undersea earthquake (the most common cause), landslide, volcanic eruption, meteorite impact, or other event that causes a rapid rise or fall in the ocean surface at a specific location. In the case of an undersea earthquake, the ocean floor moves up or down, causing a bulge or depression in the water surface. As the bulge or depression comes to equilibrium with the surrounding water surface, it sends out a series of wave crests and troughs. In deep ocean, these waves are visually undetectable, being up to hundreds of miles long, often less than three feet high, and traveling at speeds up to 450 miles per hour.

As each wave of the tsunami moves away from its generation area, it bends according to changes in the ocean floor bathymetry, orienting its wave front closer to parallel with the shoreline. At the same time, it begins shoaling, or slowing down and growing higher. Because of the tremendous wavelength of a tsunami, a wave that was less than three feet high in the deep ocean could rise to more than 50 feet in shallow water. The form of the tsunami at the shoreline could be a series of bores (a rapid rise of the water surface in which the advancing water is preceded by a step-like turbulent face), a simple rise and fall of the water surface (similar to a rapid tide) or, in rare cases, a series of breaking waves.

Certain predictions can be made about tsunamis. For example, because tsunamis are generated by a rapid rise or fall in the ocean surface, we know the earthquakes that generate tsunamis are generally those that occur on vertical displacement (dip-slip) geologic faults. Thus, the tectonic convergence (subduction) zones along the southern coast of Alaska (the source of the 1964 tsunami that struck Crescent City) and the Aleutian Islands, the northwest coast of South America, and southeast of Japan pose significant threats to California. Closer to California, the Juan de Fuca plate converges with and dips beneath the North American plate off the coasts of California, Oregon, and Washington. The area where these two plates also meet the Pacific plate (the Mendocino Triple Junction) is particularly prone to earthquakes. When seismic activity occurs in any of these areas, the risk of a tsunami goes up. Along the central and southern California coasts, the majority of the major fracture zones, including where the Pacific and North American plates meet, form transverse (strike-slip) faults that do not pose as great a threat.

Depending on the force of the generating event, as well as the nature of offshore bathymetry, shoreline geometry, and onshore topography, local tsunami inundation can vary greatly. At the shoreline, tsunami effects can range from a barely noticeable change in the water surface to astonishing rises in water level of over 90 feet, and can push inland thousands of feet. Tsunami waves cause damage by the obvious mechanisms of flooding and the force of the rapidly moving water, but may cause even more damage by the debris they forcefully carry.

Since 1737, according to NOAA data, 64 tsunamis have struck the west coast of the United States with 36 of these occurring in the last 52 years. The three most destructive occurred in 1946, 1960, and 1964. While the 1946 tsunami was barely noticed in California, both the 1960 and 1964 tsunamis left their marks on the state’s northern coast. Each time a tsunami makes landfall, visual markers are left by the receding waters: debris lines, water marks on buildings and trees, matted vegetation, and so on. Researchers rely on clues like these to help vulnerable coastal communities prepare for the next large tsunami.

Orville Magoon grew up in Hawaii and witnessed the 1946 tsunami from a precarious perch in a palm tree. Years later, he was an engineer with the U.S. Army Corps of Engineers in San Francisco when both the 1960 and the 1964 tsunamis made landfall and both times he traveled to Crescent City, Eureka, and Fort Bragg to survey inundation contours, set high water markers, and do general damage surveys. During his surveys, he spoke with as many people as possible to get eyewitness accounts. His advice to future tsunami surveyors is to get to the site as soon as possible, as the inundation zones can be hard to map once visual clues have been disturbed. He also advises interviewing eyewitnesses as close to the time of the tsunami as possible, when their accounts are the most reliable.

Recent researchers have heeded Orville Magoon’s advice. “Tsunamistas” worldwide have begun to coordinate their survey efforts in hopes of better understanding the general nature and impact of tsunamis and of helping affected areas prepare for these natural disasters. The International Tsunami Survey Team (ITST), composed of 40 scientists and more than 20 students from 15 different countries, has responded to all major tsunami disasters that have occurred since 1992. Lori Dengler, a professor of geology at Humboldt State University, has participated in several ITST surveys. According to Dengler, the team observes and documents tsunami effects, collects perishable data, and makes recommendations to both the affected country and the international community on future research, planning, and preparedness. The ITST surveys provide valuable information on what tsunamis can do.

Photographs of cars suspended in trees, rail ties driven through truck tires, etc., are powerful reminders of the forcefulness of these events. Even more important for future preparedness are the other survey results. The information on the extent of inundation can be used to anticipate impacts from similar tsunamis in other locations. However, we still are not able to predict when or where a tsunami may occur. Instead, scientists and engineers are identifying extreme inundation zones. Costas Synolakis and engineering students at USC are developing projections of the areas that could be at risk from a tsunami, and areas that would be safe. Projections have been completed for San Francisco, San Mateo, Los Angeles, and San Diego Counties, and the Santa Barbara–Ventura coast; projects for Monterey Bay will be completed soon. The state’s Office of Emergency Services has funded this work and will use it for emergency response purposes and to provide information on safe areas and evacuation routes. These products are important in preparing for the next tsunami and for making communities Tsunami Ready.

So, what if a tsunami is generated out in some distant part of the ocean? Do the residents of coastal towns such as Crescent City really have a chance to escape in time? Often, yes. NOAA is currently deploying a network of ten offshore buoys (to be fully functional by 2003) to measure water surface elevation. While tsunamis cannot be detected easily by ships at sea, they can be detected by these buoys. If a slight increase or decrease in water level is observed over several monitoring intervals, these buoys automatically switch to tsunami-monitoring mode and begin sending frequent water-surface elevation information to Tsunami Warning Centers in Alaska and Hawaii. Once a tsunami is detected, these centers issue alerts or warnings to state Offices of Emergency Services and through the weather channel of the National Weather Service.

For tsunamis generated many miles from the shoreline, people in California localities such as Crescent City will have several hours to learn about the coming waves and should be able to move away from high-risk areas onto higher ground. For locally generated tsunamis, however, the first large wave may arrive soon after the generating event and well before a community-wide alert or warning can be announced. In these cases, local education and preparedness are essential. The main thing to do if you are in a tsunami-prone area and an earthquake strikes is to “duck, cover, hold,” then immediately seek higher ground (see sidebar, “How to Survive a Tsunami”).

Evidence from the 1960 and 1964 tsunamis, such as inundation contours and high water marks, have allowed scientists to predict where the greatest flooding might occur in Crescent City, and what areas will be less affected, should a tsunami hit again. The city was smart after the 1964 tsunami, in that it developed the area closest to the harbor as a park (permanent open space) and allowed reestablishment of downtown businesses mainly inland of Front Street. The city also erected tsunami evacuation route signs on many streets. Recently, when a new hotel was being considered for an area of Crescent City that may be at risk from tsunamis, the Coastal Commission required that the developer prepare a tsunami safety plan. An information sheet is to be placed in each guest room, and hotel staff is to be trained on how to respond to a tsunami warning or alert. Thus, when (not if) another tsunami hits, the destruction should be less serious than in past events.

While tsunamis are rare events, the “Hollywood” accounts of their destruction are based on the fact that tsunamis have occurred throughout history and have repeatedly caused great devastation and loss of life. Tsunamis have struck California in the recent past and will do so again in the future. Though they are difficult to predict, scientists have developed an international tsunami warning system and are actively working to map areas at risk. The tsunami warning system and maps, along with awareness, education, and well thought out plans of action will help minimize the damage caused by the next tsunami to strike California. Crescent City has relied on these key steps in its program to become the first Tsunami Ready community in California. Hopefully it will not be the last community to recognize and plan for this type of natural disaster.

KEVIN KNUUTI is a research hydraulic engineer with the Coastal and Hydraulics Lab, U.S. Army Engineer Research and Development Center; kevin.knuuti@erdc.usace.army.mil.

LESLEY EWING is a coastal engineer with the California Coastal Commission; lewing@coastal.ca.gov.

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