Science

Frequently Asked Questions (FAQs)

This section has been designed to answer many of the common questions concerning the nature of tsunamis, their occurrence here in Hawai'i and the Pacific region, and what scientists and civil authorities have done to improve our understanding and prevent loss of life from this destructive natural phenomenon.

Please read on and discover the fascinating facts about tsunamis! If you have trouble with any of the terms, look them up in the Tsunami Glossary.

Where is Hilo Hawai'i?

Hawaiian Islands map

Hilo, pronounced (hee-low), is located on the windward (eastern) coast of the island of Hawai'i, 200 miles southeast by air from Honolulu, the state capitol. Nestled between the flanks of the volcanic peaks Mauna Kea and Mauna Loa, Hilo is situated around Hilo Bay, the second largest deep water port in the island chain. As of the 2010 census, the Hilo population is approximately 44,000 residents. Hilo is famous for its rainfall (about 120 inches per year).

The geographic isolation of the Hawaiian Islands has resulted in unique assemblages of plants and animals found nowhere else on Earth. Surrounded by Pacific Ocean, the history of the Islands has been shaped by the interactions between land and sea. Hilo occupies a unique spot in this history, having been frequently subjected to devastating tsunami waves. In terms of property damage and loss of human life from tsunamis, Hilo surpasses all other areas in Hawai'i.

Hilo is affected by tsunamis for many reasons, one of which is the local topography and bathymetry.  The orientation of the Hawaiian Ridge and coastline, with respect to the direction and approach of a tsunami, plays an important role.  Also, small funnel-shaped bays, like Hilo Bay, harness the tsunami wave energy and amplify the heights of the waves, leading to greater destruction. In years past, Hilo was built up right down to the water's edge, and these structures sustained heavy damage during past tsunami events.

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What does the word "tsunami" mean?

"Tsunami" is a Japanese word which translates as "harbor wave", now used internationally to refer to a series of waves traveling across the ocean with extremely long wavelengths (up to hundreds of miles between wave crests in the deep ocean). When these waves approach shore, the speed of the wave decreases as they begin to "feel" the bottom. It is at this time that the height of the wave drastically increases. As the waves strike shore they may inundate low-lying coastal areas, resulting in mass destruction and in many instances loss of life. Often a tsunami is incorrectly referred to as a tidal wave. Tidal waves are simply the periodic movement of water associated with the rise and fall of the tides produced by the gravitational attraction of the sun and moon. Tsunamis have no connection with the weather nor with tides.

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Is there a Hawaiian word for tsunami?

There are two words used to describe tsunamis. "Kai e'e" is a general word for tsunami waves, and "Kai mimiki" is used to describe the withdrawal of the water before the Kai e'e arrives.  The withdraw of the water is actually the trough of the tsunami reaching shore.

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What causes a tsunami?

Oceanographers often refer to tsunamis as seismic sea waves since they are usually the result of a sudden rise or fall of a section of the earth's crust under or near the ocean.  A seismic disturbance can displace the water column, creating a rise or fall in the level of the ocean above, generating a tsunami.

Tsunami waves can also be created by volcanic activity and landslides occurring above or below the sea surface.  These types of activities produce tsunamis with much less energy than those produced by submarine faulting.  The size and energy of these tsunamis dissipates rapidly with increasing distance from the source, thus resulting principally in local devastation.

Read more on the generation of tsunamis.

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Could nuclear testing create a tsunami?

Wave detection equipment showed that nuclear testing explosions at and near Pacific islands do not propagate hazardous tsunami waves.

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How is a tsunami wave different from an everyday ocean wave?

The waves you see at the beach are generated by wind blowing over the sea surface. The size of these waves depends on the strength of the wind creating them and the distance over which it blows. Generally the distance between these waves, known as the wavelength, ranges from a couple of feet to perhaps a thousand feet. The speed of these waves as they travel across the ocean ranges from a few miles an hour up to sixty miles an hour in some instances.

Tsunami waves resulting from geological mechanisms (see above question) behave much differently than wind generated waves. The magnitude of the disturbance causing the tsunami is the primary factor influencing the size and strength of the waves. The distance between successive wave crests (the wavelength) is much larger than that of a wind wave and may be hundreds of miles. Depending on the depth of the water in which the tsunami is travelling, it may attain speeds of up to 600 miles an hour. Review the table (PDF) comparing tsunami and wind waves.

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How does a tsunami behave as it approaches land?

When the waves of a tsunami approach land, their appearance and behavior become dependent on several local factors. Two of the most important factors are the topography of the seafloor (bathymetry) and the actual shape of the shoreline. As a tsunami encounters shallow waters surrounding the shoreline, its height can increase from three feet (one meter) or less to over 50 feet (15 meters). Wave heights can also increase when concentrated on headlands or when traveling into bays having wide entrances that become progressively more narrow. The presence of an offshore coral reef can dissipate the energy of a tsunami, decreasing the impact on the shoreline.

The image most people have of a tsunami is a large, steep wave breaking on the shore. This image is seldom the case. Most tsunamis appear as an advancing flood or wall of water with no developed wave face, resulting in rapid flooding of low-lying coastal areas.

Another event that may result from a tsunami is a standing wave or seiche. A seiche occurs in bodies of water that are partially or completely enclosed, such as Hilo Bay, creating a standing wave that continually sloshes back and forth. The appearance of a seiche would be very similar to what happens when you place a glass of water on the table; the water rocks back and forth before settling. When a seiche is generated by a tsunami, subsequent tsunami waves may arrive in unison with a seiche resulting in an increase in the drawdown in sea level and a dramatic increase in wave height. Seiche waves may continue several days after a tsunami. Read more about characteristics of tsunamis.

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How long does it take a tsunami to reach land?

Once generated, a tsunami wave in the open ocean can travel with speeds greater than 500 miles an hour. These waves can travel across the Pacific Ocean in less than one day. Locally generated tsunamis can reach coastlines in just minutes.

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How many waves are there in a tsunami?

A tsunami generally consists of a series of waves, often referred to as the tsunami wave train. The amount of time between successive waves, known as the wave period, is usually a few minutes; in some instances, waves are over an hour apart. Many people have lost their lives after returning home in between the waves of a tsunami, thinking that the waves had stopped coming.

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What is runup and inundation?

When a tsunami approaches a coastline, the wave begins to slow down and increase in height, depending on the topography of the sea floor. The maximum vertical height to which the water is observed with reference to sea level is referred to as runup. The maximum horizontal distance that is reached by a tsunami is referred to as inundation. See an illustration of these concepts and look at the runup maps for the Hawaiian Islands.

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How are tsunami wave heights measured?

The wave height of tsunami waves can be highly variable in a local area depending on the underwater topography, orientation to the oncoming wave, the tidal level, and the magnitude of the tsunami. Because direct physical measurement of a tsunami wave would be a life threatening event, a common method for determining tsunami wave height is by measuring the runup, the highest vertical point reached by the wave. Runup heights are measured by looking at the distance and extent of salt-killed vegetation, and the debris left once the wave has receded. Tide gauges, found in most harbors, are the other tool for measuring tsunami wave height and period (wavelength).

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What is the "wrap-around" effect?

Whether a tsunami is generated in the North or South Pacific, it has the potential to affect all shores of the Hawaiian Islands. As large tsunami waves approach the islands, they may refract or bend around the islands and diffract through the channels between the islands as well. The ability of tsunami waves to bend around and through the islands is called the wrap-around effect. Sometimes tsunami waves will reflect off of a land mass instead of bending around, thereby increasing wave height of the approaching wave. Therefore, whether a tsunami warning is issued from an earthquake in Chile, Alaska, or Japan, inhabitants along all shores of the Islands should take the necessary precautions.

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Do all oceans have tsunamis?

Yes. Tsunamis have been recorded in all the major oceans of the world. However, this phenomenon is mainly restricted to the Pacific basin, an area surrounded by volcanic island arcs, mountain chains and subduction zones earning the nickname the "ring of fire", as it is the most geologically active area on the planet. The amount of activity in this region makes it much more susceptible to submarine faulting and subsequent tsunami events, whereas the Indian and Atlantic oceans are far less geologically active, with some exceptions, and therefore the occurrence of tsunamis is rare.

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What is the tsunami warning system?

The lack of a warning during the 1946 tsunami that devastated many coastal areas in Hawai'i led scientists and governmental agencies to establish a tsunami warning system for the Hawaiian Islands and United States territories in the Pacific by 1948. The main objectives of this system are: To detect and locate the existence of all possible tsunami-causing earthquakes by the use of properly monitored seismographs; to ensure that a tsunami actually exists by measuring water level changes at water level and tide-gauging stations located throughout the Pacific; and finally, to determine the time of arrival of the tsunami and to provide an adequate warning for evacuation procedures. The Pacific Tsunami Warning Center is located on O'ahu. Read more about the warning system.

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What do I need to know to be prepared for a tsunami?

Because a tsunami can strike at any time, being adequately prepared and knowing what to do beforehand could save your life. Hawai'i State and County Civil Defense agencies provide maps of evacuation zones and information on how to be prepared for this type of natural disaster in the front pages of the telephone book. You can also use our map viewer to enter an address to see if it lies within the evacuation zone. There are useful phone applications that give alerts, such as “Disaster Alert” and “Tsunami Watch.”

Know nature's signs. If you are at the beach and you feel an earthquake or observe a rapid withdrawal of the sea or an abnormally high rise of the sea (higher than high tide), head for higher ground immediately.

When a tsunami warning has been issued and the sirens sound, turn on the radio and follow instructions given by the authorities. Do not attempt to use the telephone or go to low-lying areas to view the oncoming waves. Remember, tsunamis travel at very fast speeds across the ocean; therefore, once a warning has been issued you should evacuate to higher ground immediately.

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If I need to evacuate, what should I bring with me?

Your tsunami survival kit is generally the same for all natural disasters. Here is a list of suggested supplies:  an extra supply of prescription medicines, non-perishable foods, ice chest, a minimum of 2 quarts of water per person per day, pet food, candles/flashlight, matches, blankets/sleeping bags, extra cash, clothing, eyeglasses, personal hygiene items, special items for infants/elderly/disabled family members, quiet games/books/toys for children, important papers including your driver's license, special medical information, insurance policies, property inventories, first aid kit, and water purification kit.

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Can the arrival time of a tsunami be accurately predicted?

When a tsunami is generated offshore the wave will behave as a shallow water wave. A shallow water wave is one that travels through water having a depth less than 1/20 of its wavelength. Knowing that the average Pacific Ocean depth is roughly three miles, oceanographers can determine the speed of the tsunami, and calculate the time it will take to travel between any two points. This information led to the development of travel-time charts that make it possible to predict the arrival time of a tsunami wherever it is generated. Due to the high speeds of these waves, a tsunami can travel across the Pacific Ocean is less than one day! Areas near the epicenter of earthquakes, landslides or volcanic activity are most vulnerable to the effects of a tsunami as there is little or no time for a warning.

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What has been the most destructive tsunami to strike the Hawaiian Islands in recent history?

Early on the morning on April 1, 1946, an earthquake with a reported magnitude of 8.6 occurred in the Aleutian Islands off of Alaska. Approximately five hours later the destructive tsunami waves struck the Hawaiian Islands. Maximum runups were reported to be 54 feet in Molokai and 55 feet in Pololu Valley on the Big Island. Waves in some areas penetrated more than half a mile inland. Between wave crests, the drawdown is reported to have exposed some areas of the seafloor 500 feet in the seaward direction. A total of 159 tsunami-related fatalities resulted from this destructive event. Many were curious school children who ventured into the exposed reef area, not knowing the receding water to be a sign of an approaching tsunami. There was no warning system at the time.

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How many locally generated tsunamis have occurred in the Hawaiian Islands in recent history?

Halape On the Big Island there have been several significant tsunamis resulting from local earthquakes or submarine landslides. The most recent and devastating of these tsunamis occurred in the early morning hours on November 29, 1975. Within a little over an hour, two earthquakes jolted the island. The first, located three miles inland of Kamoamoa village in Volcanoes National Park, had a Richter magnitude of 5.7. The second, centered two miles offshore of the Wahaula heiau (also in the park area) was much more violent having a Richter magnitude later to be determined as 7.2. The result of this earthquake was a 10-foot subsidence of the shoreline and the second most destructive local tsunami ever to be recorded in Hawai'i.

Campers in the remote Volcanoes National Park coast at Halape were awakened by the violent shaking of the first earthquake. Some of them had barely gotten back to sleep when the second earthquake shook so violently that standing was nearly impossible. Within 30 seconds, the first of five tsunami waves struck Halape. Two campers, one an adult with a group of Boy Scouts, the other a fisherman, did not survive. Nineteen others were injured. The maximum runup height was 47 feet at Keauhou Landing and 26 feet at Halape, 1.9 miles to the southwest.

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How are inundation/evacuation areas determined?

Methodology was developed at the University of Hawai'i (UH) to determine the maximum expectable inundation of our shores for worst-case tsunamis, drawing on the records from many years compiled by the Joint Institute for Marine and Atmospheric Research JIMAR).  These historical data are used in mathematical analyses to predict maximum wave heights along the coast; these heights are then used in numerical models involving the topography (land contours) to map the inundation in each location.

In coordination with the Civil Defense officers on each island, a final map is prepared showing the actual evacuation zones. The zones extend inland from the inundation limit to the nearest landmark such as a road, which can be used by public and police to identify the areas that must be evacuated to ensure safety. The zones are published in the front of the telephone directories for each Hawaiian island.  When the sirens sound, people are routed to safety until officials determine that hazardous wave action has ceased.

It is interesting that for Hilo itself, there are such complete (block-by-block) records of inundation (1946-1964) that they have been used to determine evacuation zones with only minor analysis.  In fact, these records have been extensively used to test the computer models developed to predict tsunami wave heights and inundation.  If the model can adequately re-create a previous event, there is more assurance it can be used to predict future events elsewhere. Read more in the article written by George Curtis, renowned tsunami expert.

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Since I don't live in an inundation area, why should I be concerned?

The shoreline areas of the Hawaiian Islands are no doubt the main attraction for visitors and residents alike. Much of the state's commerce and recreation involves the surrounding ocean, and therefore it is very important for all of us to acknowledge the threat a tsunami would impose on our lives. Even though you may live in an area that is not threatened by a tsunami directly, you would definitely be impacted by its effects.

There has been a period of relative tsunami quiescence (the last major destructive tsunami in Hilo was in 1960), and many people have never directly experienced the destructive and deadly force of a tsunami. In Hilo, we have been very fortunate that in each of the years 2010, 2011, and 2012 there have been warnings and evacuations, but the tsunamis have been non-hazardous. On the other side of the Big Island, Kona experienced damage to homes, businesses, and infrastructure in the 2011 tsunami from Japan; fortunately, there was no loss of life. It remains essential that everyone be properly informed and aware of what to do and where to go in the event of a tsunami.

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I own a boat. What should I do when there is a tsunami warning?

Boats are safer from tsunami damage when in the deep ocean rather than moored in a harbor. U.S. Coast Guard guidelines suggest deployment to water depths of at least 1,200 feet (200 fathoms). However, do NOT risk your life and attempt to get underway if it is too close to the first wave arrival time. Anticipate slowdowns caused by traffic gridlock and hundreds of other boaters heading out to sea. Do NOT go near a harbor if there is a local tsunami.

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If I would like more information on tsunamis who should I contact?

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Last Revised August 2013