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Tsunamis are a series of giant waves created by a disturbance such as an earthquake, landslide, volcanic eruption, or meteorite. From the area where the tsunami originates, waves travel outward in all directions. A tsunami can move hundreds of miles per hour in the deep open ocean, where it may only be a few inches or feet high. As a tsunami travels inland and ocean depth decreases, a tsunami’s wave height becomes higher and higher, and it can hit land with waves as high as 100 feet or more.
Tsunamis are characterized as shallow-water waves—that is the ratio between the water depth and its wavelength gets very small. Shallow-water waves are different from wind-generated surf waves. Wind- generated waves usually have a period (time between two successional waves) of 5 to 20 seconds and a wavelength (distance between two successional waves) of about 100 to 200 meters (300 to 600 feet). A tsunami wave can have a period in the range of five minutes to two hours and an open ocean wavelength in excess of 100 miles.
When a tsunami finally reaches the shore, it may appear as a rapidly rising or falling tide, a series of breaking waves, or even a bore (a step-like wave with a steep breaking front). Although most people imagine a tsunami as a large, steep wave breaking on the shore, tsunamis generally appear as an advancing tide without a developed wave face and produce rapid flooding of low-lying coastal areas. Areas at greatest risk to tsunami impacts are those less than 25 feet above sea level and within a mile of the shoreline.
Reefs, bays, entrances to rivers, undersea features and the slope of the beach all help to modify the tsunami as it approaches the shore. Because the long-period wave can bend around obstacles, the tsunami can enter bays and gulfs with intricate shapes. Previous events have shown that wave heights increase in bays that narrow from the entrance to the head (like a funnel) but decrease in bays that have narrow entrances. Shorelines protected by reefs typically do not sustain extensive damage from tsunamis as the reefs disperse the wave energy. Further, islands in a group may “shadow” one another reducing the tsunami effect, and small islands may experience reduced runup as the tsunami waves may refract around them.
Although landslides and volcanoes cause some local tsunamis, more than 95 percent of tsunamis result from subduction earthquakes. The earthquakes associated with tsunamis are referred to as “tsunamigenic” earthquakes. The association between earthquakes and tsunamis results from the fact that both are generated by the tectonic displacement of the earth’s crust. Earthquakes generate tsunamis when the sea floor abruptly deforms and displaces the overlying water from its equilibrium position. Waves are formed as the displaced water mass, which acting under the influence of gravity, attempts to regain its equilibrium. The main factor that determines the initial size of a tsunami is the amount of vertical sea floor deformation resulting from subduction zone earthquakes. The earthquake’s magnitude, depth, fault characteristics, and coincident slumping of sediments or secondary faulting control the size of the tsunami.
The second most common cause of tsunamis is landslides. A tsunami may be generated by a landslide originating above sea level but plunging into the sea or by a submarine landslide. The water displaced by the landslide causes the tsunami. Other geologic disturbances, such as volcanic activity, can also generate tsunamis. A tsunami can be generated by any disturbance that displaces a large water mass from its equilibrium position.
Low-lying coastal areas are at highest risk to tsunamis. Areas at risk to tsunamis and extreme tsunamis are placed into tsunami evacuation zones. The extreme tsunami refers to a tsunami sourced near the Aleutian Islands in Alaska, as a study completed after the 2011 Japan earthquake found that the greatest tsunami threat facing Hawaiʻi is from the Aleutians.203 Tsunami evacuation zones are shown in the figure below. In addition to mapped evacuation zones, areas adjacent to inland waterways may flood due to surges. Further, locally tsunamis may impact different areas.
NOAA’s Global Historical Tsunami Database reported 61 tsunami events impacting Maui County between 1800 and 2020 (many of these events include multiple waves and locations impacted). Seven of these events resulted in damages, injuries and/or loss of life. These events, along with others reported by the state and previous county hazard mitigation plans or by disaster declarations, are described below.
1819: This tsunami destroyed three homes on the Island of Molokaʻi.
1837: This tsunami destroyed three homes of the Island of Molokaʻi and caused two fatalities in Kahului.
1868: This tsunami damaged two homes in Kanaio.
1878: This tsunami destroyed two homes in Halehaku and damaged two more.
1946: This tsunami generated waves up to 33 feet that struck the shoreline at Kahakuloa and destroyed three homes approximately 20 feet above sea level. At Honolua and Honokohau, 24 to 28-foot waves destroyed homes, roads, and bridges. Ten deaths were reported at Hamoa, and two deaths and two injuries were reported at Keʻanae. Further, Lower Pāʻia and Mala each reported one death. In total, this event resulted in 14 fatalities, two injuries, and three destroyed homes.
March 1957: A tsunami impacting the Wailuku-Kahului Community Planning District resulted in a disaster declaration after causing an estimated $5 million in damages.
May 1960: This tsunami (generated by the magnitude 9.5 Great Valdivia Earthquake in Chile) was most destructive in Kahului, where six homes were destroyed. The Kahului Shopping Center and immediate vicinity were damaged. One home was destroyed in Spreckelsville. The damage estimate was about $763,000 in the low coastal areas. The waves washed inland for a distance of about 3,000 feet to ground elevations of about 6 feet. This event resulted in a disaster declaration.
March 1964: This tsunami (generated by the magnitude 9.2 Great Alaskan Earthquake) had a recorded maximum run-up at Kahului of 12 feet and caused an estimated $53,000 (1964 dollars) in damage.
February 2010: Generated by magnitude 8.8 earthquake off the coast of Chile – no damages, injuries, or loss of life.
March 2011: A tsunami impacting south and west Maui resulted in a disaster declaration after causing an estimated $6.37 million in damages.
Tsunami extent, or severity, can be measured in several ways. One way to determine severity is by wave height, or the maximum water height above sea level. The maximum water height reported by NOAA for a tsunami event in Maui County was 16.5 meters (approximately 54 feet) on the north coast of Molokaʻi during the 1946 tsunami. On Maui Island, the highest recorded wave height (33 feet) also occurring the 1946 tsunami event.
Tsunami extent can also be measured in terms of damages or fatalities. The 1946 tsunami was the deadliest in Maui’s recorded history, with a total of 14 reported deaths, while the March 2011 tsunami is the costliest, with almost $6.4 million in reported damages. More severe and more damaging tsunami events are possible. With climate change, rising sea levels may result in tsunami waves and surges that reach farther inland. Further, loss of coral associated with climate change may increase the intensity of tsunami events, as coral reefs systems protect coastlines against tsunami impacts by helping to dissipate wave energy.
Future Probability
Based on available data, 61 tsunami events were reported in 220 years, meaning a tsunami event impacts the county approximately every 3.5 years, on average. Therefore, a probability of “likely” (1% to 10% annual chance) was assigned to the tsunami hazard. It should be noted that large, damaging tsunami events may be less frequent.
All current and future buildings, infrastructure, and populations in low-lying coastal areas, tidal flats, and river deltas, especially those in mapped tsunami evacuation areas, are considered at risk to tsunami.
Specific impacts to buildings, infrastructure, socially vulnerable populations, life safety, public heath, and the economy from the tsunami hazards are described below. Climate-related impacts to the tsunami events are also described.
Buildings and Infrastructure: All current and future buildings and infrastructure, including critical facilities, located within low-lying coastal areas are considered at risk to tsunami.
Health and Safety: Tsunamis present serious risks to life safety. Cumulatively, tsunamis in the Hawaiian Islands have killed the largest number of people of all natural hazards affecting the state. Drowning is the most common cause of tsunami death. Powerful currents can turn floating debris into projectiles that can cause injuries, deaths, and property damage. Fires can also ignite from broken gas lines or ruptured tanks.
Economic Impacts: Tsunamis can have wide-reaching economic impacts. Direct economic impacts may result from destroyed buildings and infrastructure, including power utility poles and lines, potable water and wastewater facilities, and roadways. The force of tsunami waves may destroy piers, breakwaters, and seawalls. Ships, marinas, and fishing fleets may be battered, sunk, moored, or stranded, impacting the fishing and tourism industries. Business interruptions can occur, and damages or changes to impacted beaches may impact tourism for periods long after the tsunami. Further, the county may attract less tourists after a tsunami, as there may be a perceived risk to visiting.
Climate Change Impacts: While climate change may not have known impacts om tsunami sources (earthquakes, landslides, and volcanoes), climate change may make Maui County more susceptible to tsunami impacts. Increased sea levels associated with climate change could mean tsunamis can reach farther inland. Further, warming ocean trends are known to cause coral bleaching episodes. Coral reefs provide protection against tsunamis by forming an offshore barrier that works to dissipate wave energy.
