Usgs geologic Investigations Series i-2761, Oahu

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47

Oahu

Oahu, the Gathering Place, is the most populated

island and Hawaii’s government and business

center. While it is removed from the active

volcanism and seismicity of the Big Island,

natural hazards associated with high waves,

storms, and flooding annually threaten Oahu’s

coastal inhabitants and infrastructure. The 

primary difference between the nature of coastal

hazards on Oahu and the rest of the islands is

the magnitude of the risk involved. While Oahu

has not witnessed a direct hit by a hurricane

force storm in its history or suffered damage

from a tsunami since 1960, extensive develop-

ment of the shoreline has continued, apparently

as a result of a high level of complacency among

developers and the public.

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48

Oahu

Index to Technical

Hazard Maps

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A

tsunami is a series of great waves most commonly caused by violent

movement of the sea floor. It is characterized by speed (up to 590

mph), long wave length (up to 120 mi), long period between successive

crests (varying from 5 min to a few hours, generally 10 to 60 min), and low

height in the open ocean. However, on the coast, a tsunami can flood

inland 100’s of feet or more and cause much damage and loss of life. Their

impact is governed by the magnitude of seafloor displacement related to

faulting, landslides, and/or volcanism. Other important factors influenc-

ing tsunami behavior are the distance over which they travel, the depth,

topography, and morphology of the offshore region, and the aspect, slope,

geology, and morphology of the shoreline they inundate. Predicting the

specific form of a tsunami at a shoreline is not yet possible because of

many factors. History has shown that these factors contribute to a wide

range of tsunami runup heights and associated damage even within a very

small geographic region. For example, the tsunami generated by the 1946

earthquake in the Aleutian Islands was recorded at Makapuu Point to be

37 ft while in Waimanalo, a mere 3 miles away, the maximum runup was

only 8 ft. An important difference between tsunami and ordinary wind-

generated waves is that the wavelength (distance between successive

crests) of a tsunami can be several orders of magnitude larger. This means

that as a tsunami passes an island the water level can rise for several min-

utes. As  a result, tsunami can cause significantly greater flooding than

ordinary wind waves of the same height.

History has also shown that damaging tsunamis in Hawaii are gener-

ated from distant locations around the Pacific Ocean margins, as well as

locally in Hawaiian waters. The source region of tsunami genesis has

important implications for the amount of damage caused by a tsunami

and for our ability to detect them and respond to their impact. Models

show that a tsunami generated in Alaska can travel to Oahu in 5 hr, while

a locally generated tsunami, like the 1975 tsunami in Halape, Hawaii, may

take only minutes or seconds. Five hours may seem like sufficient time to

detect and respond to a tsunami, however, there are few locations above

sea level between Hawaii and Alaska where its height and potentially dam-

aging effect can be detected. The island of Midway is the first land a tsuna-

mi wave originating in Alaska encounters on its way towards Hawaii.

Detection of a tsunami on Midway leaves only 1-2 hr for Hawaiian resi-

dents to respond.And even if a tsunami is detected prior to its arrival, pre-

dicting its flood magnitude and impact is a difficult challenge because of

its chaotic behavior when it makes landfall.

Twenty-six tsunamis with flood elevations greater than 3.3 ft (1 m)

have made landfall in the Hawaiian Islands during recorded history, and

10 of these had significant damaging effects on Oahu. This translates into

a recurrence interval of one large tsunami reaching Hawaiian shores every

7 yr and one damaging tsunami reaching Oahu every 19 yr. Since the ter-

rible tsunami of 1946, 6 large tsunamis have been recorded in the

Hawaiian Islands, and 4 have caused damage on Oahu. The three highest

tsunami wave runups recorded on Oahu occurred during the last 53 yr. If

one analyzes the last 53 yr in two time slices, 1945 to 1975 and 1976 to the

present, a very different tsunami history exists. Between 1945 and 1975, a

total of 7 large tsunamis hit the Hawaiian Islands, or one every 3.3 yr, and

a damaging tsunami hit Oahu every 6 yr. However, since 1976 not one large

tsunami has been recorded in all of Hawaii (1986 and 1994 had 2 small

events that were less than 3 ft). The historical record suggests that a dam-

aging  tsunami is overdue to reach Oahu’s shores. Ironically, it has been

during this last 20 yr that development within the Hawaiian coastal zone

has proliferated. If wave runups like those of the 1957 or 1960 tsunamis

occurred today, there exists a high probability that low-lying coastal areas

such as Waikiki, Waimanalo, Kaaawa, Punaluu, and Nanakuli would suffer

damage, primarily because of the risk that has been taken by developing

within these inundation zones.

Tsunamis

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Oahu

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