THE TSUNAMI OF 23 MAY 1960 IN THE HAWAIIAN ISLANDS

ABSTRACT

The tsunami of 23 May 1960 was detected in Hilo Bay at 0007 hours HST and recorded on the Honolulu tide gauge at 0033 HST. Runup heights ranged front 2 to 17 feet except in Hilo where the wave formed a bore and reached 35 feet. Sixty-one people were killed and 282 injured in spite of elaborate warning 5 hours in advance of the arrival of the tsunami. Damage and effects are discussed for all the Hawaiian Islands and recommendations are made to prevent future catastrophes. A brief history of Hawaiian tsunamis is given.

INTRODUCTION

A tsunami was generated off the coast of Chile as an accompaniment of the major earthquake of 22 May 1960. This report describes the nature and effects of the tsunami as it washed the shores of the Hawaiian Islands early the next morning, and compares them with those of previous tsunamis.
The Hawaiian Islands form an isolated archipelago running northwest-southeast over a distance of 1600 miles in the central Pacific Ocean (figure 1). Deep waters surround the islands, and great ocean deeps lie between the general floor of the ocean basin around the Hawaiian Islands and the continental margins of the Pacific basin. The margin of the basin, with but few gaps, is seismically active from New Zealand clockwise to the western coast of South America. No large land masses lie between Hawaii and this seismic rim, and only a few, small, scattered islands lie to the northwest or clockwise to the southeast of Hawaii. Hence, the Hawaiian chain is vulnerable to tsunamis that originate from earth movements along any part of the rim, perhaps particularly the northern and eastern parts.
The first historical record of a tsunami in the Hawaiian Islands was made in 1819, 41 years after the discovery of the islands by Captain Cook. Since 1819, 30 readily identifiable tsunamis have struck the islands (Green, 1946; Macdonald and others, 1947; Shepard and others, 1950; Zerbe, 1953; Macdonald and Wentworth, 1954; Eraser and others, 1959; Eaton and others, 1961), 15 of which have resulted in significant damage. More than half the waves have been derived from faulting in the Kuriles-Kamchatka-Aleutian region north and northwest of the archipelago, and about one-fourth have originated along the South American coast. The origin of three is unknown.
The preceding table summarizes the tsunamis reported since 1819, showing their region of origin and the relative damage caused in the Hawaiian Islands.
This table, except for the last three tsunamis, is adapted from Macdonald and others (1947) and shows that most of the severely damaging tsunamis have come from the southeast quadrant. In order of destructiveness, the zones of origin are (1) southeast quadrant, in which the coast of South America is the principal source; (2) northwest quadrant, in which the Kamchatka-Aleutian region is the chief source; (3) within the Hawaiian chain, probably restricted in origin to the coasts of the island of Hawaii; (4) southwest quadrant, from which only two tsunamis originated, one of which resulted in a small amount of damage and the other in none; and (5) northeast quadrant, from which no damaging tsunamis have been reported.
The disastrous tsunami of 1946, which came from the Aleutian Islands, caused 159 deaths and led to a study of tsunami phenomena by a number of observers in the Hawaiian Islands (Shepard and others, 1950). This tsunami was the first in 69 years to cause severe damage, although moderate damage had been caused by a tsunami in 1923. Since 1946, three additional damaging tsunamis have struck the islands-one from Kamchatka in 1952 and one from the Aleutians in 1957, which caused moderate damage but no deaths; and the May 1960 tsunami from Chile that caused severe damage and 61 deaths.
The general features of tsunamis are well known and have been discussed extensively in the literature. Observations on the Hawaiian tsunamis of the last 15 years indicate that they have average recorded periods of 15 to 40 minutes and travel across the open sea at average speeds of 400 to 500 mph. The maximum recorded height to which the waves have washed the shores of the islands, 55 feet, occurred during the 1946 tsunami. The following table lists some characteristics of the four damaging tsunamis that have struck Hawaii since 1946 and of the earthquakes that were associated with them. The wave heights reported in the table and elsewhere are the runup heights on island shores above mean lower low water, which is about 1 foot below mean sea level.

HAWAIIAN RECORDS OF THE CHILEAN EARTHQUAKES OF MAY 1960

The major Chilean earthquake of 22 May 1960, and its associated foreshocks and aftershocks were recorded by several seismographs in Hawaii. These seismographs are maintained on the island of Oahu by the Honolulu Magnetic Observatory, U.S. Coast and Geodetic Survey and on the island of Hawaii by the Hawaiian Volcano Observatory, U.S. Geological Survey. The shocks also were recorded by water-level oscillations in numerous artesian wells on Oahu (figures 2-4). These wells penetrate the confined portions of aquifers composed of basalt lava flows of the Koolau volcanic series. The range in oscillation in some of the wells exceeded 0.5 foot. Eaton and Takasaki (1959) have suggested, from a study of similar oscillations accompanying previous earthquakes, that the oscillations result from alternate compression and dilation of the aquifer by the passage of the Rayleigh waves of the earthquakes.
The major Chilean earthquake of 1911 hours GMT, 22 May 1960, was accompanied by a T-phase of unusual duration and intensity, as recorded by seismographs on the southeast coast of the island of Hawaii. According to Eaton and others (1961), the amplitude increased over an interval of 5 minutes after the beginning of the record, declined slightly, increased to a second maximum at 7 minutes, and then decreased rapidly.
The first large foreshock, that of 1003 hours GMT on 21 May, magnitude 7(1/2), was accompanied by a small tsunami detectable in Hilo Bay, Hawaii (Eaton and others, 1961).
The main shock, of magnitude 8(1/2), tripped an alarm attached to a seismograph at the Honolulu Magnetic Observatory, U. S. Coast and Geodetic Survey, at 1938 hours GMT, 22 May (0938 hours Hawaiian Standard Time (HST)). The Observatory personnel commenced an investigation leading to a public advisory bulletin on the possible generation of a tsunami at 1159 hours HST and an official warning of the approaching tsunami at 1847 hours HST (Symons and Zetler, 1960).

The 23 May Tsunami

Arrival times and velocities: The tsunami associated with the major earthquake shock was detected in Hilo Bay at 0007 hours HST (Eaton and others, 1961) and recorded on the Honolulu tide gage at 0033 hours HST (Symons and Zetler, 1960), only 7 minutes and 3 minutes later, respectively, than times of arrival as they were first estimated by the Honolulu Magnetic Observatory. If it is assumed that the tsunami was generated at the time of the principal shock, its travel time was 14.9 hours to Hilo and 15.4 hours to Honolulu. These times indicate average velocities between 440 and 445 miles per hour over the respective 6600- and 6800-mile paths, if the tsunami was generated at the epicenter on the Chile coast. These velocities are consistent with those to be expected on the basis of the depths of the ocean between Chile and Hawaii.
Period and relative height of successive waves: As is usual with tsunamis, the period of the May 1960 tsunami varied from place to place and with time at each place. Figures 5-8 (taken from Symons and Zetler, 1960) show tide gage marigrams for the initial tsunami activity at locations in the Hawaiian archipelago and elsewhere in the Central Pacific. Figures 9-11 are plots of observers records of water levels reached in Hilo Bay, Hawaii; _Maalaea Bay, Maui and Hanapepe Bay, Kauai. As usual, the earliest part of each record appears the simplest, and the later parts become complicated by the interference of waves following different paths and by the resonance of the immediate coastal area. The following table shows the intervals between the first and second crests on tide gage records, as a parameter of the simplest motion, tabulated by the U.S. Coast and Geodetic Survey (Symons and Zetler, 1960). These values are supplemented in the table by values obtained by observers at several additional points on Hawaii shorelines.
The fundamental period of tsunamis is a point of considerable disagreement. Munk and Snodgrass (1960) have demonstrated the dominant importance of local resonance in records with durations of 2 days. Van Dorn (1959) believes that the fundamental period is commonly less than 10 minutes. Others consider that the longer periods recorded, sometimes equaling or exceeding 2 hours, are the fundamental ones. The motion in the ocean could quite possibly be better described by a spectrum of some complexity than by a single dominant period.
In all the Hawaiian records of the 1960 tsunami the first significant feature is a crest, although an earlier slight trough is suggested on some records. The highest crest and maximum range were associated generally with the third or fourth wave, although the first wave was the largest in the long-period record from Mokuoloe Island in Kaneohe Bay, Oahu.
Considerable wave action persisted for many days after the first arrival of the tsunami. In general the later amplitudes did not approach those of the first waves, but a wave reaching 8 feet was observed by members of a U. S. Geological Survey team on the northeast coast of Oahu 14 hours after the first wave arrived. The continuous wave action was responsible for a second official tsunami warning about noon on 25 May, following one of the aftershocks of the main earthquake in Chile.
In general the wave action along Hawaiian shores was quiet, resembling that of the tide, although it had a shorter period and a greater range. Superimposed on the tsunami crests, the ordinary swell and chop attacked parts of the shorelines above their normal reach; otherwise horizontal velocities generally became significant only where appreciable areas were flooded. In Hilo Bay, however, the third wave was converted into a bore. Tremors recorded by a seismograph in Hilo and observations by Eaton and others (1961) indicate that the breaking occurred well out in the bay. They estimate that the velocity of the bore was between 40 and 45 fps.
Runup heights reached by the tsunami: The heights to which the waves of a tsunami will reach are governed by a complex of processes that is controlled by the submarine topography, the shoreline configuration, and the roughness of the bottom in shallow water and of the land surface in the area inundated. The heights to which the tsunami waves would reach themselves can be augmented by the heights of the superimposed wind-generated waves. As indices of the magnitude of the tsunami in the ocean, runup heights are of only qualitative value. However, they constitute direct indices of the shoreline zones subject to damage. Runup heights were measured around the shorelines of most of the islands in 1960, as they had been in 1946, 1952, and 1957, to obtain a qualitative indication of the energy distribution in the waves at sea and a record of coastal zones affected.
The runup heights, shown in figures 12-15, were measured to swash marks; lines of debris; limits of salt-killed vegetation; trash left hanging from trees, fences, etc.; and limits of abrasion and staining on trees and buildings. Most of the heights were measured to the land edge of the inundated area, but a few represent the maximum height reached by the waves at some point between the shore and the land edge, from which the water levels declined inland. Measurements were made from tide level by hand leveling or horizon sighting, using a steel tape or assistants stature, and the heights were later corrected to mean lower low water. In some areas the density of measurements was too great for plotting, and only representative values are shown.
Except in the vicinity of Hilo, the rump heights in the 1960 tsunami in general ranged from 2 to 17 feet, considerably lower than those of the 1946 Aleutian tsunami but comparable with those of the 1952 Kamchatka and the 1957 Aleutian tsunamis. The 1960 heights were relatively uniform, particularly in comparison with the 1946 heights. The heights averaged about 9 feet on Hawaii, 10 feet on Maui, 7 feet onOahu, and S feet on Kauai. There was no pronounced directional effect; in fact, on Oahu, the coastline with the greatest average runup height was that facing north-west, directly away from the waves coming from Chile. The contrast jn heights and their variability on the island of Oahu may be seen from the runup values for the 1946, 1952, 1957, and 1960 tsunamis shown in figure 16.
In the vicinity of Hilo, where lower than average heights had been experienced in 1946, the 1960 heights were greater than anywhere else in the islands, reaching a maximum of 35 feet near the waterfront south of the end of the breakwater. The maximum height at the limit of inundation was about 20 feet. Runup heights andinundation areas for the 1946 and 1960 tsunamis are contrasted for the Hilo area in figure 17. The pattern of 1960 heights suggests the possibility of interference between the incoming bore of the third wave and a wave reflected from the east end of the bay. The great runup heights experienced at Hilo in 1960 seem the more remarkable because Hilo is sheltered by Cape Kumukahi (fig. 12) from the direct approach of waves from Chile.
Casualties and damage: The tsunami caused 61 deaths by drowning or crushing, and 282 persons sustained injuries requiring medication or hospitalization, all in the city of Hilo. All this was in spite of the issuance of an official warning by the Honolulu Magnetic Observatory more than 5 hours before the arrival of the tsunami and the public dissemination of the warning by the Civil Defense Agencies and police via radio, television, fixed and mobile sirens, public address systems, and wardens. A study of the behavior of the residents of the inundated area, made under the auspices of the Hawaiian Academy of Science (Bonk and others, 1960), revealed that only a third of the people evacuated to high ground. The reasons for the failure of the warning to have the desired effect may be summarized in general by the statement that the information reaching the people was not sufficiently clear and precise to give them, with their previous understanding of the nature of the risk, a definite sense of appropriate response or an adequate feeling of urgency.
By far the major part of the physical damage caused by the tsunami also occurred in Hilo. Yearly 600 acres inland of the harbor were inundated, some of the area extending to the 20-foot contour above sea level. In nearly half of this area the destruction of buildings was almost complete (Eaton and others, 1961). In the area of maximum destruction, only buildings of reinforced concrete or structural steel, and a few others sheltered by these buildings, remained standing-and even these were generally gutted. Frame buildings either were crushed or floated away nearly to the limits of flooding. Dozens of automobiles were wrecked; all 11-ton tractor in a showroom was swept away; heavy machinery, mill rollers, and metal stocks were strewn about. Rocks weighing as much as 22 tons were plucked from a sea wall and carried as far as 600 feet inland. The nature of the destruction is illustrated by several photographs (figures 18-21).
Damage elsewhere on the island of Hawaii was restricted to the west and southeast coasts, where about a dozen buildings, mostly of frame construction, were floated off their foundations, crushed, or flooded.
On Maui the damage was concentrated in the Kahului area on the north coast.Although Kahului Harbor is protected by two breakwaters with a narrow entrance between them, the water rose on the shores of the harbor to 12 feet above sea level and swept back more than 2500 feet from the shore into the town. A warehouse and half a dozen houses were demolished, and other warehouses, stores, offices, and houses, and their contents were damaged. Other buildings were damaged at Paukukalo, just outside and west of the harbor. At Spreckelsville and Paia, east of Kahului, houses were damaged and one house at each place demolished. Additional damage occurred at Kihei on the south coast and at Lahaina on the west coast.
On the island of Molokai there was some damage to houses, fish ponds, and roads, and on the island of Lanai a beachhouse was demolished.
The islands of Kauai and Oahu escaped with only minor damage. Houses at Kuliouou, an eastern suburb of Honolulu, were flooded. Elsewhere on Oahu no damage was reported, even where there was inundation of areas occupied by houses. On Kauai, so far as is known, the only damage consisted of one frame building being floated off its foundation on the south coast.

PREPARATIVE MEASURES FOR FUTURE TSUNAMIS

The casualties in Hilo during the May 23 tsunami indicate that the tsunami warning system, despite its technical success, did not function satisfactorily in its primary purpose of inducing people to leave areas where risk is high when a tsunami is approaching. A number of improvements are planned. The U. S. Coast and Geodetic Survey has taken steps to increase the number of tide stations reporting to the Honolulu Magnetic Observatory. The Civil Defense communications network is being improved. A tighter control of the information broadcast by radio and television stations has been assured and formal evacuation zones are being outlined. A program of research on tsunamis has been established at the Hawaii Institute of Geophysics. Most importantly, however, an understanding by the public of the nature of and risks from tsunamis must be created and maintained through continuing education.

ACKNOWLEDGMENTS

Besides the published material cited, observations by many individuals have been used in this paper. In particular, acknowledgments are due C. H. Marsh, Richard Taylor, Joseph Swezey, R. De Mello, M. Vierra, Michio Okuda, Willard Nagata, Dan Pokipala, J. R. Porteus, and R. P. Bruce, who made or arranged for observations on Maui; Frank Fisher, Christe Zones, Ronald Lubke, and K. J. Takasaki, who contributed to the observations on Oahu; and Frank Morris, Noriaki Kojiri, and Larry Nishikawa, who contributed to the observations on Kauai. Jerry P. Eaton, Don H. Richter, and Wayne U. Ault are thanked for the use of their observations on Hawaii in advance of their publication.

REFERENCES

Bonk, W. J., and others
1960. 　　「Human Behavior During the Tsunami of May 1960」, Science, 133: 1405-1409.
Eaton, J. P., and others
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Eaton, J. P., and K. J. Takasaki
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Contribution No. 20 of the Hawaii Institute of Geophysics.
Prepared in cooperation with the U. S. Geological Survey, and published with the authorization of the Director.DOAK C. Cox GEOPHYSICIST HAWAII INSTITUTE OF GEOPHYSICS
JOHN F. MINK HYDROLOGIST-GEOLOGIST
HONOLULU BOARD OF WATER SUPPLY
(FORMERLY GEOLOGIST, U. S. GEOLOGICAL SURVEY)
Manuscript received on January 9, 1963.