The tsunami which reached islands of the Samoan group at approximately 2035 hours local time, 22 May 1960, was undoubtedly one of the largest that has been recorded in the group. Generated by a large earthquake off the coast of Chile (near 41degreeS, 73^(1/2)degreeW) at 1911 hours Greenwich Mean Time (GMT), 22 May, the tsunami was responsible for widespread damage throughout the Pacific.
This paper discusses observed features of the tsunami at various locations in the three principal islands of the Samoan group, and Tables 1-3 summarize this data in approximate chronological order. Table 4 tabulates seismic data recorded at Afiamalu, Western Samoa, on 21, 22, and 25 May, GMT. Table 5 presents a brief resume of previous tsunamis recorded in the group.
The characteristics of the tsunami observed at Rarotonga in the Cook Islands are also described.
The Samoan archipelago (fig. 1) is a chain of islands approximately 300 miles long, lying between 168degree08minute and 172degree50minuteW longitude and 14degree33minute and 13degree20minuteS latitude, about 1,500 miles north-northeast of -New Zealand and 500 miles east of Fiji. The three main islands in the archipelago are those of Savaii and Upolu, Western Samoa, and Tutuila American Samoa.
In general, the individual island coastlines are bounded by reefs, of either the barrier or fringing type, although in certain areas these have been overridden by lava flows. Detailed bathymetry of the area is sparse, but the general picture (Kear and Wood, 1959) is that of one or more offshore shelves that slope gradually to depths of 50-100 fathoms. Beyond these the sea floor falls away relatively steeply down to and beyond 500 fathoms.
It is evident that the submarine features of the group, combined with the general presence of an offshore reef, have a profound effect on the behavior of tsunamis reaching the shoreline proper. At coastlines screened by reefs, the 1960 tsunami commonly manifested itself as a series of short-period surges, rather than the long period activity normally associated with such phenomena; in such cases, striking wave amplitudes were not observed.
At Fagaloa Bay (fig. 2) and Pago Pago Harbor (fig. 3), however, the normal behavior pattern was more evident; bathymetric profiles up these relatively long bays show a gradually sloping sea floor that reaches a depth of approximately 40 fathoms at the mouth of the bays. Detailed offshore data are not available for Fagaloa Bay, but it seems probable that it is entirely unscreened by reefs. Pago Pago Harbor is partially screened, directly out from the mouth of the bay.
The great-circle distance from the epicenter to Pago Pago Harbor is 5,937 miles. The travel time, calculated from the Pago Pago marigram (figure 4), is 12 hours 24 minutes, giving a mean velocity of 702 ft/ sec. This is slightly higher than the velocity calculated for the Chile-Apia path, due in part to the time lag resulting from the placement of the Apia Observatory tide gage inside the reef, and in part to a suspected clock lag at the gage.
For a uniform ocean depth on the Chile-Samoa path, the expression
gives the mean depth as 15,312 feet. As has been pointed out by Green (1946), however, the probable path is not necessarily the single great circle from the epicenter. The 22 May tsunami arrived at Samoa well ahead of the computed estimated time of arrival.
The tsunami was not observed at Rarotonga in the Cook Islands until about the time it was recorded in Samoa, although the first arrival should have been expected in the Cook Islands nearly two hours earlier than at Samoa.
TSUNAMI IN UPOLU
The first news of an impending tsunami reached Apia Observatory, Upolu, Western Samoa (fig. 5) at approximately 2100 hours local time on 22 May. It was learned that Honolulu broadcasting stations were reporting that a large earthquake in Chile had generated a tidal wave. On tuning in to Honolulu, reports were received of the preparations being made in case the tsunami reached the Hawaiian group.
Apia Radio (ZMB) was then contacted;it advised that no tidal wave alerts had been received up to that time.
At this stage, the Director of Broadcasting, who was at Apia Observatory, suggested that the local broadcasting station (2AP) remain on the air. In view of the apparent imminence of the tidal wave, it was suggested that a preliminary warning be issued, advising people in coastal areas to be prepared to move to higher ground. Special emphasis was to be placed on warning the Aleipata and Fagaloa Bay districts, in view of their location and previous experiences during tsunamis.
At 2130 hours Apia Radio advised that an official warning (230447 Z) of Honolulu Magnetic Observatory (HMO) had just been received. This gave the tsunamis estimated time of arrival at Samoa as 2200 hours, and 2AP was advised to broadcast the preliminary warning as mentioned above. The Secretary to the Government was advised of the action that had been taken.
Rarotonga Radio in the Cook Islands was then queried as to whether tidal wave activity had been experienced there. The reply was negative. It was later found that the first, though not the most serious, wave did strike Samoa at about 2050 hours, but it seems not to have been observed. No reports of tidal wave activity in Samoa had come in by 2230 hours; at that time a further message from Rarotonga stated that tidal wave activity was in progress. It was erroneously assumed that this was the first arrival in Rarotonga, and 2AP was advised that the tsunami could now be expected at about 2330 hours. However, this was the second major wave, and the one responsible for damage at Aleipata and Fagaloa Bay at 2345 hours.
At about midnight the customs wharf, Apia Harbor, was visited. The tsunami activity observed there was very prominent, displaying a wave period of about 8 minutes and a range of about 4 feet. The effects of the tsunami could also be ob-served at Mulinuu, although, as could be expected, they were much less pronounced there, as a result of the sheltering effect of the reef at low tide. The Apia tide-gage records (fig. 6) show a wave period of about 20 minutes at this stage, and a range of about 1 foot.
Shortly after 0130 hours, a further message was received from Rarotonga that stated that the wave had arrived at 2100 hours local time and caused considerable damage to small craft. No action was necessary.
At about 0230 hours, several members of the Observatory staff left Mulinuu for Malaela and Fagaloa Bay. The party arrived at Lalomanu shortly before dawn, having stopped several times on the south coast of Upolu to observe the tsunami and to gather information where possible.
At Lalomanu, where the tide was approaching full, it was heard that at about 2345 hours the previous night a large wave had picked up two fishermen in canoes near the reef and washed them onto the beach by the road. At 2345 hours, the tide here would have been dead low, and the road in this area lies about 3 feet above high-water mark. In other words, this crest had an amplitude of about 6 feet. The period is not known. It is possible that this wave was preceded by a trough, but this has not been verified.
At Malaela, as daylight approached and the tide became higher, it was seen that successive crests of the tsunami appeared inside the reef as lines, perhaps as long as a mile, that advanced across the lagoon and onto the road, and then receded in the same form. The reef in this region is inclined to the shoreline, its distance varying from 1 to 2 miles. The tsunami waves formed parallel to the reef and, hence, arrived at the shore at an angle. The period of this wave action was unusually short-about 4 minutes-and the range about 6 feet.
After this, the party left for Fagaloa Bay and arrived at about 0900 hours. The residents of Fagaloa Bay had heard the announcements on 2AP the previous night, and had also seen unusual tidal activity, starting at about 2115 hours, with an estimated range of about 6 feet and a period of approximately 10 minutes. They had, in accordance with 2APs instructions, taken refuge on higher ground.
At about 2315 hours a great recession caused the sea to retreat beyond the reef, leaving the head of the bay free of water. A few minutes later (no reliable estimate can be given) the villagers heard the roar as the sea began its advance.
The crest that followed advanced 90 yards through the village, rising to a height of approximately 8 feet above the normal tide, which was on the ebb and approaching low water. The range between crest and trough maxima was therefore estimated at 14-15 feet.
This wave and the recession that followed was responsible for all the damage at Fagaloa Bay. It was obviously the same wave that had been seen at Lalomanu. In Fagaloa Bay the peak water level reached the roof of one of the native houses; it was reported that children had been sleeping here before the warning given by 2AP. Debris was scattered about the village, including 44-gallon drums of petrol from one of the two stores. Only one of these stores suffered apparent structural damage. No lives were lost.
Photographs that were taken of Fagaloa Bay from the Fagaloa saddle at about 0830 hours, 23 May, show the waves advancing up the bay. The range of these waves is about 4 feet, their period about 3 minutes.
On returning to the Observatory, inquiries were made as to why the tsunami warnings had not been received earlier, or, in the case of HMOs first advisory message, not received at all.
The reply from the Regional Communications Officer at Nandi, Fiji, stated that Apia had been deleted from the list of addressees for whom Nandi was automatically responsible. The only reason HMOs 230447 Z was received was that the duty supervisor at Nandi had noticed a reference to Samoa in the information on estimated tsunami arrival times, and had on his own initiative relayed the message to Apia.
This was the first intimation received that Apia had, in fact, been deleted from the list of stations to which Nandi would automatically reoriginate tsunami alerts. It was obvious that the matter needed clarification, since seismo dummies (the testing procedure for tsunami alerts) had been received in the normal way at Apia as recently as April and flay, 1960. In addition, Apia furnishes seismic data and tide-gage information by cable to the U. S. Coast and Geodetic Survey, and data on tsunamis are sent to Scripps Institution of Oceanography, San Diego, Calif. It was apparent that Apia was cooperating to the fullest possible extent with these institutions.
A message was dispatched to Honolulu Magnetic Observatory, with an information copy to Nandi, requesting clarification of the system, and also requesting that Apia be reinstated on the warning list. The U. S. Coast and Geodetic Survey replied that it was prepared to do so, and this has since been done.
SECOND CHILEAN TSUNAMI
On the morning of 25 May, the Director of Broadcasting advised that Radio Australias morning news had announced the occurrence of a second earthquake or series of earthquakes in Chile, again accompanied by tidal wave activity. The Observatorys Wood-Anderson seismograph had recorded no major earthquake the previous night. Benioff seismograph records, however, showed a distant earthquake at about 90 degrees distance.
The average velocity of the previous Chilean tsunami was about 500 mph, and it was clear that this second tsunami, if generated at the origin time of approximately 0830 hours GMT, would arrive at Samoa at approximately 2100 hours GRIT; that is, 1000 hours local time, 25 May.
The Director of Broadcasting and the Secretary to the Government were advised of the situation, and 2AP announced that it was possible another tsunami would reach Samoa at about 1000 hours, if it had in fact been generated.
Pago Pago and Nandi were contacted; both stated that no warnings had been received.
Because there was so little concrete information available, 2AP was advised that the public should be warned to stay alert until 1300 hours local time.
At 1300 hours, information copies of HMOs initial warning and subsequent ETAs were relayed to Apia Radio by Nandi.
These messages confirmed the tsunami ETA for Samoa that had been calculated earlier, and it was apparent that the danger had passed for Samoa. The local broadcasting station, 2AP, was advised accordingly.
The Observatory tide-gage record was inspected, but no positive identification of the tsunami could be made because of oscillations from the first tsunami that were still in progress.
There were, however, no reports of renewed activity as a result of the second tsunami. It seemed certain that, if generated at all, it must have been much less prominent than the tsunami of 22 May.
TSUNAMI GENERATED BY FORESHOCK
It was reported (personal communication with Doak C. Cox) that a minor tsunami had been recorded at Hilo, Hawaii, some 33 hours before the main tsunami. This minor tsunami was supposedly generated by the first foreshock in Chile, at 1002 hours GMT on 21 May.
The Apia Observatory marigram showed a small-amplitude oscillation at approximately 1150 hours local time, and this motion was again apparent on the next low tide, with a period of about 20 minutes.
Float-well observations at the tide gage, however, confirmed the existence on the lagoon of an oscillation of period 15-20 minutes, which presumably resulted from a seiche existing on Apia Harbor.
It is not possible, therefore, to state conclusively that movement on the marigram at 1150 hours was tsunami-generated, although it appears more pronounced than is usually the case.
TSUNAMI IN SAVAII
Information from Savaii (fig. 7) is incomplete, due in part to the time that elapsed between the tsunami and the collection of information, and in part to the necessary use of an interpreter.
The tsunami seems to have been first observed at Falelima, on the northwest coast, at 2100 hours. The reef at the point of observation is within 2 chains of the shore.
At approximately 2345 hours, three large waves were observed with an estimated range of 8 or 9 feet (+5 to -4) and a period of about 30 minutes. These waves represented the most active phase observed at this location.
At Neiafu, one major crest was observed that attained a reported height of 7 to 8 feet. Times given are unreliable.
At Tufutafoe, two large crests were observed at approximately 2200 hours that attained a height of 6 to 7 feet, with a period of about 15 minutes. In this locality-the reef has been overridden by lava, which terminates about 50 yards from the shore.
The bay entrance at Sasina is screened by a reef. The tsunami was not observed until approximately 0500 on 23 May. The peak range here was about 5 feet, sufficiently high to come over the low-lying road at the head of the bay. The period was reported as about 30 minutes.
The tsunami was not observed at Fagamalo.
At Tuasivi the shore is completely screened by a reef about 500 yards out. The tsunami here manifested itself as short-period surges, with peak ranges estimated at 4-5 feet. This was the same sort of effect as that observed at the eastern end of Upolu, where the placement of the reef relative to the shoreline is similar.
It was impractical to obtain information at closely spaced intervals around the Savaii coastline; nevertheless it is considered that the data obtained are representative. No outstanding topographical features similar to Fagaloa Bay, Upolu, or Pago Pago Harbor, Tutuila, are present where the effects of the tsunami could reasonably be expected to have been more pronounced.
TSUNAMI IN TUTUILA
Three warnings from Honolulu Magnetic Observatory were received in American Samoa. The first was received at 1600 hours on 22 May, Samoan Standard Time (SST), and stated the earthquake origin time and epicenter. The second warning, received at 1811 hours SST, stated that a possible damaging wave would arrive at Hawaii at approximately 2400 hours Hawaiian Standard Time.
The third warning, received at 2008 SST, gave the estimated wave arrival time at Tutuila as 2200 hours SST. On the final warning, public gathering places were closed and people advised to return to their homes. A very few evacuated to higher ground.
The Pago Pago marigram shows the first wave as a crest arriving at approximately 2035 hours (figure 4). The subsequent recession was the first stage at which visual observations took place. Observers noted that there were approximately 10 wave crests and troughs, of which the third and fifth were considered to be the largest. The wave period was estimated as approximately 20 minutes.
At the Administration boatshed, some 800 yards farther up the bay from the tide gage, the peak amplitude was 6-7 feet above the normal tide, which at thattime was on the ebb and approaching low water. The wave reaching this peak was observed shortly before 2200 hours, and can probably be correlated with the crest recorded on the marigram at 2145 hours. The subsequent trough attained a maximum of approximately 4-5 feet, giving a range of approximately 10-12 feet.
A half mile farther up the bay from the boatshed, the wave action was observed by Mr. Morse. He reports that five waves came over the top of the retaining wall behind his house, and of these the fourth was the largest. This group of five large waves is readily identifiable on the marigram. Maximum range at this location was estimated at 8-10 feet, crest to trough. A photograph taken at the Rainmaker Hotel Annex in this area is shown as figure 8.
At Tafuna airport, Mr. Elmer Wilson reported that a loud rumble was heard at about 2140 hours, the duration being approximately 1 minute. It is considered that this was a manifestation of the crest observed at the boatshed.
No tidal disturbance was noticed at Tafuna, which is screened by an offshore reef, despite the fact that observers equipped with spotlights were keeping a watch for any unusual activity.
At Pago Pago village, which is located at the extreme west end of the harbor, the tsunami reached its greatest proportions in Samoa. The peak range here was 15.5 feet (+9.5 to -6.0), and damage estimated at $50,000 resulted. Mr. E. Brunton, of the Survey Department, made the following analysis of the damage:
"One house was lifted and moved about 10 feet inland and another was washed into the bay by the outgoing wave. A school, substantially constructed on concrete piers, was rotated about a foot with consequent springing of nearly all structural members." Figures 9 and 10 are photographs of this area, taken at approximately 1000 hours on 23 May.
At Fagaalu, located immediately inside the harbor entrance, the sea rose no more than 2.5 feet. A small channel runs through the reef off Fagaalu. No reports of activity could be obtained from coastal villages, and it seems evident that Pago Pago Harbor was the only location where the tsunami was observed. It appears that the effect around the Tutuila coast was much less pronounced than it was on the coasts of Upolu and Savaii, although rough water was observed inside the lagoon at the extreme eastern end of Tutuila on the day following arrival of the tsunami.
TSUNAMI IN THE COOK ISLANDS
The tsunami was of little consequence in the Cook Islands except in harbors like Avarua and Avatiu on Rarotonga that represent reef openings with shelving bottom. No information was received from atolls of the Northern Cook group, such as Penrhyn and Suwarrow; but it is likely that energy penetrating reef passages would he dissipated in the large and deep lagoons.
According to Mr. G. Russell, Meteorological Officer at Rarotonga (in an informal communication), the first oscillation of the tsunami was observed at about 0745 hours GMT on 23 May; the amplitude increased to a peak at about 0930 hours; then noticeably decreased for at least 50 minutes. Later it built up again quickly to a second peak at about 1100 hours. There was some damage to yachts in the harbor during this second peak. The period of the oscillation was close to 4 minutes and the maximum height was estimated at 5 feet. Fortunately the arrival of the tsunami coincided with low tide, when in fact the sea level was below the level of the coral reefs.
In the Samoa Islands, the tsunami of 22 May was most pronounced at the heads of Fagaloa Bay and Pago Pago Harbor, where the maximum ranges were 15-16 feet. In the Cook Islands, the maximum ranges were at Avarua and Avatiu harbors, where they amounted to about 5 feet. A tsunami even of these proportions constitutes a considerable menace to human life and emphasizes the need for the maintenance of rapid communications in the sea-wave warning network. Reception of warnings well in advance of suspected sea-wave activity will also permit the implementation of observational programs of greater accuracy than have been possible in the past.
The author is indebted to F. Uhrle and E. Brunton, of the American Samoa Administration, for their assistance in supplying information on the tsunami effects in Tutuila and to F. Morse, for supplying photographs of the tsunami in that area; to J. Franklin and C. Capper, of Apia, for their help in recording effects in Upolu and Savaii; and to G. F. Russell, of Rarotonga, for supplying information relating to the effects on that island. The author wishes to thank Doak C. Cox, of the Hawaii Institute of Geophysics, for helpful discussion and advice.
Green, C. K.
1946. 「Seismic Sea Wave of April 1, 1946, as Recorded on Tide Gages」, Trans. Am.
Geophys. Union, Vol. 27, No. 4: 490-500.
Heck, N. H.
1941. 「List of Seismic Sea Waves」, Bull. Seis. Soc. Am., Vol. 37, No. 4: 209-286.
Kear, D., and B. L. Wood
1959. 「The Geology and Hydrology of Western Samoa」, New Zealand Geol. Bull. n.s. 63:
Symons, J. M., and B. D. Zetler
1960. The Tsunami of May 22. 1960 as Recorded at Tide Stations, U. S. Coast and Geodetic
Survey, Preliminary Report.
OBSERVER IN CHARGE, APIA OBSERVATORY
UPOLU, WESTERN SAMOA
DEPARTMENT OF SCIENTIFIC AND INDUSTRIAL RESEARCH
Manuscript received on January 9, 1963.