Will You Know Before A Tidal Wave Hits You?

Tidal Wave Hits You?

From J. P. Shortall, in Suva Fiji now is included in a Pacific wide network of tidal wave warning stations. It’s part of a system originally set up in Honolulu, by the US Coast and Geodetic Survey after a lot of eastern Pacific islands had been bashed about a bit by tidal waves after the war.

But how does this warning operate and how effective is it? Is the addition of Suva in the network of any significance?

FOR an answer, I went to Dr. N.

J. Guest, chief geologist of Fiji Geological Survey Department and to his assistant, Mr. Robert Houtz, who is the man most directly concerned with the warn ing system in Fiji.

Just as a navigator can only obtain an accurate fix of his position by the observation of several objects at the same time, so it is necessary for the Honolulu co-ordination centre to have a number of separate observation re ports of an earthquake before it can calculate the exact spot where it was centred.

These reports are obtained from such places as Suva. There are also seismographs at Noumea, Papeete, Apia and Port Moresby.

The Suva seismograph is located at the Laucala Bay weather station.

Many stations also have tide gauges, some with an alarm system, which gives an alert when any un usual variations in the sea level occur.

IGY Helped

The International Geophysical Year was instrumental in getting many seismographs and wave recorders into the Pacific Islands.

Some stations are able to report immediately to Honolulu when there are abnormalities, but in other places the instruments are unattended and the information is recorded on a strip of paper and inspected at intervals of a month or more.

The recordings of some of the seismographs—like the one at Laucala Bay have to be “de veloped” like a film before the in formation can be inspected.

The present Suva seismograph recordings are normally only in spected at twice weekly intervals.

However, a major earthquake any where will become known immedi ately in Honolulu, and at other stations where the recordings are constantly visible.

If a check is required from Fiji a message goes out by radio tele type from Honolulu to Nadi Aeradio station. It is passed to Laucala Bay by teleprinter, and by telephone to Mr. Houtz, who smartly heads for the seismograph, located about two miles from his office. The film is processed and examined, the information coded, and the reply to Honolulu despatched by the same route.

Time Check

The warning centre in Honolulu has made a check of the time it takes to receive a reply from various stations. In a test with Suva some time ago this was found to be 20 minutes. To that must be added the time taken to travel from office or home to the seismo graph, to change and process the recording paper, and to code the message. Provided there are no “technical hitches” this can be done in 30 minutes.

Since the Suva station joined the warning network there have been

Tidal Waves Start In The North

three requests for information. On these occasions slight delays in creased the time beyond the possible minimum—but these de lays would not have affected the warning so far as Fiji and South Pacific areas were concerned. And as a result of experience gained they are unlikely to happen again.

The fact is that most of the South Pacific’s dangerous sea waves, or tsunamis—as they have been named after a Japanese re searcher — originate in the far North Pacific along the arc of submarine volcanoes extending from Alaska through the Aleutians to Japan, or from other earth faults in that region.

The earthquake shock waves travel at roughtly five miles per second, but the sea wave—if any— travels at only about one-eighth of a mile per second.

Thus a severe earthquake off the Alaskan coast will be recorded in Suva about 11 minutes later, but the sea wave will arive ten hours later.

The exact speed of travel of the sea wave will arrive ten hours later, of ocean intervening, the topography of the bottom, and other factors, which also govern the possibility of damage along any particular coast-line.

Not Much Time For Suva The Suva station estimates that there would not be much time for any effective action to be taken by the population of a threatened area of Fiji if the wave originated at a point closer than about 1,500 miles—or 31 hours —away.

It is one thing receiving the warning in Suva, and another thing having it passed to the popu lation—especially if the broad casting stations are not on the air at the time. But, as mentioned, most such waves originate much farther away and there is ample time for warning. The situation is different in the North Pacific, closer to the primary danger area. Time for a warning is limited and com munications must be rapid.

However, this does not mean that South Pacific stations such as Suva serve no useful purpose, even though the wave might be dis sinated by the time it comes south.

The earthquake report from Suva, if received quickly in Honolulu, is a valuable aid in fixing the exact centre of the disturbance and in forecasting the time of arrival of the resulting sea wave which could follow. The Suva station will m fact probably serve distant places more than it will serve Fiji.

The warning network at present does not give warning to Fiji of sea waves approaching from the south-west, but some day there may be another co-ordinating

centre in Australia or New Zealand to cover the southern area more fully. Even from this direction Fiji has some protection in its extensive barrier of off-shore reefs which would take the first shock. Unless the source of the sea wave is close inshore—like the 1953 earthquake and tidal wave in the Suva area.

"Cried Wolf"

It’s also worth bearing in mind that residents of most of the main South Pacific centres which have good radio communications with the outside world are unlikely to be taken unawares by any sea wave from the north. Apart from some evidence that a sea wave did once strike the Yasawa Group of western Fiii from an earthquake in the New Hebrides, the records in dicate that no such waves have originated from other than the far North Pacific.

There has been some criticism in Suva that Honolulu has “cried wolf” on a couple of occasions when no sea wave has eventuated in Fiji.

The same criticism has been

directed at the meteorological office in regard to certain hurricane warnings—but, against that, others point out that it is better for a warning of possible danger to be issued rather than no warning a all.