LATE PHENOMENON

Rotorua Morning Post, Volume 2, Issue 464, 23 February 1933, Page 4

LATE PHENOMENON

TAUPO OCCURRENCE OBSERVED IN DISTRICT ■ LAKES ON VARIOUS OCCASIONS. SUBJECT OF RESEARCH. (Specially written for "Post" by Ralph H. Ward.) The periodical rise and fall of the i level of Lake Taupo that occasioned widespread interest a few days ago is not an uncommon phenomenon in fresh-water lakes. It has been observed at Taupo on various occasions during the past thirty years, and also on Lake Tarawera during the same period. This fluctuation of level is termed, in geological literature, a "seiche." It was first discovered in 1730 by a Swiss engineer, Fatio de Duiller, and the phenomena were first systematically studied hy Professor Forel in Geneva. During the year 1922, following the earthquakes experienced at- Taupo from May onward for several months, I observed seiches there on several occasions at several different localities, notably near the Waikato outlet, in Acacia Bay, and in Whakaipo Bay. The phenomenon, as then observed, oeeurred when the lake was palm and in weather conditions which made it certain that cairn conditions prevailed over the whole area of the lake. On several occasions I measured the amount of rise, and corresponding fall, of level, and also timed the movement. On one occasion a rise and fall both oecupied twehty minutes with a four minute interval of rest at the top of the rise, this j rhythmical movement continuing during an hour and a half's observation and being still in progress when I left the loeality. Observed on Tarawera. The late Captain Gfibert Mair, N.Z.C., informed me of the occurrence of similar fluctuations on Lake Tarawera in the period immediately following the Tarawera eruption of 1886, some of these seiches causing a rise of several f eet in the lake level. Other observers have informed me of similar occurrences on Tarawera down to recent. times. Aecording to Professor Forcl there. are both longitudinal and transverse seiches, and a curious feature is that the effect is often much greater at • one end of a lake than the other. At Geneva, the effect of longitudinal seiches is four times -as great as at Chillon at the other end. The maximum height of a recorded seiche at Lake Geneva was over six feet, in October 1841. Although seiches are not unusual occurrences at Taupo, and probably at other lakes in the district, they are generally inconspicuous and might, in most cases, easily escape the observation of the majority of residents. Volcanic Vents. In localities such as Tarawera, close to a recently active, volcanic vent, it is possible that seiches might be due to slow and fairly extensive movements of ground level. Instrumental observations, such as those of the Volcano Observatory at Kilauea, Hawaiian Islands, show that such movements of level do occur in the vicinity of volcanic vents. In the case of Lake Taupo it is perhaps significant that extensive movements of depression of the Lake bed and adjacent shores have taken place in rei cent geological times, and that such movements were observed in progress as recently as 1922. In that year the j greater part of the lake shore from the eastern side of Whakaipo Bay j and the western side of Whangamata Bay was permanently depressed by amounts ranging from four to at least ten feet. In such an area the production of fluctuations of lake level by earth movements is possible, though such a cause has not been j established, and it must he admitted i that explanations such as lunar ] attraction and earthquakes have been j shown to be untenable as a general cause of seiches. They are probably due to several factors acting together J or separately, some that have. been j suggested by seientific investigators j being sudden variation of atmospheric ! pressure and change in strength or j djrection of wind. The occurrence of j seiches, in any case, is not to be j taken as neeessai'ily, even in a volcanic area, indicating cause for alarm.