Art. IV.—On the recent Earthquakes and Wave Phenomena observed in New Zealand.
[Read before the Wellington Philosophical Society, August 25, 1868.]
One of the most important duties of the members of a local scientific Society such as this, is to obtain accurate records of phenomena of a transient character, like the disturbances of the tides and the earthquakes, by which this colony was visited between the 14th and 18th of August, 1868. I have, therefore, attempted to collect together, in the following communication, the observations which were made in different places, so far as they have been already ascertained; and although they are deficient in many respects for purposes of exact scientific enquiry, I trust that their discussion may lead to the adoption of instrumental means for recording such phenomena in future, as it is in this manner alone that sufficient accuracy can be obtained.
In the first place I will explain, in a few words, the exact nature of the recently experienced phenomena, concerning which there is a good deal of misconception.
Notwithstanding the apparent rigidity of the rocks which form the crust of the globe, they are nevertheless truly flexible and elastic; and, for the propagation of earthquake shocks, those which appear [ unclear: ] us to
be most compact and stubborn, are really the most elastic and susceptible of rapid vibratory motion.
The manner in which earthquake shocks affect the surface of the earth, and the secondary phenomena by which they are accompanied, is now well understood; but only a few of the many causes which may lead to their production are yet ascertained.
Earthquakes occur, and perhaps originate, in every part of the earth's crust, and, from the researches of Mallet, there is good reason to believe that the surface is, in some part or other, continually being subject to the jarring motion which they produce. Volcanic regions are particularly liable to them; but there they seem to be only local phenomena that fail to produce very distant effects. Volcanic energy has indeed been generally adopted as the cause of earthquakes; but applying the term to those forces by which masses of molten and chemically altered materials are heaped up on the surface of the land, or poured out beneath the ocean, it is more probable that such convulsions are not originated, but only set loose, by the passage of waves of motion through the crust of the earth, which in their origin are quite independent, of the local tension or constrained force which gives rise to the volcanic eruption.
We are rather led to look on the passage of earthwaves as the normal state of things, depending, like the ocean tides, on cosmical causes exterior to our planet. When their passage is interfered with, they become perceptible to our senses—when they interfere with, and let loose pent up forces or tensions in some portions of the earth—they lead to sudden convulsions, which, in their turn, give rise to secondary phenomena, that produce the most terrific and appalling catastrophies. These great convulsions appear to occur, nearly in every case, at the bottom of the ocean, and where it has a profound depth, at no great distance from land.
The phenomena which attend such convulsions, in the order they would appear to an observer on the shore of the neighbouring land, have been described by Mallet as follows.
First, the low hollow sound of the concussion, carried through the earth, which has been calculated to travel, on the average, at the rate of ten thousand feet per second. Following this, at very slightly inferior velocity, comes the earthwave or shock, which in its passage underneath the ocean, causes, what is termed, the forced ocean wave, which is a slight vibration communicated vertically to the water, directly over it, during its progress. It is this forced wave that causes the concussion on board ships, by which the occurrence of an earthquake is recognized by mariners.
Along the shore, the forced wave causes a gentle rise of the waters, for a short time.
If not too distant, the sound of the concussion conveyed through the water of the ocean, next reaches the observer, like the low murmuring growl of distant thunder; followed, in some cases, by the sound carried through the air at the ordinary velocity of eleven hundred and forty feet per second.
Last of all, and after a comparatively long interval, the great sea wave, caused by the mechanical displacement of the waters immediately over the seat of the disturbances, reaches the land, causing, as it approaches
the shore, a marked and sudden retirement of the waters. According to the height of the original wave and the depth of water, as it nears the shore, the wave, which may be quite unnoticed on board a ship in the offing, rises into several secondary waves, which advance with diminished velocity but increasing height, until they sweep over the low lands, far beyond the reach of the usual tides. A series of waves of oscillation following in the train of the great sea wave, varying in magnitude according to the form of the coast line, closes the series of phenomena.
The rapidity with which the ocean wave travels, depends, of course, on the depth of water, but in the open sea, it has been found in some cases to be as much as six hundred feet per second (four hundred and twenty miles per hour).
Although the slowest moving of the different impulses which originate from submarine convulsions, the oceanic waves appear to extend their influence to the greatest distance. In the case of the recent wave, this movement of the ocean was the only evidence which reached us of the occurrence of what will, I have little doubt, prove to have been a terrible convulsion in some part of the southern seas.
From careful consideration of the various accounts which have been received, it appears that the irregularity of the usual flow and ebb of the tide, was experienced along the whole of the east coast of the islands, and also in Foveaux Straits and Cook's Straits, and that it was due to the influence of three distinct oceanic waves, which reached the coast from the eastward, on the forenoon of Saturday, the 15th inst, at about the following periods:—first, between 3 to 4 a.m.; second, between 7 to 8 a.m.; third, between 10 to 11 a.m. These waves were in each case followed by smaller waves, and the irregularities did not altogether cease for forty-eight hours after their first appearance. The exact time at which the three great waves were observed, and also their distinguishing features, were modified at different points of observation, by local peculiarities due to the outline of the land, the depth of the water, the exposure of the coast line to the direction in which the wave reached the shore, and lastly, to the local time of tide.
The intervals between the smaller oscillations appear to have been generally remarked at from fifteen to thirty minutes, and to have gradually declined in extent, and frequency, until the next great wave supervened.
The earliest notice of the wave which we have recorded—beyond allusions to an extremely high tide the previous evening—was at Kaiapoi; where it was reported that at 3 a.m. the tide having ebbed for two hours, a wave four feet in height rushed up the Waimakariri river, and swept the vessels which were lying at the wharf from their moorings. This was at a distance of four miles from the mouth of the river.
At Lyttelton and Pigeon Bay, the time reported was at least half an hour later; and for the other places no exact time is reported for the occurrence of the first wave, while at several localities it appears to have escaped observation.
From this time until 7.30 o'clock, only lesser waves were remarked but about that hour a great disturbance seems to have been observed at all the stations, being described at the Bluff as a terrific rush of water. At Kaiapoi, sweeping up a line of breakers which would have been
disastrous to the town had it not passed up the south branch; and almost simultaneously, at Nelson, as having caused a reflux of the tide, at that time half-ebbed, so that it rose beyond the limits of high-water mark, and flowed into the harbour over the Boulder Bank.
A third great rush of water appears to have been everywhere distinguished from the smaller oscillations, which went on continuously, the time being variously stated from 10 to 12 o'clock, there being great irregularity in the hour reported.
In this harbour, Wellington, where I caused exact observations to be taken at frequent intervals—as might be expected from the wide expanse of water, and the narrow entrance—these waves could not be so clearly distinguished as on more exposed parts of the coast, but there is a general agreement among all the observations taken at the different stations, which leads to the above conclusions.
In the diagram (see Illustrations), it has been endeavoured to reduce to an intelligible form, the observations which were made at different parts of the coast; but from the manner in which the observations were recorded, it must not be expected that they can express the facts in a very reliable manner.
The smaller diagram shows the results of exact observations obtained in this harbour, at the end of Brown's wharf, during twenty-four hours after the phenomenon was first observed.
The altitude of each wave, as compared with its amplitude or breadth, has not been ascertained, irrespective of the degree to which it was modified by the local form of the shore upon which it expended its energy; and this element is absolutely necessary for the purpose of determining the distance at which it originated. Nevertheless, as compared with the ordinary effects of the tidal wave, we can form some conception of the gigantic force which must have influenced the ocean along the coast; when we find that the ebb and flow which these waves caused in most cases, appears to have exceeded the ordinary local rise and fall of the tide at the different localities. This leads me to expect that waves of such magnitude must have been observed at many points beyond this colony, such as the coast of Australia to the westward, and the Chatham Islands to the eastward; and that we shall receive information from these, and probably other localities, which will enable us to determine, with tolerable exactness, the focus from which they originated.
The following information appears to have escaped the notice of the Journals in the colony; it is an extract from Principles of Geology, by Sir Chas. Lyall, 10th edition, 1868, vol. 2, p. 409. “Even in the present year (November, 1867) a submarine volcano has burst out in the South Pacific, at a point 1200 geographical miles from New Zealand, and 1800 miles from Australia, between two of the most easterly islands of the Samoa or Navigator's Group, an archipelago where there had been no tradition of an eruption within the memory of man. The outburst was preceded by numerous shocks of earthquakes. Jets of mud, and dense columns of volcanic sand and stones, rising 2000 feet, and the fearful crash of masses of rock hurled upwards, and coming in collision with others which were falling, attested the great volume of ejected matter, which accumulated in the bed of the ocean, although there was no permanent protrusion of a new volcano above its level.”
An earthquake shock appears to have been felt throughout the colony a few minutes before 10 o'clock on Monday morning, the 17th inst., of a character very different from the local shocks to which we are accustomed in this place. From the appended record of telegraphic announcements, it appears to have occurred about three minutes earllier in the north-east, at Napier, than at Hokitika, on the west coast of the Middle Island. Napier is situated in lat. 39° 29′ S., long. 176° 55′ E.; Hokitika, in lat. 42° 41′ S., long. 170° 59′ E. This gives a horizontal distance of four hundred and two miles; but as we do not know whether the wave was travelling from the east or the north-east, it is impossible to infer its velocity.
The following table shows the times at which the shock is reported to have been felt in various parts of the colony.
|Hour. a.m.||Latitude, South.||Longitude, East.||Distance from Napier.||Westing from Napier.|
|Napier||9h. 55m.||39° 29′||176° 55′||—||—|
|Castle Point||9h. 56m.||40° 54′||176° 13′||106 miles||37 miles|
|Wellington||9h. 56m.||41° 17′||174° 49′||169 miles||111 miles|
|Nelson||9h. 57m.||41° 15′||173° 17′||229 miles||195 miles|
|Christchurch||9h. 57m.||43° 32′||172° 38′||367 miles||225 miles|
|Hokitika||9h. 58m.||42° 41′||170° 59′||402 miles||315 miles|
This shock was also preceded and followed by minor shocks, felt in Wellington at the times shown in the following table:
|Friday, August 14th, at 10.45 a.m.|
|" " " at 3.10 p.m.|
|Saturday, " 15th, at 3.0 a.m. tidal disturbance|
|Sunday, " 16th, at 3.15 a.m.|
|" " " at about 11.0 a.m.|
|Monday, " 17th, at 9.56 a.m. great shock|
|Tuesday, " 18th, at daylight|
|Wednesday, " 19th, at daylight|
Telegraph stations reported the shock as from the south; but for mechanical reasons, we may conclude, that their sensation would suggest the opposite of the true direction.
The periodical character of these shocks was at once evident, and they appear to be in some degree dependent on the period of maximum and minimum pressure, as indicated by the hourly fluctuations of the barometer.
In conclusion I may add, that I have attempted to state facts in a col-
lected form, deferring any fuller discussion of the matter until further details are obtained from Auckland and other places, where, no doubt, the tidal disturbances have been observed.