Art. XXII.—On the recent Earthquakes on Land and Sea.
[Read before the Philosophical Institute of Canterbury, September 9, 1868.]
Before proceeding to make a few remarks upon the contents of the valuable paper of Captain Gibson, to the reading of which we have just listened, I think that some general observations on earthquakes, as experienced on land and sea, their causes, and effects, would not be here out of place.
I should also like, with your consent, to test by the observations which we were able to make in New Zealand, some of the theories by which the origin and propagation of these most formidable and greatest phenomena of nature have been explained.
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The first sign of disturbance experienced in Christchurch was a slight shock of an earthquake, felt by several of our fellow-citizens, in the early morning of Saturday, the 15th of August, amongst whom, Mr. A. T. W. Bradwell gave me, the same day, the best account. It was about 3 o'clock in the morning that he felt a slight shock of an earthquake, travelling apparently from S.W. to N.E., accompanied by a slight subterranean rumbling sound. As buildings move generally in the direction from which the vibratory movement reaches them, it is highly probable that this earthquake came from the N.E., from which direction the earthquake waves in the sea, appeared also afterwards on our coasts.
There is no doubt in my mind that we can associate the earthquake waves, in the sea at least, with the minor shocks experienced on the land. Unfortunately, we do not yet possess the necessary data to calculate the velocity of these minor shocks, which are without doubt the last pulsations of a very severe volcanic earthquake, the focus of which is situated in a N.N.E. or N.E. direction from New Zealand.
And I may observe here, that a volcanic central or linear earthquake may be very severe at or near its focus, although its effects are confined to a comparatively limited area round about it. Thus the slight earthquake shock, experienced in the early morning of the 15th of August, might have been of a very local character only, although the disturbance on the sea bottom, near its focus, was so enormous that the effects were felt, as far as we know already, on the coasts of New Zealand, Australia, and the Chatham Islands. And whilst the disturbance in the level of
the sea, from the impetus given, was such that it was felt over a tract of country several thousand miles in diameter, the oscillations of the earth's crust may have been confined to as many hundred miles only. Supposing that the Chatham Islands are situated near the centre where the disturbance of the sea level originated, and the coast of South Australia as one of the most westerly points where it was perceptible, and allowing it the same power, which, without doubt it possessed, to move as far in an easterly direction, we shall then find that its effects were felt over more than 90 degrees of longitude, or a fourth of the circumference of the globe.
From the valuable communication of Capt. Gibson, we obtain reliable information of what took place in Lyttelton harbour. It appears that four distinct waves, of which the second was the smallest, entered that harbour:
|Hour. A.M.||Intervals. H.M.|
|The first and highest wave reached the Lyttelton wharf at||4.30||—|
|The second and smallest||7.15||2.45|
|The fourth and last||11.0||1.30|
after which only, minor disturbances took place. It will also be seen from this list, that the difference in time became less as these waves succeeded each other.
The next bay, of which we possess reliable accounts, is Pigeon Bay, in which Mr. G. H. Holmes has recorded the principal facts.
Here, the first rush of water entered about 4 o'clock, but no exact time was observed, the overseer of Messrs. Holmes and Co., only hearing the rumbling noise as if of rushing water. Therefore, we may fix it at the same time it arrived in Lyttelton, 4.30 a.m., rising 4 feet above spring tides. The second wave was observed at 7.15 a.m., giving an interval of 2 h. 45 m.; the rise of water above spring tides was 1 foot. The third rush came at 9.45 a.m., giving an interval of 2 h, 30 m.; the rise of water above spring tides was 4 feet. So far the observations agree with those of Lyttelton harbour, but instead of now only showing minor oscillations, the water continued its rush into that smaller bay with even greater velocity. Thus, about mid-day, a fifth wave came with still more force and velocity, rising 5 feet. The sixth and highest arrived forty minutes later; at 12.40 p.m., rising 7 feet above spring tides. And lastly, the seventh, at 1.5 p.m., after which, the waves diminished considerably, arriving at intervals of three-quarters of an hour. These facts are corroborated by Mr. Robert Townsend, District Surveyor, who happened to be in Pigeon Bay at the time, and who measured the vertical height of the water above spring tides.
Advancing towards the south-east, along the Peninsula, we reach Okain Bay, where the earthquake waves were also high and destructive. I owe the information which I possess about that locality to Mr. George Bishop, who resides there. This gentleman did not observe the early waves, but obtained the information from several of the inhabitants living close to the sea shore.
The first wave came alout 3 o'clock, but there is no certainty about the exact time. It was followed by three others, with intervals of about
a quarter of an hour between them, and of which the last was the highest.
A fifth wave came about 8 o'clock, but it was not so high, nor was the rush of water so violent, as of those which entered the bay afterwards. Between 10 and 12 o'clock in the forenoon, another succession of waves, at intervals of fifteen to thirty minutes, were experienced. They were irregular, but were quite as high as those in the early morning.
A very high wave rushed in about 2 o'clock in the afternoon, which Mr. Bishop considers to be the most formidable and highest of all, and which rose 6 feet above the highest spring tides; the altitude of the Government bridges above high-water mark offering the necessary data for that assertion. My informant considers this wave 2 to 3 feet higher than any of the previous ones. All the succeeding waves, which entered the bay, were smaller, and continued to flow in until Tuesday afternoon, when the tides took their regular course. It appears, therefore, assuming the time kept in Pigeon and Okain Bay to be the same, that, in the latter locality, the highest and most destructive wave arrived three-quarters of an hour after it had been observed in Pigeon Bay.
Not having, as yet, authentic accounts from the other bays of Banks' Peninsula, I may only observe that the highest rise of the water in Akaroa was towards 12 o'clock, midday on Saturday, the 15th August, but that it did not enter the harbour in the form of a high wave, and, altogether, did not occasion so much damage as in the smaller and more exposed bays.
The position of Akaroa harbour, opening to the south, being narrow at its entrance, and the water being deeper than in the more northern bays, may easily account for this. Before leaving Banks' Peninsula, I may observe, that the principal cause of the earthquake waves being there so much higher than on other portions of the New Zealand coast, may be sought in the form and shallowness of the sea bottom around it. For a long distance easterly the sea is comparatively so shallow, that the 50 fathom line lies more than 40 miles from the land.
The 100 fathom line, beginning south of the Kaikoras, close in shore, makes a great curve round Banks' Peninsula, approaching the coast again a little to the north of Otago Peninsula, where, as at the Amuri Bluff, the older palæozoic rocks reach the east coast. From these two points they recede gradually inland, so that to the westward of Banks' Peninsula they are at a distance of forty miles from the coast. The lower portion of that volcanic system below the level of the sea, sloping gently down in all directions, has at the same time formed a favorable locality for the deposition of enormous masses of shingle, sand, and silt, brought down by the large glacier rivers from the Southern Alps. Consequently we find that even as far as Timaru, the 100 fathom line lies about 100 miles from the shore. The fact that the set of the currents is in a northerly direction, or nearly opposite to that of the earthquake waves may perhaps account for the great disturbances experienced in Pigeon and Okain Bays long after they had reached their maximum in Lyttelton harbour. This is confirmed by the observations of the “Stormbird” on her passage from Lyttelton to Timaru. This steamer, in the early hours of the 15th of August, was struck by two or three heavy seas, the sea being calm at the time, and was driven about ten miles to the northward
by such a strong current that it was impossible to make headway against it. The effects of these disturbances of the sea were felt severely in Timaru. From the accounts published in the newspapers, it appears that a huge wave, which has been variously estimated at 6 or 8 feet perpendicular height, rushed upon the shore at 4.45 a.m., and rose several feet above the highest spring tides. Just as quickly as it appeared, it retreated, leaving the bottom of the sea exposed for a long distance. Another wave rushed in at 5.4 a.m., which was followed during two hours by several others, but of a lesser size.
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In Hokitika, on the western side of this Province, nothing unusual was observed during the day, but at Westport towards the time of low water, several bores (as the newspaper of that place calls them), or waves of unusual size were seen to ascend the river, running in rapidly from the sea. The largest and heaviest “bore,” 4 to 5 feet in perpendicular height, occurred between the hours of five and six in the evening, ascendding the river rapidly, and causing a vessel to break from its moorings at the wharf, and travellers, going by Cobb's coaches along the beach, observed the tide to rush very far back, and to come up again in very high rollers.
What is here very important is the fact that also there the heaviest wave was the last, and occurred so late in the afternoon. We are therefore compelled to believe that the earthquake waves passing round Cape Maria van Diemen, in the North Island, and round the southern point of Stewart's Island, and in a minor degree, through Cook and Foveaux Straits, united, only some distance from the west coast of New Zealand, on their westward course; and that they sent some waves back to invade, although in a minor degree, the opposite side of these islands.*
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Before, however, leaving this part of the subject, I wish to state a few facts observed in or near Lyttelton harbour, so as to preserve them from oblivion.
A skeleton buoy, placed near the breakwater in course of construction, and attached to an anchor of 4 cwt., was removed up the harbour during the rush of the water for a distance of half-a-mile, with anchor and chain, thus showing the enormous force of the wave.
I may observe at the same time that it would be interesting to find by soundings it well known spots, if that harbour has not been deepened to some measurable extent, as the vehemence with which the water rushed in and out must have been able to remove a great quantity of the fine silt or mud of which the bottom of that harbour is formed. It appears from the accounts given to me by some of the passengers of the steamer
[Footnote] * Since these notes were written, I visited Lake Ellesmere, which is separated from the sea by a bank of shingle and sand. This bank is lowest at its southwestern end, and is there about 8 feet above high-water mark. Consequently when the lake rises to such a height, it breaks through this shingle wall and empties itself rapidly. There is a Maori Kainga near this spot, forming a favourite fishing station. The natives informed me that the earthquake wave in the sea had not crossed this shingle bank; in fact they had not observed any disturbance, but they felt distinctly the earthquake shock of the 17th of August. The lake, being near its highest level at the beginning of this month, broached the shingle bank on the 8th or 9th.—Sept. 18, 1868. J. H.
“Taranaki,” which arrived in the morning of that memorable day in Lyttelton harbour, that, when about thirty miles from that port, and approaching the Peninsula, the water of the sea became very muddy; moreover, it was covered with driftwood, cut timber, and now and then with what they supposed to be portions of wrecks. Except a very strong N.E. swell, the steamer had not experienced the least disturbance of the usual movement of the surface of the sea, and therefore crew and passengers did not know how to account for this strange and unusual appearance of the sea. At last they came to the conclusion that a tremendous and destructive flood must have taken place inland, similar to that which visited the interior of this island in February last, when the sea had a similar discolored appearance near land, carrying at the same time an enormous amount of driftwood, and remains of human habitations and industry.
Entering now into a consideration of the second subject of this paper, the earthquakes experienced on the morning of the 17th, it would appear, at least at a first consideration of its direction, that the focus of this disturbance on land was situated near the spot where the earthquake waves in the sea had radiated, as the direction of both was apparently the same.
But when we examine the subject more closely, with the aid of the exact time at which that shock was felt in different localities in New Zealand, we shall at once find that it will not answer. We shall observe that the vibrations of the ground advanced at such an enormous velocity, that the usual explanation of a central or linear earthquake, the focus of which would be situated in a N.E. direction from New Zealand, cannot be adopted; and that we have to seek for another explanation which will satisfy us more fully.
But before entering into a consideration of this subject, I will first offer a short description of that phenomenon as observed in Christchurch and its neighbourhood. According to a communication of Mr. Bird, the telegraph inspector residing in this city, the shock was felt at the Christchurch Telegraph office as nearly as possible at 9.56½ a.m. on August 17. It lasted, according to the generally received reports, about four seconds, although it appeared to me that all was over in about two seconds. It consisted of two slight shocks, moving apparently from north to south, with an easterly tendency, but opinions about its direction are greatly divided. I myself stated that it had advanced from south to north, without doubt owing to the fact, that the movement of my house, in which I was sitting at a writing table, was in the direction opposite to the shock, as this is generally the case. Many observers state that the oscillations came from the east, and pointed to pendulums and other objects which were able to swing in the direction imparted to them by the shocks; but we ought to remember that the vibratory jars are very often turned locally from their main direction by a difference in the physical features of the country.
No damage was done by these oscillations of the ground, although they were powerful enough to cause the ringing of bells, and the cracking of the timbers in the houses, to such an extent, that many of their inhabitants ran into the streets, expecting some still greater convulsions.
It is a fortunate circumstance that the clocks in the telegraph offices throughout New Zealand keep Wellington time, because we have thus
the ready means of knowing, with some degree of certainty, the exact moment when the shocks were felt in different parts of the colony.
Thus, the Wellington shock is noted as having occurred at 9.56 a.m., whilst the observations of the Christchurch station fix it at 9.56½ a.m., and consequently only thirty seconds elapsed between them. Mr. Bird made me a very interesting communication, which shows not only that the shock was first felt in Wellington, but also that the intervals as stated must be correct, at least within a few seconds.
The Wellington operator was at that time sending a telegram to Christchurch, when he suddenly stopped for a short time, which, as the Christchurch operator thinks, was about a quarter to half a minute. When beginning again after that interval, he asked—“Did you feel that?” after which the shock reached the Christchurch office. The transmission of the four words “Did you feel that,” takes about eight seconds; if, consequently, we take the mean between a quarter and half a minute, and add the eight seconds, we shall obtain as nearly as possible the results of the clocks, namely, half a minute. Dr. Hector, in his lecture upon the same subject, gives the exact time when the earthquake vibrations were experienced at all the different stations, and also the distance of some of them from each other. (See p. 97.) To the latter I have added a few more, in order to obtain more data for calculation.
From the results obtained by other observers as to the velocity of the usual earthquake vibrations, which in the mean may be stated to be about 1800 feet per second, it will become evident from my own calculations which I am going to give, that the New Zealand earthquake has had some unusual features. It will be seen that it moved, on the average, about five times as quick, and therefore we can neither call it a central nor a linear shock, the velocities of which we know with a moderate degree of certainty.
It thus appears that the points most distant from each other at which the earthquake was felt were, Napier (9.55 a.m.) in the Northern, and Hokitika (9.58 a.m.) in the Southern Island, the distance being 402 miles, the difference of time three minutes, or, per second, 11,791 feet.
[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]
|Napier to Christchurch, 1½ minute.|
|Distance, 367 miles, or 21,530 feet per second.|
|Wellington to Christchurch, 30 seconds.|
|Distance, 172 miles, or 33,455 feet per second.|
|Christchurch to Hokitika, minute.|
|Distance, 102 miles, or 5984 feet per second.|
|Wellington to Nelson, 1 minute.|
|Distance, 76 miles, or 6688 feet per second.|
I think these results are sufficiently clear, to show, in the first instance, that these vibrations were not the result of a volcanic earthquake, either central or linear, because irrespective of the quistion of direction, they-travelled much quicker than the vibrations of those forms of earthquakes are usually propagated.
It will be seen that the N.E. to S.W. direction is that in which the
shocks were felt almost simultaneously along the whole line. Should we even adopt the east and west direction as more correct, the fact will appear at a glance that, although more time elapsed between the shocks, also in that case the velocity of the shocks was far too high for such origin.
Another point of importance not to be lost sight of, is the absence of similar news from Auckland, and from Dunedin and the Bluff, so that we may fairly conclude that no shocks were experienced in the northern portion of the Northern, and in the southern portion of this Island.
Consequently, a broad belt, running in a N.E. and S.W. direction across New Zealand, and following the direction of the central chain in both islands, appears to have been visited nearly simultaneously by the vibratory movement.
There is, however, one form of propagation, and of a truly plutonic nature—the transversal earthquake—which, if admitted in this instance will satisfactorily explain all the apparent anomalies.
If, therefore, we accept the hypothesis that an earthquake has reached the surface of the earth simultaneously along a line several hundred miles in length, and running parallel with the central chain in both islands, and through a fissure deep below the earth's crust, we shall be able to find sufficient explanation of all New Zealand occurrences. Let us therefore draw a line, passing through New Zealand, so that Napier, Waipukarau, Greytown, Featherston, (White's Bay?), and Blenheim, in which the vibratory jar was experienced at the same time (9.55), are situated at an equal distance, either on the same or on both sides of it; we shall then find that the other stations, such as Wellington, 9.56, Nelson, 9.57, Christchurch, 9.56½, and Hokitika, 9.58, may be grouped along with it, at greater or less distances. Owing to the occurrence of mountain chains, across which the shocks have to pass, a change of rocks, etc., many other causes of retardation or acceleration may arise, by which they may arrive at greater or lesser speed, at the stations where they have been registered.
And a calculation for each station from such an adopted line would show, that in every case the shocks moved from it, laterally, in a much diminished ratio, and more in accordance with the velocity of such class of earthquakes observed in other parts of the world.
It is also evident, from the occurrence of an earthquake shock in Sydney, experienced on the 18th of August, where such phenomena are of such rare occurrence, that a deep seated disturbance in or below the solid crust of the earth must have occurred over a great portion of the globe, and for which a volcanic eruption cannot account.
Thus the abysso-dynamic forces, or tides, may have acted upon the earth's crust at any given spot more readily, owing to its weakness, when compared with others. They may have been able to form a vent, from which volcanic eruptions of unusual magnitude took place on the 15th, and probably on the following days. But such a catastrophe could not happen where the crust of the earth was too solid, nevertheless, at many of such localities, the fluid magma below the former was sufficiently disturbed to act upon the interior of the shell by vibratory jars, and principally in those lines, where, as along the axis of longitudinal mountain chains, or their declivities, weaker zones were exposed to that influence.