Third Communication on the same subject. [Read by Dr. Hector, October 12, 1868.]
In America, there appears to have been two distinct shocks of great magnitude, although smaller ones were felt at frequent intervals between the 13th and 16th, as was the case in New Zealand. The first great shock was experienced in Peru at 5 p.m. on the 13th of August, which time corresponds in Wellington with 9.30 a.m. on the morning of Friday, the 14th. This shock is described as coming from the south and west, and there is no doubt that it was the result of a great submarine eruption, at a considerable distance from the coast; as within a short time it was followed by three ocean waves, which destroyed the towns along the coast of Peru for a distance of 1000 miles, between Int. 12° and 23° S. There is no reason to doubt that it was the westerly propagation of the same three waves, the first of which reached New Zealand at 2.30 on the Saturday morning, having traversed the width of the South Pacific Ocean (over 6000 miles) in seventeen hours, giving an apparent average velocity of six miles per minute. This agrees with the velocity formerly calculated for the wave, from the difference between the time at which it broke on the Chatham Islands and the Australian coasts. A wave, having its origin a few hundred miles from the coast of Peru, say in lat. 25° South, as appears to have been the case in this instance, would reach the Australian seas by the shortest route, following what is termed a great circle, and would appear to reach us, not from the north-east, as might be expected, but from the south-east; the reason of which can be readily assigned by examining a globe. The force which originated such a wave must have been tremendous; and there is no instance on record in history, of any earthquake wave of equal extent and magnitude. The second calamitous event, as far as we yet know, only affected the Province of Ecuador, where, at 1.20 on the morning of the 16th, or at 6.10 on the evening of Sunday, the 16th, in Wellington, a tremendous earthquake shock passed slowly from east to west, devastating the districts lying on the western slopes of the Andes, between the Equator and latitude 5° S. There is no reason to suppose that this shock, although its direction was the same, had any connection with the shock which was experienced in New Zealand on the following morning of Monday the 17th, at 9.56; but it is interesting to find, that at that particular period the cause which gave rise to earthquakes was in activity at wide distances apart on the earth's surface, giving strong support to the view advanced in the lecture before the New Zealand Institute, that the primary cause of earthquakes is an influence external to our planet; so that earthquakes are to be considered as the remote cause, rather than the effect of, volcanic phenomena.
The following remarks on the Earthquake Wave were communicated by Captain Hutton, F.G.S.
The earthquake wave that crossed the Pacific Ocean in August, from Peru to Australia, is, I believe, the largest wave of its class yet recorded; the only ones that can at all compare with it being the one caused by the earthquake at Lisbon in 1755, which was propagated across the Atlantic to the West Indies, a distance of 3500 miles; and the more recent one of December, 1854, caused by the great earthquake in Japan, and which traversed the North Pacific to San Francisco, a distance of 4500 miles.
The wave that lately visited our shores appears to have originated somewhere about latitude 20° S., and longitude 70° W., at 5 p.m. on the 13th of August, according to the reckoning at the place, or at 9 a.m., 14th of August, according to our time. The first wave reached New Zealand at 4 a.m. on the 15th of August, having therefore travelled about 6700 miles in nineteen hours, or at the rate of 5.87 miles a minute, The three waves reached us at three-hour intervals, and must, therefore, each have been about 1000 miles in breadth. The velocity at which waves travel over the ocean depends upon the depth of the water, and varies as the square root of the depth, so that the deeper the water the quicker the wave will travel. The wave raised by the earthquake at Lisbon travelled to the Barbadoes at the rate of 7.8 miles a minute, while it went to London at very little more than two miles a minute. Professor Airey has shown that a fixed relation exists between the breadth of a wave, its velocity of progress, and the depth of the water on which it travels. The earthquake wave of December, 1854, was 217 miles in breadth, and travelled at the average rate of 6.1 miles per minute, from which Professor Rache concluded that the mean depth of the North Pacific was 2365 fathoms, or 14,190 feet. In the same way, by the progress of the tidal wave, the Atlantic, from 50° N. to 50° S., has been calculated to have a mean depth of 22, 157 feet.
Applying the same theory to the late wave, we find that the South Pacific has an average depth of only 9721 feet, or not quite 1454 fathoms; or, in other words, the South Pacific is much more shallow than either the North Pacific or the Atlantic. This fact, if it should hereafter be established, has a very important bearing both on geology and the geographical distribution of plants and animals, which, however, it would be quite out of place to enlarge upon here.
F. W. Hutton.
Auckland, October 6, 1868.
In the above estimate of the depth of the South Pacific, as deduced from the breadth and velocity of the earthquake waves of 15th August, it will be observed, that while the velocity is the same as that of Rache's wave of December, 1854, or six miles per hour, Captain Hutton assumes that the wave had a width of 1000 miles, on the ground that they occurred at three hours' interval. This is, however, hardly correct, as at the Chatham Islands the great waves followed at intervals of half an hour; and at different parts of the New Zealand coast at very unequal intervals, amounting generally to four hours. From this I infer that we have not the data for ascertaining the width of the wave in crossing
the Pacific, as it appears to be modified to a great extent as it approached the New Zealand and Australian coasts. The deduction respecting the depth of the ocean must, therefore, in this case, be accepted with great caution.
Tidal disturbances in New Zealand, August 15, 1868.
Abstract from local newspapers, letters, etc.
Bluff. Tide rose higher last night (14th) than ever known before. Between 8 and 9 this morning terrific rush of water, rose very high, although the time of high-water was not till 10.27. At 11 a.m. the tide fell nineteen inches in twenty minutes, and rose again a short time after.
Port Chalmers. At 10 a. m. water rose one foot, and fell again in a few minutes. Large ships in the harbour swung round their anchors three times in one tide. Ordinary high tide at 1.40 p. m., water then rising and falling continually, at the rate of two inches per minute.
Oamaru. At 11.50 a.m. extraordinary change in the sea level, the tide rising and falling fifteen feet perpendicularly at intervals of fifteen minutes, and receding eight or nine feet perpendicularly below low-water mark. Agitation continued all day.
Timaru. At about 5 a.m. sudden rise of the sea of about six feet, which in the course of five minutes fell to a lower level than has been ever witnessed in Timaru. Sea rose and fell on the beach rapidly for the space of four hours, with a strong current, changing with the rise and fall of the tide. Succession of whirlpools in the offing, very distressing to ships.
Pigeon Bay (Banks' Peninsula). At about 4 a.m. loud rumbling noise from the sea—first waves were not witnessed otherwise. At 7 a.m. water considerably lower than ordinary low-water mark, great rush of water, in ten minutes it was a foot higher than ordinary high-water mark. Fish of all descriptions were thrown up by the water. Water kept rising and falling about five feet every half hour, until ten minutes to 10 a.m., when it rushed in very quickly and rose four feet above highest high-water mark. At twenty minutes past 10 a.m. it fell equally fast, and was at its lowest point at 10.35; again at 11.15 the water came in with a great rush, carrying away a small jetty and some garden fencing, which was about four feet above high-water mark. At ten minutes to 12 it retired to its lowest point. Advancing again, it came in with greater force than any of the previous waves, carrying away a boat house, more garden fencing, and forty thousand feet of timber, stacked five feet above highest high-water mark. At five minutes past 12 the water retired once more; at 12.40 the water having risen seven feet above the ordinary high-water mark, carried away another jetty, also the ketch “Courier,” 30 tons, which was brought back within a few feet of her old berth by the next rush at 1.5 p.m. After this the water continued to rise and fall at intervals of about three quarters of an hour, each wave getting smaller as the tide retired.
Port Lyttelton. At 4 a.m. the harbour was observed to be quite dry from the wharf to Officer's Point, and the vessels lying on their sides.
In a few minutes an immense wave came up the harbour, tearing the vessels from the wharves, and breaking their warps and eight-inch hawsera. Much injury was done to vessels. Water rose and fell for hours. At 6 a.m. the tide was below low-water mark, and in a quarter of an hour it was above high-water mark. At 9.30 a.m. another roller came in. The water sometimes rose and fell three feet in five minutes.
By other observers: At 11.30p.m. on Saturday (15th) the water was half-flood, stood at this some time; at 12.30 a.m. on Sunday it rose to the usual high-water mark; at 1 a.m. it fell back to half-ebb; at 4.30 it was half-tide again. After this the water gradually fell back; at 6 a.m. it was down to low-water mark, making three times high-water in six hours; at noon it was high-water; at 1 p.m. the water dropped suddenly four feet. At 2 p.m. the tide rose higher than usual, covering the Reef. The agitation is still (6 p.m.) going on.
Hokitika (West Coast), No particular disturbances of the tide noticed.
Kaiapoi,—four miles from the mouth of the Waimakariri. At 3 a.m. —two hours after ebb—an immense wave, four feet high, rushed up the river, doing much damage to the ships lying at the wharf. First wave at about 3 a.m., followed by two others at intervals of about fifteen minutes. Up to 4 o'clock, seven in all, but not of such magnitude as the first two or three, had rushed up the river. The greatest force of the waves went up the south branch of the river, thus saving the town from disastrous consequences. Waves continued to come up at intervals during the day.
Picton. Nothing unusual in the tide.
White's Bay. At 10 and 10.20 a.m. sudden recession of the sea, about forty yards down the beach from the usual tide mark. It rose again suddenly with a heavy swell and surf. High and low tides all the morning alternately.
Nelson: time of ordinary high-water, 6.30 a.m. The tide receded in its customary way. At 7.54 a.m. water suddenly returned, rising rapidly until 8 a.m., rushing in, in all directions, over the Boulder Bank, which appeared to have then more water upon it than at the time of high-water. Estimated height of wave, four feet. About noon there was another considerable rise and fall of the water. Also, at 5.10 p.m., with a sudden fall of about two feet. Sunday morning (16th) high-water, ordinary, at 7.40. Ebbed for about ten minutes, then suddenly returned and covered former high-water mark. Sunday afternoon further irregularities, which continued till Monday, in a slight degree.
Wellington. See page 96, et seq.
Castle Point. Tide last night higher than it has been for ten years, with one exception. Time of ordinary high-water 2 p.m. At 11.53 a.m. tide was as high as ordinary high-water.
Napier. At 10 a.m. tide rose higher and fell lower in ten minutes, in the “Iron Pot,” than has ever been known before. Water continued to ebb and flow every hour, from three to six feet.
Opotiki. Tidal disturbances on the 15th August. First appearance at 8.30 a.m., it was then low-water, and a wave about six feet high rushed up the river at the rate of six or seven knots, filling the river up to high-water mark; remained thus for a few minutes, then rushed back,
and in fifteen minutes it was low again. Several smaller waves followed. Next day slight oscillations still continued.
At Opapi and Tirirua, similar disturbances.
At Cape Runaway, great wave between 4 and 5 a.m., doing much damage. Indications on the beach show this wave to have been nearly ten feet high.
Auckland Harbour. No perceptible difference in the tides, but at Orewi and Port Charles, tide rose to an unprecedented height, rising and falling about six feet several times.
Mongonui. Great tidal disturbances, which continued in a minor degree till the 17th; water frequently rose and fell from four to five feet between 9 a.m. and 2 p.m. Such an occurrence never observed here before.
Sydney. The tide water rose and fell repeatedly fully four feet. No serious damage done.
Melbourne. Marine disturbances were not observed in or about Port Phillip.
Chatham Islands. Early on the 15th, three immense waves rushed in, causing great destruction of property and the loss of one life (a Maori). At Tupanga, on the northern side of the island, the phenomenon was felt with the greatest force, the settlement being entirely destroyed. Similar disasters occurred at Waitangi, and great loss was occasioned.