DR. HECTOR ON THE RECENT EARTHQUAKES AND TIDAL PHENOMENA.

Wellington Independent, Volume XXIII, Issue 2729, 8 September 1868, Page 3

DR. HECTOR ON THE RECENT EARTHQUAKES AND TIDAL PHENOMENA.

At the last meeting of the Wellington Philosophical Society, Dr Hector road tho following paper, which, under existing circumstances, will be of great interest to our renders . — One of the most important duties of the members of a local scientific Society such as this, ib to obtain uecurato records of phenomena of a transient character like the disturbances of the tides and the earthquakes by which tho colony was visited bet ween the 14tli and 18th inst. I have, therefore, attempted to collect together, in the following preliminary communication, the observations which were mndo in different places so far as they have been already ascertained; nnil ulthough they are deficient in many respects for purposes of exact scientific enquiry, I trust that their discussion mny lead to tho adoption of instrumental means for recording 6uch phenomena in future, as it is in this wmnner alono that sufficient accuracy can bo obtained. As there is no reason to beliero that the tidal disturbances are in this instance directly connected with the earthquakes which weexperienced, I shall, therefore, allude to the two classes of phenomena separately, although they are only different manifestations of tho same paroxysmic energy, but occurring in different parts of the earth's crust, and at different times. Respecting the tidal disturbances, from cavoful consideration of the various accounts which have been received, it appears that they were ex-

perienced along tho whole of (.lie Eusfc const of tho - ' islands, and also in Foveaux Straits and Cook's f Straits, and that tho irregularity of tho usual > ' flow and ebb of the tide was due to the influence , of three distinct oceanic waves which reached tho i coast from eastward on the forenoon of Saturday, j the 15th inst.. at about tho following periods : — | Ist between 3to 4 a.m. ; 2nd between 7 to 8 a.m.; I i .3rd between 10 to 11 a.m. These waves were in ! each case followed by smaller waves, and the irregularities did not altogether cease for 48 hours • ul'ter 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 pecu- , liariries due to the outline of the land, the dopth of the water, the exposure of the coast lino to the direction in which the wave reached the ehore, and lastly to the local time of tide. The intervals between the smaller oscillations appear to have boon generally remarked at from fifteen to thirty minutes, and to have gradually declined in extent, and frequency until the next great vavo supervened. The earliest, notice of the wave which wo lmvo recorded beyond allusions to an extremely high tide tho previous evening, was at Kaiapoi, where it was reported that at 3 a m. the tide had ebbed for two hours, a wave four feet in height ru?hed up tho Waimikariri river, and swept tho vessels which were lying at the wharf from their moorings. This was nfc 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 tho other places no exact, time is reported for the occurrence of the first wave, while ot several localities it appears to have escaped observation. From this time until 8 o'clock only lesser waves were remarked, but about that hour a great disturbance seoms to hare been observed at all the slat ion?, being described at tho 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 halfebbed, so that it roso beyond the limits of high wiiter mavk, and flowed into the harbor over tho Boulder Bank. A third great rush of wafer appears to have been everywhere distinguished from tho smaller oscillations which went on continuously, the time being variously stated from 10 to 12 o'clock, there being great irregularity in tho hour reported. In this harbor, 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 bo so clearly distinguished as on moro exposed parts of the coast, but there is a general agreement j among all the observations taken at tho different stations which leads to the above conclusions. Tho altitude of each wave, as compared with its amplitude or brendth, has not been ascertained irrespective of the degreo 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 tho distance at which it originated. Nevertheless, as compared with tho ordinary effects of tho tidal wave, wo can form somo conception of the gigantic force which must have influenced tho ocean along tho coast; when we find that the ebb and flow which these waves caused in most cases, appears to havo exceeded the ordinary local rise and fall of the tide at the different localities. This leads mo 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 tho focus from which they originated. Tho following information appears to have escaped tho notice of the journals in the colony ; it is an extract from " Principles of Q-eology," by Sir Chas. Lyell, 10th edition, 18GS, vol. 2, p. 409. "Even in tho present year (November 1807) 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, vising 2000 feot,jind tho 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 tho bed of the ocean, although there was no permanent protrusion of a now volcano above its levcl."_^ An earthquakeshock which appears to havo been felt throughout the colony a few minutes before 10 o'clock on Monday morning, tho 17th iust, of a character very different from tho local shocks to which wo are accustomed in this place. From the appended record of telegraphic announcements, it appears to have occurred about three minutes earlier in the north-east at Napier than at Ilokitika, on the West Coast of the Middle Island. Napier is situated in Jat. 39d. 29m. S., long. 176d. 55m. B. ; Hokitika, in lat. 42d. 41m. S., long. 170d. 59m. E. This gives a horizontal distance of 402 miles ; but as we do not know whether tho wave was travelling from the east or tho north-east, it is impossiblo to infer its velocity. The following telegraphic record shows tho exact time at which tremulations were felt at the different telegrap'no stations in each island, and tho distance of tho principal stations from Napier, which appear to bo tho northerly limit of tho influence : — Napier, 9.55 a.m. ; Waipukarau, 9.55 a.m. ;' Castle Point, 9.56 a.m., lOg miles; Grroytown, 9.55 a.m. ; Featherslon, 9.55 a.m. ; Wellington, 9.56 a.m., 169 miles ; White's Bay, 9.55 and 9.56i- a.m. ; Blenheim, 9.55 a.m. ; Nelson, 9.57 a.m., 229 miles ; Kaiapoi, 9.57 a.m. ; Christchurch, 9.57 a.m., 367 miles; Lytteiton, 9.57 a.m. ; Hokitika, 9.58 a.m., 402 miles. This shock was also preceded and followed by minor shocks, felt in Wellington at the times shown in tho following table : — Friday, 14th August, 10.45 a.m. ; 3.10 p.m. ; Saturday, 15th, 3 a.m., tidal disturbance ; Sunday, 16th, 3.15 a.m. and about 11 a.m.; Monday, 17th, 956 a.m., great shock ; Tuesday, 18, daylight ; Wednesday, 19, daylight. Telegraph stations reported tho shock as from tho South, but for mechanical reasons wo may conclude that their sensation would suggest tho opposite of tho true direction. The periodical character of these shocks waß at once evident, and they appear to bo in somo degree dependent on the period of maximum and minimum pressure as indicated by the hourly fluctuations of tho barometer. With regard to this I may observe, that the . fluctuations of the barometer during tho oscillations of the tide, indicate a frequent variation, amounting to inches, which is not beyond tho usual daily variation, but from tho irregularity with which this oscillation occurred affecting a rising barometer, it is evident that somo influence must have been at work beyond tho ordinary secular variation duo to the sun's position. The author then explained that he had only attempted to state facts in a collected form, deferring tho full discussion until further details are obtained from Auckland and other places, where no doubt the tidal disturbances wero observed. ; He then described simple forms of solf-registor-ing tido-guagos, which might bo distributed to different places, for tho purpose of getting exact information in future. Eeferring to tho great earthquake wave which i occurred in the North Pacific in 1854, and com- ■ paring it with the phenoma recently observed, he , was led to infer that the late tidal disturbances '■ must havo been duo to some convulsion of un- ' usual magnitude. Describing tho periods of the > year when earthquakoa occurred with the greatest > frequency in different parts of the world, with the assistance of a map, he indicated the particular • systems or areas of the earth's surface, disturbed i : by peisinio energy, and that particularly to which ■ Zow Zealand belongs.