3. “On Earthquakes in New Zealand,” by Sir James Hector.
This paper is an attempt to place on record all earthquakes that have been noted in New Zealand. The author held that earthquakes, like other
natural phenomena, required to be classified. Until within the last few years this had been attempted only by theories of their origin. What is required is accurate description and record, so that the varieties may be differentiated. A classification somewhat of the following nature might be adopted: Thus, in the first place, earthquakes and volcanic action are popularly connected, and their relation is easily understood, but applies only in a comparatively few cases, as there are many-. other kinds of earthquakes that do not appear to be connected with volcanoes. Secondly, we may have a primary impulse radiated from a deep-seated focus of small diameter—of which the Charleston earthquake of 1st September, 1886, was a marked example. This spread 1,000 miles in all directions from a focus 12 miles beneath the surface. The area within which the vertical displacements exceeded the lateral was 26 by 18 miles, but the focal points were along a curved line only 12 miles in length. Even in this earthquake we thus see a tendency to a longitudinal extension of the focus, but the impulse that caused the primary shock was simultaneous. This leads to the next class of heavy earthquake shocks that have great longitudinal extension, and which generate fresh impulses successively as they are propagated through a tract of country in which the undulations meet with strata in a condition of stress. It is to this class we must refer the only two great earthquakes that have been recorded in New Zealand, as they were propagated from N.E. to S.W. for a distance of at least 800 miles, while the lateral propagation was not more than 100 miles to the N.W. or S.E. A short abstract was given of the principal features of these two earthquakes, which occurred in October, 1848, and January, 1855, from the descriptions published at the time by Sir William Fitzherbert, and the late Judge Chapman. Lastly, we have slight tremors that frequently reach us, evidently from a great distance to the S.E., and which, except where they locally encounter superficial conditions of instability, do no material damage. They are widespread; and, if the undulations are circular, they must have an enormous diameter, as they affect New Zealand nearly as if they were straight lines.
The early records are necessarily very incomplete, and commence with the earthquake felt near D'Urville Island by Captain Furneaux on the 11th May, 1773. Subsequently only prominent shocks are referred to, until from the beginning of 1846 to October, 1848, when the shocks felt in Wellington were recorded. From that date until 1868 the record is very imperfect; but since 1868, when the present Meteorological Department was organized, and the telegraph brought into operation, the record has been tolerably complete for the whole Colony. The earthquakes during this latter twenty years have therefore been scheduled, the Colony being divided into six districts, each having a characteristic structural peculiarity, as shown on the map exhibited. An analysis of this schedule showed that during the period 537 earthquakes have been recorded: of which only 2 were recorded in the northern district of Auckland; 184 in the central district of the North Island; 183 on the east coast of the North Island; 88 on the west coast of the South Island; 98 on the east coast; and 30 in the extreme south. But of the above number 142 were felt only in the middle section of the North Island, between the South Taranaki Bight and the Bay of Plenty; 147 only in the district between the East Cape and Wellington; and 115 were confined to the east and south coast of the South Island. Of the whole number, only 7 could be identified as having been felt in places outside the Colony. In conclusion, a short reference was made to the modern views as to the causes of earthquakes, and especially to the important bearing of a recent paper by Mr. Autray Strachan, F.G.S., regarding the destructive effects of explosions from “slickensides,” or smooth surfaces formed by motion along deep-seated faults, when these faults have been brought within reach of the miner's pick by the elevation of the land. It appears that these smooth-faulted surfaces are in a state of intense molecular tension, probably acquired through slight movements when under intense pressure at pro-
found depths. The force thus stored may be liberated under certain circumstances by insignificant vibrations, and thus become an important factor in the generation of violent earthquake shocks. It at least points to a method, hitherto unsuspected, by which intense force may be stored up as a consequence of earth movements, without calling in the aid of the plutonic fusion of rock masses.
On the walls were exhibited specimens of the dally weather charts for the whole Australasian Group, which are now issued by the Government of Queensland under the superintendence of Mr. Clement Wragge, the Government Meteorologist.