SLIPPING FAULT.

Putaruru Press, Volume VII, Issue 294, 27 June 1929, Page 1

SLIPPING FAULT.

ORIGIN OF QUAKE. Phenomena Explained. Geologists’ Views. Irresponsible speculators as to tile cause of last week’s earthquake embrace in their theories volcanic eruptions and similar phenomena, but there is little, if any, evidence to show that the shake was due to anything more than a displacement along one of the numerous earth tauits which are to be found in the Murchison district. All the evidence available at the present time points to the centre of the earthquake as Saving been close to Murchison and not out to sea or elsewhere, as has been suggested in some quarters. Geologists, in talking of this earthqualm as having been caused by displacement along a fault line, would not use the word epicentre, which is n term used to denote a theoretical spot on the earth’s surface immediately above the supposed centre of the disturbance. If an earthquake Is caused by displacement along a fault line is it obvious that the term epicentre can hardly be justified.

The Kiverview Observatory in Sydney, from the very complete record it obtained of the earthquake, placed the centre of the disturbance at roughly 100 miles out to sea. Undoubtedly there is an unstable area out to sea beyond Westport, and colour can be lent to the theory of submarine origin by the fact that the west coast suffered much more severely than the east coast, the suggested origin being to the west of the northern part of the South Island. With an earthquake of such severity as last week’s, had its origin been submarine, a tidal wave might have been expected, but there ■was no sign of one and apparentlynothing abnormal in the tides. As has already been said, all the evidence points to the seat of origin being at a point about equidistant from Wellington and Christchurch, somewhere in the neighbourhood of Murchison. The country around Murchison, according to officers of the Geological Survey Department, Is divided into well-marked earth blocks owing to differential elevation and depression at previous periods in the geological history of that part of the country. One of the biggest of these faults runs down the Wairau Valley and thence south along the foothills of the Alps. One branch from the direction of Nelson joins it at Tophouse. To the west Is a comparatively large downfaulted area subdivided by other meridional faults. The relief of accumulated stresses caused the tremors and vibrations which preceded the main shock and was also the cause of the noises heard and of the main shock itself.

It seems quite unnecessary to call in the aid of volcanoes or other agencies to account for the shake. That it was felt severely in many parts of the West Coast and Nelson is due to the fact that the whole of that district is full of faults. When a place is situated near a fault it is liable to feel a shock more severely, even if the nearby fault is what is termed “ dead,” that is, not actively concerned with the earth movement actually causing the earthquake. If the damage done by last week’s shake is analysed with reference to a map of faults it will be seen that the damage has been greatest along the lines of the faults. One runs along the coast close to Westport. Nelson College is close to the Waimea fault, and the Takaka Valley formation is determined by two faults. Among other facts of interest to be gleaned from an analysis of the earthquake is the nature of the movement, described in various reports as a rotatory one. This would Indicate a complication in the vibrations. The long and undulating nature of the movement would point to an origin deep in the earth : a sharp- and vertical movement usually denotes an origin near the surface. Alluvial or built-up ground always moves more in an earthquake than more solid ground, hence one

reason why Westport felt the shake very severely. Other shocks which have affected Westport have been definitely shown to have had an origin at an earthquake centre some way off the coast, but on this occasion the accumulating evidence would seem to rule this out. In places where there are long lines of well-defined physical features stresses in the strata are nearly sure to exist, and in these places, therefore, earthquakes may be expected. This is especially so where high land runs down steeply into the sea, as in South America. New Zealand is fortunate in the respect that no powerful submarine centres of earthquake activity lie very close to the shore, hence tidal waves when an earthquake occurs are not at all likely. An • explanation of eye-witnesses’ stories about whole hills being seen to collapse may be found in the fact that in the Murchison area are several limestone hills which may be said to be fairly honeycombed with caves. With the occurrence of the earthquake it is quite possible that one o-r more of these hills collapsed like a pricked balloon, the debris being hurled across the valleys. It has been suggested on more than one occasion that meteorological influences may play their part in starting an earth movement, low barometric pressure being conducive to the relief of stress. At the time of last week’s earthquake, however, the barometer was high and pressure did not become low \ until two days later, so on this occasion, at any rate, there would not seem to he any connection between seismology and meteorology. The Noises Explained.

The booming noises, something like muffled explosions, heard before and at the time of the earthquake, and also subsequently, may at first sight seem to lend colour to an explosion or volcano theory. But as a matter of fact such noises in the Murchison district are not at all uncommon, and can be explained on geological grounds. These noises are familiar to miners as occurring when there is 1 relief of pressure in the earth’s crust owing to their borings, and they seem to occur also in places where there are faults. Near Murchison they have .been heard and investigated on several previous occasions, especially by these who have connected them with so-called mud volcanoes and- therefore the presence of oil. These noises obviously increased in intensity at the time of the earthquake, as would be expected when it is considered that they are caused by the grating of Wock surfaces as stress is relieved, Were our knowledge greater about such things their increased incidence before the earthquake might have been regarded as a warning. A fracture of the earth, or movement along a fault, resulting in an earthquake, takes place much like the snlitting of a branch off a tree. First there are a few preliminary cracks and then the main split, followed by lesser of greater splits. Signs of the coming fracture were received early on the morning of the earthquake, when the seismographs registered small tremors. But in the present state of human knowledge about earthquakes we cannot decode nature’s signals with certainty, hence the warning passed unheeded. Before the comparatively young sciences of either geology or seismology can enter the field of prophecy both geologists and seismologists will be the first to admit that there is very much more to be learned about earthquakes and their incidences and causes. Wellington Evening Post.