ORIGIN OF QUAKE

Hokitika Guardian, 25 June 1929, Page 5

ORIGIN OF QUAKE

SLIPPING FAULT

GEOLOGIST EXPLAINS

WELLINGTON, June 24.

The Rivcrview Observatory in Sydney, from the very complete record it obtained of the earthquake, placed the centre of the disturbance at roughly lUO miles out to sea. Undoubtedly mere is an unstable area out to sea bcyoiul 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 Soutn 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 apparently nothing abnormal in the tides. There is liitle, if any evidence to show the shake was due to anything more than a displacement along one of the earth—faults which are to be found in the Murchison district. Ali the evidence points to the centre of the quake as having been close to Murchison and not out to sea or elsewhere, as suggested in some quarters. Geologists, in talking of this earthquake as having been caused, by displacement along a fault-line, would not use the word epicentre, which is a term used to denote a theoretical ,pot on the earth’s surface immediately above the supposed centre of the disturbance.

The eviuence points to the seat of origin being at a point about equidistant from .Wellington and Christchurch, somewhere in the neighbourhood of .iLurchis..n. The country around Murchison, according to officers of the ecological Survey Department, is divided into well-markin’ earth-blocks owing to differential elevation and depression at previous periods in the geological history of that part of the '•ountrv. One of the biggest of these faults runs down the Wairan Valle.' >nd thence south along the foothills of the Alps. One branch from the direction of Nelson joins it at Tophou.se. To the west i<; a comparatively large lowji-faulted area sub-divided by othei meridional faults!

It is considered quite likely that the root of the trouble will lie traced t( he fault the slipping of which was the cause of the Otira earthquake earlier in the year. Earthquakes habitually occur along lines of fault, and evidence from other parts of the world show that the centre of the disturbance travels* along a fault, an earthquake taking place at one time at one spot on it, and at another time at another spot. It has yet to he shown definitely that the (Mini earthquake and last week’s earthquake are connected with the same fault or system of faults, but it is considered probable that they are. That each or either ol the Otira or Murchison quakes has any connection with the recent earthquake centred near Ilunterville is more problematical. ..HERE THE FAULT LIES. It seems quite unnecessary to call in the aid of volcanoes or other agencies to account for Monday’s shake. That it was felt severely in many parts of tlio West Coast and Nelson is due to the fact that the whole of that district is full of faults. When a place is situated as near a fault it is liable to feel a shock more severely, even it the nearby fault is what is termed “dead,” that is, not actively concerned with the earth movement actually causing the earthquake. Jf the damage done by Monday’s quake is analysed with reference to the map ol faults, it will lie seen that the damage has been greatest along the lines of the faults. One runs, along the coast close to Westport. Nelson' College is quite dose to the Waimea fault, and the Takaka Valley formation is determined by two faults. The map also shows plainly how faults lie alongside and form tno valleys reported as having suffered the worst.

The booming noises, something like muffled explosions, beard both before and at the time of the earthquake, and also subsequently, may at first sight seem to lend colour to a.n explosion of 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 arcfamiliar to miners as occurring when there is relief of pressure in tlio earth’s crust owing to their borings, and they seem to occur also in places where there arc faults. Near Murchison they have been heard and investigated on several previous occasions, especially by those who have connected 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 lie expected when it is considered that they are caused by the grating of rock 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 fraulmv of the earth or movement along a fault, resulting in an earthquake, takes plaee much like the splitting of a branch off a tree. First there are a few preliminary cracks and then the main split, followed by lesser or greater splits. Signs of the coming fracture were received early on Monday morning, when the seismographs registered small tremors. But in the present state of human knowledge about earthquakes we cannot decide nature’s signals with certainty, lienee the warning passed unheeded. , Before the comparatively young sciences of either geology or seismology can enter Die field of prophecy, both geologists and seis-

mologists will be the first to admit that there is very much more to he learned about earthquakes and their incidences .and causes.

The movement of the ’quake has been called a rotary one. This would indicate a complication in the vibrations. The long and undulating nature of tho movement \vould 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 big shake very severely. Other shocks which have affected Westport have been definitely shown to have had an origin at an earthquake. centre some way oft' the coast, but".on this occasion the accumulating evidence would seem to rule this out.

Ju places where there are long lines of wejll-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 arc 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 Murr-hison area are several limestone hills which may he said t n be fairly honeycombed with caves. With the occurrence of the earthquake, it is quite possible that onor more of these hills collapsed like a oricked balloon, the .debris being hurled across the valleys.

NOT VOLCANIC.

EXPERTS’ OPINION

CHRISTCHURCH, June 23

In spite of the conviction of some of the residents in the affected areas that the ’quake was due to volcanic action because they saw sulphurous black water oozing out of tho ground there is no proof that volcanic action had anything to do with the mattei it all, according to Professor R. Speight, Professor of Geology at Can terhury College. Sulphurous clays vore frequent in that part of tnc •ountry and would account for the effect on the water.

Mr H. F. Skey, of the Magnetic Observatory, was of the same opinion, lie thought 'that possibly a hidden lmt spring had been uncovered by the 'quake; it was no indication of volcanic action. Tlio earth movement was ■irobably due to pressure from below in the earth strata. It must have 'icon some such movement that once nished up the Kaikouras. Though the maximum movement would he at the surface, the original movement would i )0 deep down and might actually ]>e verv minute. Had the ’quake been lue to volcanic action this would have been continuous and there would certainly have been an' outbreak.