’QUAKE CAUSES EXPLAINED

Nelson Evening Mail, Volume LXIV, 24 February 1931, Page 6

’QUAKE CAUSES EXPLAINED

MANY FAULT LINES GRADUAL SETTLING MARES AFTER SHOCKS BIG UPHEAVALS RARE (By Professor Speight in the "Christchurch Times”.) Earthquakes arc usually associated with the movements of blocks of the earth's crust, but the motive power behind these movemens still awaits discovery. The opinion of one very competent observer is that at some deepseated level in the crust, probably in the vicinity of twenty miles deep, serious changes occur which manifest themselves near the surlace in adjustments, that these adjustments are tho immediate cause of the ’quakes, but the deeper movements are the true source. What is the nature of these movements or their prime cause is entirely unknown, though there are various interesting speculations concerning them. We can suggest that they take the form of injections of the liquid substratum into the overlying solid crust, and that this- client is gradually transmitted upwards till it causes changes in the surface strata. Such movements are intimately associated with the foldings of rocks responsible for the lormation of mountain chains, and there is a close connection between the general run of earthquakes and mountain building, for ’quakes are most severe in those regions of the earth where mountain formation is known to be going on.

ROCKS SNAP UNDER PRESSURE

Whatever then may bo the prime cause, a slow and irresistible pressure affects the upper layers of the earth’s crust, and when the strain becomes too great the rocks snap and the disturbance is propagated through the earth in the form of vibrations which are the special feature of an earthquake. If the vibrations are rapid and the swing of the particles is great the ’quake is destructive, but there are numerous ’quakes in which the swing of the particles is small, so small at times that they can only be detected by a delicate seismograph. The effect is complicated also by the presence of vibrations of different character, propagated at various speeds and in different ways. The movement of the ground responsible for the disturbance may be upward sr. downward or horizontal, either in a straight line or circular, and it is generally the case that the intensity of the ’quake depends on the amount and suddenness of the movement. The break which takes place is usually located along a fault line, and earthquakes are generally due to the slipping of blocks of the crust along such a line. Faults are so frequently mentioned in connection with earth disturbances that something should be said at this stage concerning them.

HOW FAULTS ARE FORMED A fault occurs when one part of a bed or set of beds moves relatively to the parts in its proximity. If there is no relative movement it is not a fault, but a fissure, and the formation of fissures in the earth’s crust without relative movement is rare. Thus if you place two blocks of wood side by side and then move one relative to the other it forms a fault, and if one of the blocks is broken so that a similar displacement takes place it is also a fault. And whatever occurs on a small scale in the case of a block of wood occurs on a large scale when blocks of the earth’s crust are involved. The amount of relative movement

may be a few inches or many feet — thousands of feet at times —but tho total displacement does not take place all at once, rather by a scries of fits and starts or sudden slips, each of which causes an earthquake. Thus a fault with a relative vertical movement of ICO feet may involve a score or more ’quakes. These may occur at very long intervals, perhaps hundreds of years, and the great differences of level Quit are produced are only possible if we allow such an age to the earth that we cannot possibly comprehend it. Geologists believe that the development of (lie earth as we see it has taken hundreds of millions of years.

LIVE AND DEAD FAULTS Now there are hundreds of faults in New Zealand; fortunately, only a very few are active or likely to be active. In the great majority of cases no movement may now be. expected in connection with them—they are quite dead. But some faults are alive and these are associated with present day quakes just as the dead ones were associated with quakes of by-gone geological ages. Although it lias been freely stated that two great fault lines occur in New Zealand this is quite misleading. There are a number of faults, with a south-west-north-east trend, occurring in North Canterbury. Marlborough, and the southern part of the North Island. These lie parallel to the chief physical features of the area, such as the various ranges of the Kaikoura Mountains, the river valleys, and steep escarpments of limestone—indeed, these features are the result of movements along the faults. They also affect the bed of the sea, but their presence is merely a matter of inference and not of observation. This series of fractures is the most prolific breeder of earthquakes in New Zealand. There is, however, no definite line of fault of a major character.

WHOLE COUNTRY INTERSECTED

There are others with a north-south trend, such as those associated with the Murchison quake and its developments, and we have yet another system with a north-west-south-east trend, which are apparently inactive; and there are still others crossing these at right angles. Thus tho whole country is intersected with faults, but it must be emphasised that only a very small number of these are active, and that actual movements along a particular fault occur only at very wide intervals, sometimes measured by hundreds of years, for stresses must have time to accumulate before a snap of the beds occurs. There is unfortunately no method of knowing when a fault may prove active. Earthquakes in connection with the Arthur’s Pass or Murchison region were practically unknown till about three years ago, and faults known to occur in those places and believed to be dead, have in experience proved to be only dormant or sleeping, Likewise we do not know when other faults may prove dangerous.

MERCIFUL UNCERTAINTY There is a well-defined earthquake rent running through Porter’s Pass and down the valley across Lake Lyndon, but bow long ago that produced quakes is unknown. there is another neai Rakaia Gorge, and the small earthquake craters from which water was once discharged are still clear and defined. Similar manifestations occur widespread in the northern parts of the South Island and especially in the Kaikoura region. The only parts of New Zealand which are relatively free from quakes are the extreme north and the extreme south, not that those regions are free from faults but any Quit occur are apparently dead. In the case ot Otago there is one region where they are still active, the far north-west, and I suppose the minor quake recently icported from Otago rose in that area of instability. The important question then arises as to what faults arc dead and what are only dormant, and whether there is anv wav of distinguishing them. If a fault is'suspected it can be carefully

watched, and accurate surveys may indicate changes in level or tills of the ground, which indicate coming trouble, but one cannot say definitely when the crust is approaching tho breaking point, and it is perhaps really as well that we do not know. It is possible, however, with the development of seismplogy that reasonably accurate predictions may he made, but that time has not arrived yet. This ,may be a possibility when wo know that a fault is really alive, but it is impossible to watch all, and specially so when a fault occurs under Lhe sea, and may be many miles off the coast.

HAWKE’S BAY QUAKE The earthquakes felt in Hawke's Bay up to the present have usually come from changes taking place some distance off the coast, generallv along the line of a submarine cliff, where tho sea bed suddenly dips down from comparatively shallow water to great depths, but it is reasonably certain that the recent disturbance was due to the upward movement of a block of land in close proximity to Napier itself, and probably lying along the floor of the sea under the bay. The definite location of the origin whence the earthquake came is a matter for seismologists to determine, but the geological evidence points to its being not far from the coast-line. ENORMOUS ENERGY The energy of such a dislocation must be enormous, especially when one considers the volume and weight of the disturbed part of the crust, hundreds of square miles in area. The mere effort to raise this slowly must be very great, but to lift it suddenly for several feet requires tremendous force. No wonder that destruction of man’s works follows such an effort of Nature. All the deep-seated energy may not be expended at the first effort, and the earthquake may be succeeded by others, arising from the same or a different level or at. other points along the fault plane. These are usually called aftershocks. Most of the after-shocks arise from movements along subsidiary faults, which must occur before such a block of country reaches the final condition ol stability. Such after-shocks are rarely as intense as the first, usually they affect a smaller area; they may last for months and even years when the first movement has been great, and they gradually fade away as the stresses become completely satisfied.

LEVELS SAGGING BACK Thus after-shocks still occur at Mm chison, no doubt due to the gradual settling back of the block of country raised at the great ’quake. When this occurred the land was tilted to theeast over a considerable area. At the White Creek fault, some seven miles west of Murchison, it was raised bodily fifteen feet, at Murchison it was raised about seven feet, and this died away to the east, so that at Glenhope there was no apparent rise at all. This was determined because the carefully levelled route of the Buller Gorge railway went right across the tilted area. Now this is slowly sagging back, as disclosed by re-surveys, and the resulting adjustment causes the after-shocks. Similar phenomena must occur at Napier, but they will gradually get weaker, except when some sudden slipping of a block may be responsible for a tremor of more than ordinary intensity. The public belief that earthquakes and volcanoes are intimately connected is not based on a sound foundation. It is true that, earthquakes occur near volcanoes, and that they precede volcanic outbursts, but such are usually quilc local and very rarely indeed are they at all severe. The great earthquakes of the world occur in regions where there are no volcanoes or in volcanic regions where the two manifestations are entirely independent. Thus the country affected by the Arthur’s Pass and Murchison earthquakes is not only free from volcanoes, but is singularly free from volcanic rocks, and the same is true of the Napier and Wellington region. Volcanic rocks do occur in Marlborough and North Canterbury, but they date from a bygone geological age. 'There is no direct connection between the volcanoes of the middle of the North Island and the chief seismic regions of the country.

N.Z. WILL NOT DISAPPEAR There is another point which might be mentioned. It is commonly believed, especially by people who live in Australia and other countries, that .New Zealand is pre-eminently the land oi earthquakes, that the whole land is quivering like a jelly, and that it is only a matter of time before it disappears beneath the waves. 1 his lias no foundation whatsoever in fact. Neither the intensity nor the frequency of our earthquakes equals those ol Northern India, Japan, Italy or South America, and these countries still survive. New Zealand has had as long a geological history as any of them, and there are no grounds for thinking that such a disaster will ever occur. Though we certainly do know that a large part of the country is subject to earthquakes, perhaps a little more severe than wo anticipated, its general stability nevertheless is undoubted. All the same, we must recognise our danger such as it is and meet it with all the resource at our disposal, so that when shocks do occur the loss of proprety and life may be minimised, there is, perhaps, one saving clause. Nature takes a long time in carrying out her designs, and the life of a generation is as nothing in the history of the world, so that if we may expect such convulsions as the Napier ’quake, they are likely to occur only at long intervals. However, we must not forget the lessons taught by one disaster before the next is at hand, for nature never spares those who are deaf to her warnings. And the question is to ask whether we are taking, and will take, these warnings seriously, or whether they will, in a little while, he forgotten till the next occasion falls.

lIOW CAUTION DIES The following from the _ 1929 Bulletin of the Seismological Society oi America may he quoted in conclusion : “Only a catastrophe such as the San Francisco earthquake and fire, the Tokio disaster, or the Florida hurricane arouses a lethargic public. Then there results an immediate realisation of the necessity for more stringent building regulations; the engineer and architect are called up to suggest proper revisions for greater safety, and the building codes of those cities affected by the disaster are revised, after which the cycle of events pursues its course. Ihe ruins are cleared away; damaged buildings are repaired and new ones built; no more disasters occur; the race between expense and profit is under way, and little by little the more stringent regulations are less completely enforced and then allowed to lapse. This has been the history of Californian cities since 1906.” Is it to he the history ol New Zealand?