SCIENTIFIC.

Otago Witness, Issue 1921, 14 September 1888, Page 35

SCIENTIFIC.

EARTHQUAKES.

(Written for the Witness by F. G. S.) No. 11. The effects of an earthquake are dependent not only upon the violence of the concussion, but upon the structure of the ground and the character of the buildings erected thereon. Thus it is sometimes found that one portion of a city may be more damaged than another; and this is not necessarily due to any diminution of intensity in the shock, but to the fact that the differently affected areas consist of dissimilar geological formations. As an instance, we may cite the town of Port Royal in Jamaica, one portion of which was erected on solid limestone, and another of more recent date on ground gained by drifted sand. During the earthquake of 1692 the latter was shaken to pieces, while the former remained comparatively uninjured. In this catastrophe the ground swelled and heaved like the rolling sea, and the earth was traversed by numerous cracks in which many people were swallowed up, some totally, and others only partially, their heads remaining above ground ; some were left alive in this horrible position and perished slowly ; subsequently the projecting parts of their bodies formed food for dogs.

It is usually found that buildings erected on loose inelastic foundations, such as sand or clay, suffer much more severely than those built on solid rock. 'Professor Milne, however, gives numerous instances where the rule here enunciated has apparently been reversed. It must not be supposed that the severe shaking experienced at the time of an earthquake is the measure of the ultimate damage. This is, no doubt, sufficiently severe, and, in the case of an inland tract of country usually forms the beginning and end of the destruction. In addition, however, to the immediate effects of the shake, huge blocks of rock may become dislodged from positions they have long occupied, and come crashing down, causing landslips, which scatter desolation and death. As has been already mentioned, fissures are formed in the earth, which may engulf trees, houses, quadrupeds, and human beings. In the 1848 earthquake of New Zealand, a fissure 18in wide was formed, running parallel to the main axis of the Kaikoura mountains, and extending for a distance of 60 miles. The 1855 earthquake, in the same locality, caused a fracture which could be traced for 90 miles. The Japanese have a proverb, which advises .people to take to a bamboo grove during an earthquake. This is because the soil beneath a grove of these trees is so matted together with fine roots that it is almost impossible for it to be rent open. Terrestrial waters are very frequently Subject to Sjcvere Change ;

old springs suddenly cease to flow, or on the other hand become greater in volume, or

muddy and discoloured ; in other cases new springs burst forth. One of these in a mining district of America was within two hours taken up as a water right under the laws of the State in which it appeared. Lakes are even more sensitive than running water. Sudden oscillations in level occur even at a distance of hundreds of miles from the seat of the disturbance. Sometimes the waters of a lake are suddenly drained, and the bed is left dry ; in other instances, depressions of the ground are caused, which when filled with water, form new lakes. In the earthquakes of 1811-12, in the Mississippi Valley, lakes 20 miles in extent were formed in the course of an hour. Occasionally this effect is produced by large slips of rocks and stone, which form dams in the beds of rivers and cause immense accumulations of water ; if the barrier be strong enough to resist the pressure, a permanent lake is formed, which in process of time becomes filled up ; but if the dam be too weak a sudden rush of the pent up water may occur and cause untold havoc. In other earthquake disturbances rivers have been almost dried up, as in 1158, when the Thames became so low that it could be crossed on foot, even at London ; and in 1110 the Trent at Nottingham became equally shallow. The effects of ■

Seismic Phenomena at Sea are naturally less open to observation than those occurring inland, but many instances are on record of shocks felt by vessels far from land, which coincided in time with earthquakes observed on shore. The feeling is variously described as resembling sharp tugs at the cable, when vessels were at anchor, and in other cases as being like sudden blows, or like the noise which would be produced by large rocks dropped into the hold. Sometimes the rigging and masts have been damaged and cannon jerked up from the decks on which they were resting.

As was mentioned in a preceding article, one form- in which the force manifests itself is as the sea wave caused by the direct transmisssion of the vibration in the crust of the earth to the overlying water. The arrival of this wave at places distant from the seismic centre is usually subsequent to the sensation of tremor in the crust. At the great Lisbon earthquake about an hour elapsed between the two, and the sea wave in that case completed the most awful and sudden destruction of human life ever known in the history of the world. There was absolutely no warning. On the Ist November 1755 a noise like subterranean thunder was heard, and a violent shock overthrew the greater part of the city. In five or six minutes 60,000 people perished. It was a holiday, and many people were in the churches, which fell upon them ; others rushed for safety to a magnificent marble ' quay, then recently erected, for there, they thought, no buildings could fall upon them. Alas I for their vain hopes of safety. In a moment the quay with its living burden was swallowed up as in a whirlpool and no tra.ce of the unfortunate refugees was ever again seen. Over their grave is 600 ft of water. " The sea wave which succeeded this shock was 50ft or more above its ordinary level, and; as in ■ most other instances, the waters first retired and then rushed back with terrific force. The height of this was, however, nothing to one which occurred on the coast of Lupatka, and reached the extraordinary altitude of 210 ft. The connection between the sea and earthwaves is not thoroughly understood; the great difficulty arising from the fact that they are , not always associated, and that sometimes the sea wave precedes the other. For instance, the great earthquake of June 1855, in New Zealand, was unaccompanied by any oceanic disturbance, while the small shock of February, in the same year, was followed by a motion of the sea. In the recent occurrence of similar events, we have received no records of tidal irregularities.

In addition to the destruction wrought among the works of man and the fractures in the earth's surface already referred to there are Great Paroxysmal Movements' of the crust,

which remain, as enduring monuments of the great forces at work, after all traces of the other and more easily obliterated effects are effaced by the lapse of time, and by those natural agencies which smooth down the scars left by slips and cover them with a mantle of verdure, which wash down the waste of the hills into the fissures and valleys, and which gradually restore to the landscape the features of repose and peace which it originally possessed.

These sudden movements leave their record on the history of the earth by permanent elevations and depressions of land. It is naturally somewhat difficult to register these changes of level in continental areas, or even in the inland portions of islands. When long stretches of level water, as, for instance, canals, traverse a country we have some facts to go upon ; and it is stated that in the delta^ of the Indus in 1819 a large area was depressed, and the watercourses were suddenly changed. When, however, we have the sea level as a datum we can with considerable accuracy determine what changes have taken place. In this colony in the year 1855 a tract of land, comprising very nearly the area of Yorkshire, is estimated to have been raised frpm lft to 9ft. The alteration in level was not perceptible on the coast 16 miles North of Wellington, but from that point to Pencarrow Head the elevation increased until it finally reached 9ft. In the South Island the movement was reversed, being downward, and the valley of the Wairau is supposed to have subsided about 5f t,so that the tides flowed several miles further up the river, and ships l > taking in fresh water were obliged to go three miles farther from the sea.

The great object sought to be attained by seismologists is (putting aside the prediction of earthquakes) how to lessen their

Disastrous Effects Upon Life and Property.

To this end much attention has been devoted to discovering what class of buildings are most likely to offer a resistance .to damage. And here again we meet with some striking differences of opinion ; but it appears to be particularly important erections that should be of either one class or another — either solid, substantial buildings, strong enough to defy the shocks, or else light, yielding structures so unsubstantial as to escape injury. In all cases it is advisable 'to build low, and avoid heavy turret-topped chimneys. " Turret-like brick chimneys," says Mr H. E. Warner, writing of Caraccas (or the city of earth-

quakes), " with iron-top ornaments, would expose the architect to the vengeance of an excited mob." On no account should composite buildings be allowed — that is, those in which the resisting power of the materials ia unequal; as, for instance, wooden houses with brick chimneys attached. In some cases houses are placed upon round stones or cannon balls, so that they can to a certain extent rock when subject to earth tremors.

Don't sit t~o near the footlights at the theatre, for it has been observed that nearly all the men who do so have become bald and urey, evidently from the effects of the glare. The best way, however, to prevent ths hair from turning grey, or to restore its natural colour and freshness when the Bilver streaking has already begun, is to use Mrs S. A. Allen's World's Hair Restorer, which never fails to absolutely renew tho colour of youth. Sold everywhere.