EARTHQUAKES.

Press, Volume L, Issue 14974, 22 May 1914, Page 9

EARTHQUAKES.

AND VOLCANIC ZONES. * PROFESSOR DATTD'S VIEWS. Roforring to tho disaster which recently visited the villages iv tho noigtibourhootl of Mount Etna, Professor David, of Sydney, remarked to an "n----te.-viewor from tho "Daily Telegraph" that the earthquake, which had led to such appalling suffering and loss of life, appeared to have had a somewhatdifferent modo or origin from that, which occasioned the still moro terrib'.o catastropho of Messina and Reggie, in 1909.

"The latter earthquake," said tho j Professor, "commonly known as tho Mussina earthquake, affected a largo j area, and was due to au imraeuse crack in the earth's crust leading to a bodily ; displacement of tho crust on oithei side of tho crack. It was probably accompanied by a slight compensating elevation on the earth's crust on tho opposite side of tho crack. Tho prescut earthquake seems to have been oxtreniely local in its effect, and perhaps connected with either severe steam explosions in Mount Etna, or to tho collapse of tho roof of some subterranean reservoir, through which the supplies of lava for Etna have been drawn. Obviously in a case whero » volcano has been., pouring out lava for generations, many cubic miles in bulk, the withdrawal of so vast an amount-of material from underneath the earth's crust leaves the crust unsupported, so that under the forco of gravity sooner or later the crust collapses, in order to .ill up that vacuum which Nature abhors. Such fallings in of the crust in tho neighbourhood of volcanic centres are very common, and invariably give rise to'sharp earthquake shocks. Itshould bo possible later for geologists to ascertain by an actual (survey whether or not such a collapse has taken place in the neighbourhood of tho crater of Mount Etna, inasmuch ns ustiallv the cracks ascend to tho surface and affect the surface displacement, • which can bo very easily traced. it may bo mentioned that Etna is situated on a great earthquake and volcanic zone. This runs from Etna eastwards through Reggio, and then bends around, following the courso of the Italian Apounino Mountains, and, as is well known, Vesuvius is situated on a part of this zone. SNAPPING OF THE CRUST. "But this is not tho only great earthquake crack whi-ft runs througa Etna,*' continued tho Professor, "for a second important fissure extends northwards, from Etna through tho Island of Vulcano, from which tho generic nanio -of volcano was originally derived, to the volcanic island of Stromboli. It will thus bo seen that Etna is situotodl at tho junction between two powerful volcanic and earthquake ?<ones. Iho I smallness of tho area said to have been 1 affected by this recent severe earthquo ko suggests also that tho point of : origin of tno earthquake was a shallow ! one. It was formerly supposed that earthquakes originated at depths of from 10 up to W, or even as much :is j3O miles below,the earth's surface, This ! view is now known to be erroneous. I Most earthquakes originate at a depth ' not much exceeding five miles. __ and oven distinctly less than that. Earthquakes which affected very largo areas, ; like that of Messina, and that of San I Francisco, in 1906, aro due to a snapping of tho earth's crust along fiomo I huge more or loss vortical lino of fracture, running more or less in a straight i lino often for over 100 miles. Tbo 1 crack is produced, as geologists believe, 1 through slow changes of temperature involving loss of heat, moro rarely perhaps increase of heat, causing certain parts of tho earth's crust to be put in a state either of tension like a pioco of india-rubber when it is stretched, or of compression. It is estimated that tho actual area of rock crack during tho development of an extensive earthquake fissure is something of the order of 100 square miles, when the fact is lome in mind that a bar of Gib rock eryonite from near- Bowral, having a cro-*section of about one square inch, will stand a pull cf nearly a ton beforo it snaps, one can imagine tho enormous forces which aro needed to 6nap the prism of rock with a cross-section of 700 souare miles, and ono cannot wondor that tho sudden releaso of this enormous stretching force such as that which piace at the mom on t when au earfcr-diiak© fissure opens, sets up violent vibrations, which strike buildings vertically, obliquely, or horizontally with .astlymore than steam-hammer forco. WELL-MARKED "ONES. "Of late years seismologists have observed that not infrequently when one part of tho earth's a list is affected by a violent earthquake shock or violent volcanic eruption, the region that is antipodal to it is frequently affected almost simultaneously, or a trifle later in timo. Tho area of tho earth's crust that is antipodal to Etna lies_ in the Pacific Ocean to tho cast of Kermadee Islands. If tho recent Etna eruption had boon of tho magnitude of that of Messina there might havo been a serir ous risk of a sympathetic outburst 'n that region. At the samo time ii is not always tho antipodal regions that are specially affected. It has been proved now that thero are certain wellmarked zones followed by earthquakes. These are mostly associated with parts of the earth's crust whore active movement is taking place- Such movement is very noticeable around the shores of almost the whole of the Pacific and Southern Oceans, which is thus described as being girdled with fire, for the earthquake belt is also a volcanic zone.

■/'Recently Father Pigot, S.J., of Riverviow. has shown that the chief shocks which reach tie eastern side of Australia originate in movements of tho {cep ocean basin between New Calelonia and Auckland, cr in that deepest known portion- of the ocean, the Great Trench, which lies imroediatelv to 10 east of tho Tongan Islands. This - over 30.000 ft deep, so that if Mount Everest could bo placed in it it would bo completely overwhelmed. These extra deep narrow trenches in the ocean floor, which indicate where slices if the earth's crust have recently subsided. '*""\~'''r.ly give rise to earthA .iake shocks.".