EARTHQUAKES.
The following interesting article on the subject of earthquakes is taken from a late number of the University Echo, and is from the pen of Professor Joseph Le Conte, of the University of California. There is no subject connected with dynamical geology which has so excited the interest and baffled the power of science, as that of earthquakes. Until about twenty years ago thex-e xvas really no scientific knowledge on the subject at all. Observations there were, it is true —an ever-increasing mass of rubbish in the form of loose popular observation—but not a single truly scientific observation, not a single attempt even to redxxce to order the crude materials already ou hand. The reason of this slowness of scientific development in a subject of such vital importance is partly the real difficulty and complexity of the phenomena, deep beneath the sxxx-faoe of the earth, and therefox’e far removed from direct observation ; but principally their suddenness and the terror they inspire utterly unfitting the mind for scientific observation. We never can reduce to order the chaos which still exists, until we have observers which cannot be affected with teiTor—until we have self-recording instruments or seismometei’s—distributed in abundance over every earthquake region. Within the last twenty years, however, and mainly by the use of seisinoinetei-s, the foundation of a true science of earthquakes has, we believe, been laid. FREQUENCY OP EARTHQUAKES.
Mallet, iu his earthquake catalogue, has collected the records of 6830 earthquakes, occurring in the 3456 years previous to 1850, or an average of about two per annum for the whole period. Of this number, 3240, or nearly onehalf, occurred in the last fifty years, or about sixty-five per annum ; and during the last four years the record gives us about two per week, or one hundred per annum; not because earthquakes were more frequent, but records were more perfect. When we recollect that three-fourths of the earth-surface is covered with the ocean, and that but a small portion of the remaining one-fourth is occupied by civilised man, we may safely say that there is certainly not a day and probably not an hour but the earth is trembling in some portion of its surface. CONNECTION WITH OTHER I’ORMS OF IGNEOUS AGENCY. That there is a close connection between earthquakes and volcanoes is plainly shown by the following facts :—l. Violent volcanic eruptions are always preceded and accompanied by earthquakes. 2. Earthquake regions are also volcanic regions, or have been so in very recent geological times. If we have two maps ; one shaded to exhibit the volcanic intensity, and the other the earthquake intensity of different portions of the earth-surface, the portions of the earth shaded and the depth of the shading will be found nearly the same in two cases. 3. But while there is an undoubted association of the two phenomena, as if they were produced by similar causes ; yet there is also a complementary relation, as if volcanoes are vents through which the forces which cause both find relief. Thus when the smoke wreath disappears from Cotopaxi the inhabitants expect earthquakes. Stromboli is a rare example of a volcano in incessant activity ; in 1783 it ceased for the first time in the memory of man ; the great Calabrian earthquake followed soon after. The great earthquake of lliobamba, in South America (1797) followed close upon the sudden checking of activity in a neighboring volcano. The neighborhood or the town of Caracas was for several years infested with earthquakes, one of which destroyed that town in 1812. These troublesome shakings ceased on the bursting out of a volcano in the neighborhood.
These facts would seem to indicate that in both volcanoes and earthquakes a powerful force of some kind is generated within the earth ; if the crust breaks and the force finds vent we have a volcanic eruption ; but if not, an earthquake is produced. There are also some phenomena which connect earthquakes with another form of igneous agency, viz., gradual bodily elevation and depression of the earth’s crust. In 1835, after a great earthquake which shook the coast of South America over, an area of 600,000 square miles, the whole coast line of Chili and Patagonia was found elevated above the level of the sea 210 feet. Again, in 1822, iu tho same region, after a similar earthquake, the coast lino was again raised 2 to 7 feet above the sea level. Now in this very region there are old beaclx marks or sea terraces from 100 feet to some 1300 feet above the sea, and extending 1200 miles along tho coast on each side of the southern point of South America, plainly showing tlxat tho whole southern eud of this continent, during very recent geological times, has been bodily raised out of the sea to that extent. It seems impossible to doubt that the force which px-oduced this continental elevation was also the cause of the accompanying earthquakes. Again, 1819, after a severe earthquake which shook the whole region about the mouth of tho Indus and Ganges, a large tract of land, 2000 square miles in area was sunk, and became a salt
lagoon, while another area fifty miles long and ten to sixteen miles broad was raised ten feet. In commemoration of this wonderful event, the raised portion was called “ TJllah bund,” the Mound of God. Again, in 1811, a severe earthquake (probably severer than any yet felt in California) shook the valley of the Mississippi River. In the region a little below the mouth of the Ohio River, where it was most severe, large tracts of land were sunk bodily several feet below their former level, and have been covered with water ever since. It is now called the “ Sunk Country.” In the two cases last mentioned there was evidently formed a fault or dislocation, i.e., there was a fissure of the earth’s crust, and one side dropped down lower than the other. Such fissures and faults are found intersecting the strata almost everywhere. We see them in these cases formed under our very eyes in connection with earthquakes. ULTIMATE CAUSE OF EARTHQUAKES. The connection of earthquakes with the two other forms of igneous agency suggests each of them a possible cause. Preceding and accompanying volcanic eruptions, occur subterranean explosions so violent that they are heard a distance of hundreds of miles. In the great eruption of Tomboro, in 1814, these explosions were heard over 900 miles. Also, it is well known that in volcanic eruptions, especially of the exjdosive type, gas and (especially) steam escape in such enormous quantities as to leave little doubt that the'foroe of the eruption is in fact the elastic force of steam. These facts, together with the association of earthquakes with volcanic eruptions, has suggested the idea that the sudden revolution or the sudden collapse of vapor may be the cause of earthquakes. According to this view, an earthquake is, on a grand scale, a phenomenon similar to the jar produced by the explosion of a keg of gunpowder buried in the earth. But the association of earthquakes with the bodily elevation or subsidence of great areas of the earth's crust suggests another, and we believe a much more probable cause. It is well known that the earth’s crust is slowly rising in some places and sinking in others. This is particularly the case in earthquake regions, as for example on the coasts of. the Mediterranean and in South America ; but also in regions not troubled with earthquakes, as on the shores of the Baltic. Now suppose we have a force of any kind tending to elevate or depress the earth’s crust in any locality ; if the crust yields steadily we have gradual, quiet elevation or depression, as on the shores of the Baltic ; but if, on the other hand, the crust resists, then the force accumulating and the tension of the crust increasing, rupture of the crust must eventually take place, with sudden elevation or depression, as in the case of the coast of Chile. In case of elevation the fissure would be formed on the surface, but in case of depression far beneath the surface. Such a sudden formation of a fissure, either with or without displacement or fault, would if necessity produce a concussion or jar which would propagate itself until it reached the surface, and then spread outward on the surface.
This is, we believe, by far the more probable cause of earthquakes. When we remember that fissures often extend many thousands of feet, and even many miles into the earth, we easily perceive that the sudden rupture of such thickness of rock would produce a concussion fully adequate to produce the dreadful effects of earthquakes. As to the cause of the elevation and depression, the discussion of this point would carry us entirely too far. Suffice to say that it is most probably due to slow secular contraction of the earth still going on, more in the interior than the exterior, by which the crust by lateral pressure is thrown into great wrinkles. PROXIMATE CAUSE. But whatever be the true or ultimate cause —whatever it be that produces the original conclusion —whether it be in the nature of an explosion, or, more probably, a sudden rupture of the earth’s crust ; this original concussion propagates itself from the origin or focus in every direction as an elastic spherical wave, or as a series of such waves, precisely in the manner of sound waves, until it reaches and emerges on the surface. Prom this point of first emergence it then spreads in every direction, reaching more and more distant points successively. It is this progressive earth jar or tremor, this series of elastic earth-waves, successively emerging and spreading on the surface, which is the immediate cause of the observed phenomena of earthquakes—which in fact constitutes the earthquake itself. ELECTRICAL THEORY. There has arisen in the last few years and become quite prevalent in California a theory, or rather a notion—an opinion—that earthquakes are in some way due to elecricity ! The argument, as we have heard it, goes somewhat as follows ; Thunder and lightning are rare in California, while earthquakes are not unfrequent; therefore earthquakes in California take the place of thunderstorms and summer rains ; therefore since thunder storms are attended with electric discharges, earthquakes must be produced by electric discharges ; therefore earthquakes are a sort of subterranean thunderstorms they are produced by subterranean lightning. Let us examine this so-called theory. There is a strong tendency in the intelligent popular mind, and even in many scientific minds not thoroughly imbued with the true spirit, nor thoroughly trained in the true methods of science, to attribute whatever in nature is mysterious or imperfectly understood to electricity. Vital phenomena, especially those connected with the nervous system, are attributed to electricity certain curious physiological iihenomena are attributed to animal magnetism ! —thought, feeling, will, are the product of electricity ! So also in geology, slaty cleavage—mineral veins, foliation of metamorphic rocks, &c., have been attributed to electric currents. Thus electricity is made the scapegoat for the sins and shortcomings of other sciences. Now, it is true that our knowledge of electricity is still very imperfect, yet there is certainly much that we do know about electricity —rthere is a largo body of ascertained facts and well established laws. Electricity has already assumed the form of a positive science, as is sufficiently shown by the varied applications of this wonderful agent. It is in the highest degree unphilosophioal and irrational, therefore, to attribute any phenomena to electricity unless they are similar to the known phenomena, or subject to the known laws of electricity. Now suppose the earth charged to any extent with electricity—imagine a gigantic electric machine pumping inconceivable quantities of electricity into the earth until it is charged like a great leyden jar. If now it can be shown how this electricity can produce a concussion, giving origin to elastic earth waves, then it will be time to consider seriously this theory—then it will begin to deserve the title of a theory. Currents of electricity doubtless there are within the earth —thermo-electric, chemico-electric, magnetoelectric ; but electric currents cannot produce concussion. A concussion such as gives origin to earthquake waves, if produced by electricity at all, could be produced only by disruptive discharge ; but a discharge within a good conductor, in the present state of science, must be regarded impossible.
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New Zealand Times, Volume XXXII, Issue 5010, 14 April 1877, Page 1 (Supplement)
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2,068EARTHQUAKES. New Zealand Times, Volume XXXII, Issue 5010, 14 April 1877, Page 1 (Supplement)
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