EARTHQUAKE, the term applied to any tremor or vibration of the ground produced by subterranean causes. Many earthquakes are so gentle as to pass al most unrecognized, others excite general alarm without causing damage, while some spread destruction over wide areas. Probably no part of the earth's surface is wholly free from vibration, but de structive earthquakes are confined to comparatively limited regions. The al most universal succession of phenomena recorded in the case of notable earth quakes is first a trembling or vibration, next a severe shock, or several in quick succession and then a vibration which gradually but rapidly becomes insensib.
In most cases each shock lasts only a few seconds, but the vibrations that follow may be continued for days, weeks, or even months. Noises of various kinds usually precede, accompany, or succeed an earthquake. Some of these have seemed to those who have heard them to resemble the howling of a storm, the growling of thunder, the clanking and clashing of iron chains, the rumbling of heavy wagons along a road, or the shat tering of enormous masses of glass. Such noises are transmitted through the ground, the sea, or the air.
Some earthquakes are not attended by subterranean sounds. At the time of the terrible shock which destroyed Riobamba in Ecuador on Feb. 4, 1797, a complete silence reigned. Subterranean sounds may be heard without any earth-tremor being perceived. Earthquakes are felt either as vertical shocks, from below up ward, as horizontal or lateral shocks, or as undulatory movements. At the time of the great earthquake of Riobamba, the bodies of many of the inhabitants were projected across the river and fell upon La Culla, a hill more than 300 feet high. During the Calabrian earthquake of 1783, the undulatory movement was well marked by the motion of the trees, which swayed to and fro, their branches touching the ground. The same appear c nee was noted at New Madrid, Mo., (luring the earthquake of 1811-1812, where the trees bent as the earth-waves passed under them, immediatly after ward recovering their position.
Observations of this kind have led physicists to the belief that an earth quake is a wave or true undulation of the crust. The wave produced by the original impulse travels outward in all directions from the "focal cavity," or "centrum," in successive spherical shells. The point or area on the surface of the ground directly above the "origin" or centrum, is called the "epicentrum"; it is at this point that the shock is usually felt as a vertical stroke coming from below upward. As we recede from this point, the direction of motion becomes more and more nearly horizontal, and also gradually decreases in intensity till it becomes insensible. Away from the epicentrum, the earth-wave at every point comes up obliquely from below— the radial lines along which an earth quake is propagated from the centrum being called "wave-paths." The direc tion of motion is also influenced by the configuration or varying topographic features of the disturbed district.
Most earthquakes occur during the cold months or in winter, at which time barometric fluctuations are most numer ous. Among memorable earthquakes may be noted that of Lisbon, Nov. 1, 1755, destroying 60,000 lives, which left the city in a heap of ruins, and was felt from the Madeiras to Great Britain; and that which destroyed Aleppo in 1822.
Within the United States only five great earthquakes are known to have oc curred since the first settlement.
The earliest occurred in the year 1755, in Massachusetts, which, though very forcible, was much less so than the other four, but the information about it is scanty. The second, the most energetic of all, was that of New Madrid, Mo., in 1811, folowed by many forcible shocks in subsequent months and years. The third was in the Inyo Valley of Cali fornia in 1868; and the fourth at Charleston in 1886, which was from 107 to 120 kilometers deep and was felt at a distance of 1,000 miles. The fifth was the San Francisco earthquake of 1906, given below.
The severe and destructive earth quakes since the Christian era are as follows: A. D. 79. Destruction of Herculaneum and Pom peii. Destruction of life very great, includ ing Pliny the Elder.
A. D. 526. Around the Mediterranean. Esti mated fatality, 120,000 people.
Dec. 18, 1631. Naples. Fatality, 3,000 people. June 7, 1692. Port Royal. Fatality, 3,000 people.
A. D. 1693. Sicily. Destruction of Catania, 300 villages and 60,000 people.
Oct. 28, 1724. Lima and Callao. 18,000 people destroyed.
Nov. 1, 1755. Lisbon. Fatality, 60,000. It was felt over a twelfth of the earth's surface. Feb. 5, 1783. Calabria generally. 30,000 people. Feb. 4, 1797. Riobamba, Quito, Cuzca. 40,000 people.
March 26, 1812. Caracas. More than 20,000 people.
Jan. 11, 1839. Martinique. 300 people.
Aug. 13, 1868. Peru and Ecuador. Especially at Arica, 70,000 people destroyed and $300, 000,000 worth of property.
April 3, 1880. Chios. 14,000 dwellings and 3,541 people.
July 28, 1883. Ischia. 2,400 people.
Aug. 27, 1883. The Krakatoa destruction was peculiarly volcanic, not seismic. It de stroyed about 35,000 people by a tidal wave.
Aug. 31, 1886. Charleston earthquake described above.
Oct. 28, 1891. Central Japan (Owarl-Mlno).
Destroyed 7,000 people, 200,000 dwellings, and 822,500,000 worth of property. Feb., 1902. Caucasus. 4,000 people.
May 8, 1902. Guatemala. Was very fatal, but numbers unknown. It was simultaneous with the outbreak of Mt. PeIN on Martinique which destroyed St. Pierre and was fatal to 20,000 people.
April 18, 1906. San Francisco Bay. Destroyed San Francisco and Santa Rosa, uncounted buildings, about 800 lives and more than (est.) $300,000,000 worth of property. Dec. 28, 1908. Messina. Destroyed the city and caused the death of 200,000 people, the worst earthquake disaster in history. May 5, 1910. Cartage, Costa Rica, destroyed. Oct. 11, 1918, Porto Rico, loss of 150 lives and property valued at $1,000,000.